Numerical Groundwater
Flow Modeling
Of the
Dire Dawa Area
By
Minalah Bushra Yousuf
A thesis submitted to the School of Graduate Studies of Addis Ababa
University in partial fulfillment of the requirements for the Degree of
Master of Science in Hydrogeology (AAU)
February, 2007
Acknowledgements
First of all I would like to thank Dr. Tamiru Alemayehu, my advisor and initiator of this
work. Further more I would like to acknowledge him in special way that advised and
generously shared his experience and giving me reference materials. His critical reading
of the zero draft and his invaluable comments give the present shape of this thesis.
I am grateful to Oromiya water works construction enterprise (OWWCE), especially to
Ato Tariku Negera (General manager of OWWCE) for giving me the chance to resume
my post graduate study, encouraging me during study time and facilitating the necessary
logistics through east Hararge project office.
I gratefully acknowledge all organizations and individuals who directly or indirectly
supported me in my study. First I would like to thank my friends Ahmed Jemal, Adane
Bekele, Legese Geda and Mesele Siyum for all their financial and material supports
during my study. I would also like to thank Ingida Zemedagenyehu, WWDSE, for all the
data he gave me .I also would like to forward my hear felt thanks to Japan International
Cooperation Agency (JICA) and ministry of water resources for giving me the opportunity
to participate in 3rd International groundwater modeling training, without which this work
was not possible; Dire Dawa Water resource office; Dire Dawa town water supply
sewerage Authority and Harar water supply project office.
Last but not least, I would like to forward a profound gratitude to my wife, Sr. Seada
Amin, for her blessing, guidance, encouragement and support which gave me strength to
carry out the work.
2
Table of Contents
pages
Acknowledgement ................................................................................... i
List of figures ......................................................................................... v
List of tables ......................................................................................... vi
List of annexes ...................................................................................... vi
Abstract ................................................................................................ vii
1. Introduction
1.1. Back ground ............................................................................... 1
1.2. Objective .................................................................................... 3
1.3. Methodology .............................................................................. 3
1.4. Pervious works .......................................................................... 6
2. General overview of the study area ............................................. 7
2.1. Historical overview of Dire Dawa and the surroundings ............ 7
2.2. Location ..................................................................................... 7
2.3. Physiography and drainage ....................................................... 9
2.4. Land use/Land cover ............................................................... 12
2.5. Climate .................................................................................... 13
3. Regional geology and hydrogeology ......................................... 17
3.1. General ................................................................................... 17
3.2. General geologic setting .......................................................... 17
3.2.1. Precambrian Basement rocks ........................................ 18
3.2.2. Mesozoic Sedimentary rocks ......................................... 18
3.2.2.1.
Adigrat Sandstone/ Lowe Sandstone ................... 19
3.2.2.2.
Hamaneli Limestone ........................................... 19
3.2.2.3.
Amba Aradam sandstone/ Upper sandstone ...... 19
3.2.3. Tertiary volcanic ............................................................. 19
3
Table of contents ------ Continued
3.2.4. Quaternary sediments .................................................. 20
3.3. Structural and Litho-logical features ......................................... 22
3.4. Effects of faulting system on the modeled zone ...................... 23
3.5. Regional Hydrogeology ........................................................... 23
4. Conceptual model ........................................................................ 25
4.1. Local geologic setting and geologic structures ......................... 26
4.1.1. Local geologic settings ................................................... 26
4.1.2. Local geologic structures ................................................ 27
4.2. Hydro-geologic setting of the area ........................................... 28
4.3. Aquifer system ......................................................................... 33
4.4. Boundary condition .................................................................. 35
4.5. Geometric characteristics ........................................................ 36
4.6. Hydraulic Properties ................................................................ 39
4.7. Groundwater recharge zonation .............................................. 42
4.8. Groundwater Discharge .......................................................... 43
4.9. Groundwater level and movement ........................................... 47
4.10. Conceptual groundwater balance ......................................... 50
4.11. Model in put Data processing ............................................... 57
5. Numerical simulation of groundwater flow ................................ 59
5.1. General concept and modeling approach ................................. 59
5.2. Well distribution ....................................................................... 61
5.3. Governing Equation and model code ....................................... 62
5.4. Spatial discretization ............................................................... 65
5.5. Boundary condition .................................................................. 66
5.6. Mode input parameters ........................................................... 71
5.6.1. Initial and prescribed Hydraulic heads ........................... 71
5.6.2. Hydraulic properties ....................................................... 72
4
Table of contents ------ Continued
5.7. Model simulated stresses ........................................................ 74
5.7.1. Recharge ....................................................................... 74
5.7.2. Discharge ...................................................................... 75
6. Calibration and sensitivity Analysis ........................................... 76
6.1. Model calibration ..................................................................... 76
6.2. Data used for calibration .......................................................... 77
6.3. Steady-state calibration ........................................................... 77
6.4. Calibration Results .................................................................. 81
6.5. Simulated water budget ........................................................... 89
6.6. Sensitivity analysis .................................................................. 90
6.7. Scenario Analysis .................................................................... 92
6.8. Model limitations ...................................................................... 94
7. Conclusions and Recommendations ......................................... 96
7.1. Conclusions .............................................................................. 96
7.2. Recommendations .................................................................. 99
References---------------------------------------------------------------------------101
5
Lists of figures
Pages
Fig.1.1 Flow chart showing the general methodology followed------------- 5
Fig.2.1 Location map of the study area------------------------------------------- 8
Fig.2.2 Profile showing elevation drop along south-North------------------ 10
Fig.2.3 Drainage map of the study area----------------------------------------- 11
Fig.2.4 Mean annual rainfall of Dire Dawa area------------------------------- 15
Fig.2.5 Average yearly rainfall at Dire Dawa area---------------------------- 15
Fig.2.6 Mean monthly rainfall & evapotranspiration of the area----------- 16
Fig.4.1 Geologic map of the study area & its surrounds--------------------- 31
Fig.4.2 Hydro-geologic map of the study area & its surroundings--------
32
Fig.4.3 Operating principle of water circulation in the study area---------- 34
Fig.4.4a 3D map of layer-1----------------------------------------------------------- 37
Fig.4.4b 3D map of layer-2----------------------------------------------------------- 38
Fig.4.5 Transmissivity values of different geologic formation---------------- 41
Fig.4.6 Map of the study area showing water points used in the Model--- 49
Fig.4.7a Static water level contour map of layer-1------------------------------ 51
Fig.4.7b Static water level contour map of layer-2------------------------------ 51
Fig.4.8 Color composite land sat TM image of the study area--------------- 58
Fig.5.1 Quasi 3dimensional and full 3dimensional models-------------------- 61
Fig.5.2 Location map of the existing water points------------------------------- 64
Fig.5.3a Model area showing boundary condition of layer-1------------------69
Fig.5.3b Model area showing boundary condition of layer-2------------------70
Fig.6.1a Distribution of observation well in layer -1----------------------------- 79
Fig.6.1b Distribution of observation wells in layer -2---------------------------- 80
Fig.6.2a Scatter plot comparing observed and simulated heads-------------82
Fig.6.2b Histogram showing error distribution-------------------------------------83
Fig.6.3a Comparison of observed and simulated contour map of layer-1--82
Fig.6.3b Comparison of observed and simulated contour map of layer-2--83
Fig.6.4 Plot of result of sensitivity analysistest on head ------------------------92
Fig.6.5a Simulated hydraulic conductivity zonation of layer-1-----------------84
Fig.6.5b Simulated hydraulic conductivity zonation of layer.2-----------------85
6
Lists of tables
Pages
Table2.1 Mean monthly values of Dire Dawa
Meteorological elements-----------------------------------------------14
Table3.1 Generalized geological events of Dire Dawa area --------------21
Table4.1 Classification and description of hydro-stratigraphic units--- -34
Table4.2 Summary of transmissivity of the different geologic units----- 41
Table4.3a Summary of daily groundwater abstraction rates form
Wells, Boreholes, and Springs--------------------------------------52
Table4.3b Daily groundwater abstraction rate from water point,
and Discharge contribution of each wells------------------------53
Table4.4 Summary of location and Spring discharges ---------------------56
Table6.1 Calibrated and observed water levels------------------------------ 87
Table 6.2 Model simulated steady-state hydrologic budget--------------- 90
Table6.3 Results of sensitivity test on water level--------------------------- 91
Lists of Annex
ANNEX-1-------------------------------------------------------------------------------105
7
Abstract
Groundwater pumping from the study area is currently increasing and hence there is a
concern that future water demands associated with expansion of urbanization,
industrialization and increasing trend of local population in utilizing groundwater for their
irrigation demand could cause water level declines. To address these concerns a
steady–state groundwater flow model of the Dire Dawa area was developed to help
better understand the aquifer system, assess the long-term availability of groundwater,
and evaluate groundwater conditions owing to current pumping and to plan for future
water needs from the area.
A two layer groundwater flow model with a confining unit in between was developed.
Boundary conditions, hydraulic conductivity, altitude of the bottom of the layers, storage,
coefficient, recharge, and discharge were determined using exiting geohydrologic data.
Rates and distribution of recharge and discharge were determined from existing data and
estimated when unavailable.
Steady-state groundwater flow is simulated in the model by the finite-difference method
using MODFLOW-96+interface with advective transport. The finite-difference grid
consists of two layers, 87 rows and 125 columns. The model uses a uniform grid size of
400m by 400m and contain 8011 active cells in both layers.
Model calibration was accomplished by varying parameters within plausible range to
produce the best fit between simulated and observed hydraulic heads. Two calibration
criteria were set for this calibration. The first criterion was visual matching of simulated
contours to those of observed contours. The second criterion sited was matching
simulated hydraulic heads at 80% of the point to within 10m of the observed hydraulic
heads. Both criteria are found sufficiently achieved.
8
The importance of each input parameter and their effect on simulation results were
evaluated through sensitivity test, in which the value of hydraulic parameters such as
hydraulic conductivity and recharge were adjusted above and below their calibrated
values. The model was found most sensitive to hydraulic conductivity and recharges
Water management alternatives were evaluated by simulating hypothetical scenarios of
increased withdrawals and altered recharge of the steady–state condition with in the
model.
9
CHAPTER - 1
INTRODUCTION
1.1. Background
In recent decades it has become evident in many countries of the world that groundwater
is one of the most important natural resources. As a source of water supply groundwater
has a number of essential advantages when compared with surface water: as a rule it is
of higher quality, better protected from possible pollution including infection, less subject
to seasonal and perennial fluctuation, and much more uniformly spread over large region
than surface water. Putting groundwater well fields in to operation can be gradual in
response to growing water demand while hydro-technical facilities for surface water use
often require considerable one time investment.
These advantages coupled with reduced groundwater vulnerability to pollution
particularly have resulted in wide groundwater usage for basic human needs, agricultural
and industrial developments in the world. To meet the increase demand of water due to
rapid growth of population, urbanization and industrialization especially in developing
countries, it is very important to evaluate groundwater resources quantitatively to make
management strategies.
Generally, evaluation of groundwater resources starts from basic studies such as
geological and hydro-geological investigations, meteorological analysis, and water
balance analysis. Based on these studies, groundwater basin and aquifer units are
identified and aquifer characteristics are evaluated. After these studies, groundwater flow
system and its potential is evaluated by means of numerical groundwater flow modeling
using the above parameters and finally the management strategy is established for
economical utilization of the resources.
10
Numerical models have become important tools, in modern hydrogeology. There are two
areas of this discipline where we need to rely up on models of the real hydro-geologic
system to understand why groundwater flow system is behaving in a particular way and
to predict the future behavior of such a system.
Groundwater hydrologists are often called up on to predict the behavior of groundwater
systems by answering questions like: what changes can be expected in groundwater
levels in the aquifers beneath some city?; how will a change in stream stage affect the
water table in an adjacent aquifer?; What is the most likely pathway of pollutant if there is
a leak from particular source? And so on. Providing answers to these seemingly simple
questions involves formulating a correct conceptual model, selecting parameter values to
describe spatial variability within groundwater flow system, as well as spatial and
temporal trends in hydrologic stresses and past and future trends in water levels.
Although some decisions can be made using best engineering or best geologic
judgment, in many instances human reasoning alone is inadequate to synthesize the
model. Moreover, due to their flexibility and easy implementation the use of models
permits us to generate and simulate hypothetical situations of the flow system to gain
insight in it.
In Dire Dawa area, groundwater is exploited by different industries and institution in
addition to wells and boreholes that are operated by Dire Dawa Council Water Supply
and Sewerage Authority and used for public services. In long terms, extended and
uncontrolled withdrawal may result in water level declines, which causes imbalance
among hydrologic stresses. So, this groundwater flow model simulation may project the
risk of such uncontrolled withdrawal on the hydrologic system, based on which
necessary action to be taken would be proposed to alleviate such a problem. In addition,
this thesis work gives an insight about the response of the Dire Dawa groundwater basin
groundwater flow system to different possible occurring scenarios like decreased recharg
and increased groundwater abstraction. So, this model may be used as a tool to water
resource managers to assess the regional affects of change in stresses to the steadystate system. Moreover, it improves the understanding of the groundwater system and
11
the regional affects of various groundwater use alternatives on the water resource of the
area.
1.2. Objectives
The general objective of this study is to establish groundwater management system
through subsurface flow simulation and aquifer sensitivity analyses for Dire dawa area.
This management system establishment helps for effective and economical utilization of
groundwater resources and environmental protection in order to attain the regional and
national water policy.
The specific objectives of the study include:
-
Conceptually model the aquifer parameters and the hydrologic condition of the
study area.
-
Simulate groundwater flow to wells and natural flow condition depending on the
aquifer characteristic of the area.
-
Calibrate the simulated groundwater head data with the observed head data
under steady-state condition.
-
Predict the consequences/ effects on the local and regional groundwater
conditions when Dire Jara well filed start pumping water to Harar town.
-
To evaluate the behavior of the groundwater system under possible future
utilization scenario.
-
Propose the future groundwater management system so that to keep the natural
balance of the environmental condition.
-
Recommend the way by which groundwater condition of the study area can be
enhanced.
1.3. Methodology
In order to attain the objective of the study the following methods are used;
12
. Analysis of available in formation:
- Checking and updating well databases.
- Selection of appropriate wells and well parameters.
- Study of previous work and literature.
- Preparation of a new data base.
- Estimation of well parameters.
. Filed observation and desk tasks:
-Pumping test data collection and evaluation.
-Description of well litho-logic logs.
-Geologic observation.
. Modeling:
-Construction of conceptual model.
-Selection of a specific steady-state numerical model.
-Model run.
-Model calibration.
-Interpretation of results and conclusion.
A flow chart of this methodology is presented in fig (1)
13
Figure1.1 The general methodology followed (Anderson and Woessner, 1992).
Field Data
Conceptual Model
Mathematical Model
Numerical Formulation and Code Selection/Computer Program
Field Data
Comparison with
field data
Model Design
Calibration
Prediction and Sensitivity Analysis
Results
Presentation
14
1. 4. Pervious works
Previously several geological, hydro-geological, pollution studies, and other related
works have been performed for different purpose in the Dire Dawa area. The main hydrogeological work were to plane and design water supply facilities for Dire Dawa and Harar
towns as the ever increasing water demand far exceeded water supply. Most of the
previous works deal with specific issue but comprehensive work was done by WWDSE
(Water Works Design and supervision Enterprise).
Same of the relevant work done on hydro-geology were:
Associate Engineers (1990), Groundwater exploration and production well construction
and testing at Dire Dawa.
Contaminant
of
the
hydro-geologic
system
in
Dire
Dawa
town
by
Taye
Almaayehu(1988).
Dire Dawa Administrative council integrated resource development master plan study
project by WWDSE (2004).
Hydro-chemical variation and aquifer characterization in the Dire Dawa and its
surroundings by Eyilachew Yitayew (2004)
Harar water supply and sanitation project groundwater development hydro-geological
report by BCEOM in association with EEDSE and CECE (2004).
Hydro-geology of Dire Dawa area by Tesfamichael keleta (1961) and several ground
water augmentation reports by mezmure Hailemicheal (1980), ketema Tadesse (1981),
Ereitzer (19 61), and others.
15
CHAPTER - 2
GENERAL OVERVIEW OF THE STUDY AREA
2.1. Historical overview of Dire Dawa and the surroundings
The history of Dire Dawa is rather recent since it owes its existence to the famous
"chemen de fer Franco-Ethiopian" railroad, which was built by the French between the
years 1897-1917 and founded 1902. The railroad connects Addis Ababa with the port of
Djibouti and at one time was intended to go all the way to the White Nile. Dire Dawa
hosts headquarter of the railroad with repair shops and maintenance facilities and
emerged as the major town in the eastern part of the country.
Since its foundation Dire Dawa have had a considerable number and divers population.
Part of the population was drained from the central region of Ethiopia and the first
settlement pattern within Dire Dawa is closely connected to the railway terminal.
(Eyilachew Yitayew, 2004).
2.2. Location
The study area encompasses an area of 885km 2 in Awash River basin in eastern part of
country. Geographically it is located between 795000m – 845000m Easting and
1045200m – 1080000m Northing (Projection: UTM, Datum: world geodetic system 1984,
zone – 37) at about 520 km East of Addis Ababa (fig 2.1) location map of the study area.
It is found at the Awash River basin and has an elevation generally ranging from less
than 1000 M.A.S.I. to more than 2300 M.A.S.I.
16
Fig.2.1 Location map of the study area.
17
2.3. Physiography and drainage
The study area is the southern margin of the Afar depression; and therefore,
physiography of the area is mainly controlled by volcano-tectonic rather than erosional
activities. The area is characterized by successive short running E-W oriented step faults
forming half graben and horsts. The aggregate throw of the fault made the area to drop
its elevation from more than 2200m at Dhangago to below 1000m at the north part of
shinile (fig 2.2 profile showing elevation drop along south-north direction). The
geomorphology of the study area can be classified into three major features: the
escarpment, the transitional and the alluvial plains. There is an altitude difference of
about 1120 m between the alluvial plain and the mountain peaks of the escarpment over
a distance of about 13,300m.
The escarpment area is characterised by steep slops, gullies and dry wadies mainly
underlain by sedimentary and metamorphic rocks. As the result of step faults tilting the
sedimentary rocks towards the south, a number of bench shape plateaus with southerly
dip form the escarpment areas. Wadies mostly cut across these rocks mainly following
the NE trending fault systems. In these areas runoff is high due to the steep slope and
the rock surfaces permitting low retention period.
The transitional areas are mainly characterised by small outcrops of sedimentary rocks,
basalts and some recent coarse alluvial sediments. In this area the topography is gentle
and the rocks are close to the surface.
The alluvial plains are characterised by gentle to flat topography. Except some volcanic
hills of younger age, the Mesozoic and the tertiary rocks are buried deep inside the
sediment.
The presence of high relief in the area made it dissected by many tectonically controlled
small intermittent rivers, which are tributaries to the main perennial river Awash. Most of
the rivers run N-S to NNW-SSE following the trend of the major NNE-SSE running
lineaments which cross the rift margin, (Fig.2.3, drainage map of the area). Because of
the faults and faulted blocks the drainage has generally rectangular drainage pattern in
the mountains.
18
Fig.2.2 Profile showing elevation drop along south-north direction
19
20
2.4. Land use / Land cover
Present land use pattern of the area were mapped by Water Works Design Supervision
Enterprise on land use / land cover study of the Dire Dawa administrative council. The
land use is dominantly agricultural land use pattern vary geographically with climate,
physiographic, and population density. In general the land suitable for crop production is
being used for that purpose.
Generally the land use / land cover type of the area have been grouped by WWDSE in to
four major classes. They are designated as urban build up, cultivated land, physiognomic
vegetation types, and bare land.
The land use systems of the area can also be classified on the basis of agro-ecological
conditions in to crop, livestock and tree production components, and socio–cultural and
economical characteristics.
The cereal farming system occurs in the valley bottoms. Sorghum, maize and sweet
potato are the major crops grown below the escarpment in the valleys. Chat is also
grown as an important cash crop. Eucalyptus plantation around the homesteads is also
growing for energy as well as construction purposes.
The agro-pastoral systems occur in the foot of the mountains, particularly in south
eastern and northern part of the study area. Where the farming system is agro–pastoral,
the main subsistence component is cropping but extensive livestock rearing is also
important.
The land use practice of the study area had brought negative impact on soil conservation
and management. High populate increases over the limited land area of the study area
resulted in the indiscriminate forest clearing, overgrazing, absence of soil conservation
and poor soil management and land use practice coupled with the erosive nature of the
area have caused much amount of soil erosion. The major constraint to agricultural
21
development (livestock, crop, forestry production as well as wild life development)
identified for the area is the harsh climate condition. Moisture stress and high
temperature affects the development of the region.
2.5. Climate
Climate, which can be defined as prevailing weather conditions of an area, is a long–
term view of the weather pattern of a particular locality. It is frequently more useful as an
environmental tool. In order to understand the environment and the possible impact of
human activity on it, a basic knowledge of climate is required.
The climate of the study area is dominated by various inter–related factors. But the main
factors are the near equatorial location of the area and altitude.
There is only one 1st class metrological stations in the study area that is located at Dire
Dawa town that measures all meteorological elements including continues rainfall and
mounted with automatic rain gage. The station is founded at 811000 Easting and
1061000 Northing at altitude of 1260 m (M.A.S).
As explained in pervious section lowland and highland areas characterize the study area.
There is more than 1000m altitude difference between the two. The mean temperature
varies between 22oc to 28oc in the lowland area and 14oc to 16oc in the highland area.
Both have similar temperature pattern with the maximum mean temperature in June and
the minimum value in January. The hottest months are from the months of May to
Septembers, where as the coldest months are from November to February. (table2.1)
22
Table 2.1.Mean monthly values of Dire Dawa Meteorological element
Jan. Feb.
Mar
Apr
May
Jun
Jul
Max. Temp.
28.2
29.9
30.2
31.7
33.6
34.8
33.3
Min. Temp.
15.3
16.6
19.2
20.5
21.6
22.6
Relative humidity
39.3
42.5
44.4
47.5
35.2
Sun Shine(hr)
8.8
8.0
7.8
7.4
Evaporation
217.
199.
283
245.
6
8
(mm)
Aug
Yearly
Sep
Oct
Nov
Dec
32.6
31.4
32.0
30.2
28.6
31.50c
21.1
20.6
20.9
18.9
15.9
15
19.00c
30.0
41.4
40.8
34.6
27.6
29.6
29.9
41.4%
8.4
8.1
7.5
8.0
7.7
8.3
9.4
9.3
283
323
293.
283
266.
283
242.
221.
(open
2
3
water
7
.
8
8
mean
portion)
Wind Speed
4.2
3.8
4.4
4.6
4.1
5.5
5.6
5.1
4.2
4.2
4.2
3.6
4.5m/sec
20.7
21.6
84.5
68.3
45.3
20.6
91.8
146.
85.3
32.2
12.9
11.1
640.3
(m/sec)
Rainfall (mm)
0
*
Average Maximum Dire Dawa Rainfall (daily) in mm = 56.3 mm
*
The Maximum of Daily maximum Dire Dawa Rainfall = 115mm (in 1955)
*
The minimum of Daily maximum Dire Dawa Rainfall = 23mm (in 1972)
N. B. Taken from WWDSE Climatology and Hydrology Report.
23
Mean rainfall (mm)
1100
1000
900
800
700
600
500
400
300
200
100
1950
1960
1970
1980
1990
2000
Year
Mean annual rainfall series
Mean annual rainfall
Fig 2.4.Mean annual Rainfall of Dire Dawa
Tatal annual rainfall trend of Dire Dawa station (1981-2002)
1000
900
rainfall
800
Rainfall (mm)
700
600
500
400
300
200
100
19
81
19
82
19
83
19
84
19
85
19
86
19
87
19
88
19
89
19
90
19
91
19
92
19
93
19
94
19
95
19
96
19
97
19
98
19
99
20
00
20
01
20
02
0
Year
Fig.2.5. Average yearly rainfall (mm) at Dire Dawa (1981-2002)
24
Rain Fall & PET at Dire Dawa
PET/Rainfall (mm)
250
200
150
100
50
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Month
Mean Monthly Rainfall
ETP (NMSA)
ETP (CESEN & ANSALDO)
Fig.2.6. Mean monthly Rainfall and evapo-transpiration of the Dire Dawa area.
The mean rainfall in the study area varies between 500mm and 1000mm with the
minimum values in the lowland and maximum in the mountains around Dhangago. The
area experiences two rainfall seasons. The small rain of March to May and the summer
rain of July to September (Table2.1)
The annual mean rainfall of Dire Dawa town is 631 mm. (Fig.2.4). The potential evapotranspiration for every month is greater than the monthly average rainfall for the study
area Fig (2.6). This implies there is no infiltration to the groundwater system. However,
there is infiltration that is explained by the vast groundwater reservoir in the well fields of
the study area. The groundwater recharge in the well fields of the study area is hence
mainly from the heavy rains at the vicinity and runoff from the escarpment areas that
spreads in the alluvial plain as flood water and from heavy rain in short periods to
recharge the groundwater reservoir before there is loss as evaporation due to high
temperature.
25
CHAPTER - 3
Regional Geology and Hydrogeology
3.1. General
One of the most remarkable features of the Earth's crust is the rift valley of eastern
Africa. The valleys, rather than just a single valley, form a more or less continuous scar
from Israel and Jordan in southwestern Asia to Mozambique in southeastern Africa (Ase
et al, 1986). One of the prominent part of the valley system is the so called the main
Ethiopia rift valley. The main part of the Rift valley faulting took place during the middle
Pleistocene times (Wiberg, 1974), possibly along older fault lines and continued in to
upper Pleistocene, although some investigation suggest that the development of the fault
system continues now a days (Ase et al, 1986).
3.2. General geologic setting
Ethiopia is a country with a broad range of geomorphic provinces:- A high and rugged
mountainous core cut by deep gorges and incised river valleys; fault bound plateaus and
basins; a prominent rift valley that hosts a number of lakes; and bordering plains that
range from the harshest of deserts to subtropical jungles. Elevations range from 4620m
above mean sea level at Ras Dashen to 110m below mean sea level in the Afar
depression.
In terms of their surface exposure, the main lithologic units of the country have been
grouped as follows (Dr. Tamiru Alemayehu, 2006).
i
Precambrian metamorphic basement rocks,
ii
Mesozoic sedimentary rock,
iii
Tertiary volcanic rocks (largely basalts) and,
iv
Quaternary volcanic rocks, largely ignimbrites and sediments
26
These all geologic units of Ethiopia are also exposed in the Eastern part of the country
and at the study area. Generally the regional geology in comparison with local geology is
described based on their regional stratigraphic position, form the older to younger as
follow:
3.2.1 Precambrian Basement rocks
Precambrian metamorphic rocks are exposed in the northern, western, southern and
Eastern parts of the country. In the Eastern part of the country, the basement rocks
consist of crystalline acidic migmatites, gneisses and granites. The main components of
this formation are migmatic quartz rich gneisses and the intrusive pegmatite-feldspathic
granite (Tamiru Alemayehu, 2006). The Precambrian metamorphic rocks of the study
area belongs to Archean complex of Alghe group and is mainly composed of high grade
quartz-fieldspar-biotite, gneiss, biotie-hornblende gneiss and pegmatite.
3.2.2. Mesozoic sedimentary rock
At the end of the Precambrian, the crust was uplifted, so very few Paleozoic sedimentary
rocks are found in Ethiopia. Conditions changed at the onset of the Mesozoic, when
shallow seas spread initially over the Ogaden region and then extended further north and
west as the land continued to subside. Sandstone was deposited on the old land surface,
followed by the deposition of shale and limestone as the depth of water increase. In the
west of the country sedimentation ended up with the deposition of clay, silt, sand and
conglomerate as the sea reduced during the Jurassic. The sea invaded again in the
lower cretaceous, and the sedimentation sequence was repeated (table 3.1 shows the
Generalized geological events of the study area). Mesozoic sedimentary rock of the
study area which is the result of transgression and regression events are grouped in to
three formation known as Adigrat sandstone, Hamaneli limestone and Amba Aradam
sandstone.
27
3.2.2.1. Adigrat sandstone/ Lower Sandstone
This formation is unconformable overlying the basement complex and is exposed mainly
in the southern parts of the area under laying the Hamaneli limestone. Triassic to lower
Jurassic in age, this sandstone is medium to coarse–grained and red to brown in color.
The thickness of the Adigrat sandstone in Dire Dawa area does not exceed over 50m.
But kazmin (1975) indicates in his study records a maximum thickness of 800m for
Ethiopia.
3.2.2.2. Hamaneli Limestone
The Hamaneli limestone comformably, overlies the Adigrat sandstone and is the
dominant formation in the study area. It is exposed in the southern part of the area
forming different step ridges extended in the east-west direction and decreasing
topographic elevation from south towards the north. In the absence of the Adigrat
sandstone, it unconformably overlies the Precambrian basement rocks. Hamaneli
limestone is composed of alternating layers of shale and chock at the lower part and
brown to gray color limestone at the upper part. The overall thickness is not known
although Gretzer (1970) cited in his study estimated to reach 175m for the upper
limestone.
3.2.2.3. Amba Aradam sandstone/upper sandstone
This upper Jurassic to lower cretaceous formation is unconformbly overlaying the
Hamaneli limestone and is exported mainly in the west, central, north and northeastern
part of the area. This unit is estimated to have a thickness of about 200m (Tesfamichael
Keleta, 1974).
3.2.3. Tertiary Volcanic
The Tertiary volcanic rocks mapped in the study area are Alaji basalts and the stratoid
basalt of the Afar series. (seife Mikael Berhe, 1985).
28
The Alaji basalt is unconformably overlying the Amba Aradam sandstone and is exposed
mainly in the northwest, central and northeastern part of area. This unit mainly consists
of a phyric flood basalts associated with rhyolites and subordinate trachytes and ranges
in age between 36-13 Ma (kazmin, 1979).
The stratoid basalt exposed in the study area is the upper part of the Afar series and
represented by the transitional type basalt (selfe Mikael Berhe, 1985). The age of the
upper part of the Afar series range between 4 and 0.37 Ma (Mengesha, 1996). This rock
unit is exposed in the northwest and northeast of the study area forming an east-west
trending continuous ridge, which is most likely fissural eruptions and serves as a barrier
for the surface run off in the Dire Dawa area.
3.2.4. Quaternary Sediments
The quaternary sediments of the area consisting of alluvial sediments, lacustrine deposit
(travertine) and river sand deposit.

Alluvial Sediments: The alluvial deposit, which is the weathering product of the
existing rocks in the area, is mainly covering the lower most elevation of the study
area. This deposits consisting of boulders, gravel, sand, silt to clay sizes of the rock
units of metamorphic rocks, sandstone, limestone and basalts. The alluvial sediment
is mainly deposited over the tertiary basalts and has variable thickness.

Travertine: Lacustrine deposit, travertine, is out cropped in Dire Dawa and Melka
Jebdu area. The rock is associated within the alluvial sediments. In both areas, the
travertine is used for construction of houses and fences.

River sand deposits: These are the most recent sediment in the study area; it is
deposited in the flood plains of the main streams of the area. Significant deposits are
associated with Goro, Dechatu and Lega Hare intermittent streams crossing the Dire
Dawa town. The sand deposit is the weathering product of the rocks with in the
29
watershed of the main streams and their tributaries. The deposit is composed of
fragments of metamorphic, sedimentary and volcanic rocks of the area.
Table3.1. Generalized geological events of Dire Dawa area
Chronology
Quaternary
Events
Litho-logical
formation
Fluvial activities and
Alluvial deposits and
young volcanic
basic volcanic rocks
eruptions
Pliocene
Cenozoic
Volcanic eruptions and Pyroclastic and lava
rift formation
Tertiary
deposits
Trap volcanoes
Uplifting
Miocene
erosion
Eocene
Paleocene
Mesozoic
Cretaceous
Regression
Upper sandstone
Antalo Limestone
Jurassic
Transgression
Adigrat sandstone
Intrusion and
High grade
metamorphism
metamorphic rocks
Triassic
Paleozoic
Precambrian
30
3.3. Structural and litho-logical features
Structural features such as faults in the rocks often optimize storage, transmissivity and
recharge, particularly when they occur adjacent to or within surface drainage system.
Faults will have the highest impact on hard and massive rock types, elastic formations
such as tuffs and weakly consolidated deposits will bend rather than break. As a result,
they tend to suppress the radius of influence and the magnitude of the damage caused
by tectonic events. In the study area a series of parallel faults runs in east-west, forming
normal faults down throw towards north (rift valley) with steep to gentle dips. The general
trend of these faults is related to the main rift system. These faults are visible only in the
escarpment. Due to the cover of thick superficial, there is no visible fault indication in the
northern plain. But the scattered hills of the upper sandstone suggest that, fault blocks
are probably buried by superficial deposits. The faults are normal faults down throw to
the north as part of the synthetically faulted margin resulting from crustal attenuation of
the Harar plateau. They strike east west in the south and east, gradually change to ESEWNW towards the NW and W. Some NW-SE trending cross faults are common in the
central part of the basin.
3.4. Effects of faulting system on the modeled zone
Although the faulting process has been intense and of long duration its effect on the
shallow aquifer formation in the modeled zone does not appear to be of great importance
with the exception of the boundaries of the model, which in some cases, have been
selected according to the existence of fault line. As mentioned before, the effect of
tectonic activity upon the upper unconfined aquifer, which is unconsolidated sediments,
will be suppressed and the influence of faults can be safely neglected. In addition, during
the field work no evidence was found that could indicate that, the fault system plays an
important roll in the upper shallow groundwater flow.
In the case of lower more productive aquifer, the case is different. The fault structure in
the area strongly affects the movement of groundwater. In the escarpment area, these
31
rocks are highly fractured and especially the limestone is karatstified and generally offers
a good recharge area. The disadvantage is that they are situated on the ridges and hills
and part of the groundwater percolating in this formation is found emerging as springs at
the base of sedimentary rocks or on the surface of metamorphic rocks. The step E-W
trending fault system at some areas resulted in the lateral discontinuity of sedimentary
rocks and making them to have lateral contact with the basement complex and also with
tertiary basalts (Fig 4.3). This lateral discontinuity is also one of the disadvantages for
confined aquifer in the rift valley, which would reduce the rate of lateral flow of the
groundwater recharge from the mountain areas towards this lower aquifer. However, the
faults oriented in NE and North direction facilitates the flow of groundwater from the
mountain towards the deep confined aquifers. These fracture medium consists of solid
rocks with some primary porosity by a system of cracks, micro cracks, joints, fracture
zones and shear zones that create secondary porosity and form a network for flow when
interconnected. Therefore, forming a conceptual model for such system requires a gross
simplifications or a detailed description of the aquifer properties controlling flow. However
in this study conceptual model is made on the assumption that the flow is like flow in
porous medium due to numerous fault system that is interconnected.
3.5. Regional Hydrogeology
From the standpoint of groundwater development, the rocks of the Precambrian
metamorphic complexes are notoriously problematic. Fracture rock aquifers exist within
them, but in shallow reaches can only produce very modest amounts of water, often
barely sufficient to satisfy the drinking needs of small settlements. The deeper reaches of
this aquifer could have higher yields but exploration and deep drilling will be expensive
and time consuming.
The Mesozoic sequence is much more promising in terms of groundwater development.
For example, springs are often found at the contact between the Adigrat sandstone and
the Precambrian basement rocks, so this contact is a key exploration target. As far as
large-yield aquifers needed, the limestone of the Antalo group is by far the most
32
promising exploration target in the country. They have prominent secondary permeability
in the form of solution cavities. In the study area too the main aquifer is the upper
sandstone and the Hamaneli limestone.
The tertiary flood basalts can be major sources of groundwater, which under some
circumstances are easy to tap. For example, high-yield springs are common at the
contact between the flood basalts and Mesozoic rocks, and at the contact between
individual lava flows. Furthermore, many springs are high enough above the valley floors
that water can be delivered by gravity with relative ease. On the other hand, most spring
have modest yield so they are not a viable alternative for irrigation water or larger
settlements. Extraction through wells would undoubtedly afford higher yields. But in the
study area this unit is are impervious and only serves us confining unit.
The Quaternary sedimentary units host some of the most promising groundwater
resources. Groundwater development in alluvial filled graben has highest potential for
making a difference in further development of the agriculture of Ethiopia, because these
valleys are scattered throughout the mid elevations of the country, where most of the
population lives and are recharged every year by mountain streams, so their water tables
are shallow, and they are filled with coarse-grained alluvial with good hydraulic
conductivities. In the study area this unit is represented by upper low permeable aquifer
and yields water for dug wells and shallow drilled wells.
33
Chapter - 4
Conceptual model
Groundwater models attempt to represent an actual groundwater system with a
mathematical counterpart.
A conceptual model is a pectoral representation of
groundwater flow system, frequently in the form of a block diagram or a cross section. It
is a representation of the understanding of the real physical system.
The purpose of building a conceptual model is to simplify the field problem and organize
the associated field data so that the system can be analyzed more readily. Simplification
is necessary because a complete reconstruction of the field system is not possible.
In theory, the closer the conceptual model approximates the field situation, the more
accurate is the numerical model.
It is critical that the conceptual model be a valid
representation of the important hydro-geologic conditions; failure of numerical models to
make accurate predictions can often be attributed to errors in the conceptual model.
However, the conceptual model is nothing else than a simplified representation of reality,
because it can not reflect all the complexity of the real system even if it is based on a
large amount of information about the system it describing, " our conceptual model will
always be less complex than the real system, a conceptual model could never fully
describe all details of the real system" (Fetter, 1994)
The steps followed in development of conceptual models for the Dire Dew area are: -
1) Definition of geologic settings, Aquifer and confining units,
2) Identification of hydraulic properties of the aquifers,
3) Identification of sources and sinks of water, and
4) Identification and delineation of hydrologic boundaries encompassing the area of
interest.
34
4.1. Local Geologic settings and Geologic Structures
4.1.1. Local Geologic Settings
Geology of Dire Dawa area (according to Dire Dawa Administration council integrated
Resource Development Master Plan Study Project) can be classified as follows (Fig4.1):
Basement complex rocks: composed of gneiss, pegmatite and granodiorite of
metamorphic rocks. Fractured and weathered part of this formation may have very little
water. Practically it is impervious.
Mesozoic sediments:

Adigrat sandstone (lower sandstones) un-conformably overlies the basement
complex with a thickness of not more than 20m, fractured and pervious formation.

Hamanlei (Antalo) limestone: varies in thickness up to 200m and its lower part is
interbeded with shale and overlain with oolitic limestone. This formation together
with upper sandstone makes the main water bearing horizon in the area.

Amba-aradam (upper) sandstone:- composed of quartzos sandstone, thickness
from 150 to 200m at some places intercalated with basaltic flows ( lava flows
within the sediments or sills) and limestone intercalation. This formation is the
main water bearing horizon in the area.
Tertiary volcanics: Alaji formation (lower trap basalts) predominantly basalts and
stratoid basalts. The Alaji formation overlies the upper sandstone unconformabley. It has
a wide spread area coverage and practically impervious. This formation is considered as
confining layer for the bottom confined aquifer.
35
Quaternary formation: All rivers and streams descending from the escarpment have
built large aerial extent and thick alluvial deposits. These deposits consist of cobbles
and coarse-grained sediments near the escarpment, while they consist of fine detrital
sediments in plain area. The alluvial sediment is one of the water bearing formation in
the area. This unit represents the upper unconfined aquifer of the area with relatively low
yield.
4.1.2. Local Geological Structures
The major structural features of the study area and of the region are faults. Numerous
faults that runs east west forming normal faults down-thrown towards north with steep to
gentle dips. The great East African Rift System started to develop in Miocene. Rifting
began from the presently oceanic Red Sea and Gulf of Aden Rifts, which join with the
younger and continental Main Ethiopian Rift (MER) at the complex proto-oceanic Afar
Triple Junction (Afar Depression) where three rift systems; the Main Ethiopian Rift, the
Gulf of Aden and the Red Sea Rifts intersect. The study area is located in the southeastern escarpment of the Afar Depression (Huchon, 1989). According to Habteab Zeray
and Jiri Sima (1986) and Mesfin Aytenfisu, (1981), the area consists of three tectonic
units: the plateau, the escarpment and the depression. The river headings toward the Rift
Valley are highly influenced by series of faults, which are forming the escarpment. The
plateau and escarpment is dominated by E-W or ENE-WSW trending faults and
perpendicular to the drainage system in the area. Abrupt change in slope gradient on the
escarpment caused decreases in velocity of surface water, which resulted in forming
coalescent fan at the foot of the escarpment. This normal fault that forms graben-horst
structure strongly affects the movement of groundwater.
Other prominent structural feature in the limestone area is a joint of tectonic origin. The
faulted blocks of limestone are highly jointed and fractured. These are common in
limestone especially in the area where these units are exposed to the surface, and
solution cavities (Karst is well developed) are clearly visible.
36
4.2. Hydro-geologic setting of the area
An aquifer is a saturated permeable geologic unit that is capable enough to yield
economic quantity of water to wells or springs, where as a confining unit has low
permeability that restricts the movement of groundwater and limits the usefulness of the
unit as a source of water supply.
The Dire Dawa area groundwater occurrence, distribution and flow regime is highly
governed by topography, geological formation, and aerial and topological relationship of
the geological formation. Based on major factors, the study area can be categorized into
two groundwater systems:- that is, the escarpment and the down throw block (ground
water basin of Dire Dawa)(Eyilachew Yitayew,2004).
The escarpment groundwater system
The escarpment occupies the southern, southeastern and eastern parts of the study
area. It is highly rugged areas and intensively faulted by east-west trending tectonic
lineaments. Geologically, the western part is dominated by sedimentary formations
(limestone and sandstones), at the central part and south of Dire Dawa town
sedimentary and basement rocks dominate, to the east, volcanic and sedimentary rocks
dominate. Generally, the groundwater of this part of the study area is the recharge area
for the down throw block at the foot of the escarpment. However, at the eastern part of
the escarpment downstream of the sedimentary formation, basement rocks outcrops and
the topography highly enhances complete drainage of the groundwater of the
sedimentary formation. To the west, the sedimentary formation forms extensive
distribution from watershed divide to the foot of the escarpment, which highly facilitates
the recharge condition of the groundwater of the down throw block (Dire Dawa
Groundwater Basin). At some localities the groundwater is partly discharged as springs
at the contacts, joints and depression from the various geological formations. The
groundwater potential of this part of the study area is evaluated by the discharge of the
springs.
37
Groundwater of the down throw parts (Dire Dawa groundwater basin)
It occupies the plain part of the study area. This area is considered to have high
groundwater potential, where Dire Dawa town water supply source is found and where
Haseliso well field, the future water supply source of Harar town is located. The
groundwater occurrence and distribution in the basin is mainly a function of the
geological formations, geomorphology and tectonics. Here the hydrological condition of
each aquifer is presented. The Adigrat sandstone and Basement rocks are not
encountered by drilling in the basin, therefore these formations are not characterized
here.
The hydro-stratigraphic units that are identified with in the study area are: -
Alluvial aquifers
The process of denudation, transportation and accumulation of the major rocks from the
escarpment forms the alluvial deposits. These alluvial sediments are distributed in Dire
Dawa groundwater basin, overlaying the basalts, upper sandstone and limestone at
different places. These deposits occur in the basin as river sand deposit along the river
channel with limited aerial extent; travertine with limited aerial extent and alluvial
sediments with large aerial extent. From hydro-geological point, these formations are
categorized as alluvial sediments by the dominant aerial coverage.
The thickness of alluvial sediment varies in thickness and composed of clay, silt, sand,
gravel and rock fragments. The groundwater depth varies in the alluvial sediments from 5
to 45m. The top elevation of this layer is ground surface. The bottom elevation of this
layer is estimated to be 900m a.s.l. The discharge of wells from this aquifer varies from
dry to a specific well discharge of 3lit/sec/m. The Transmissivity of the alluvial formation
as obtained from pumping test results varies from 8 to 700m2/day.
The estimated
average value is 27.5m2/day (Hydro geologic Report, WWDSE, 2005).
38
Tertiary volcanic rocks
Tertiary volcanic rocks in the study area are mainly stratiod basalts and Alaji basalts
outcrops that occupy the elevated areas at the north and northeastern part of the area.
This formation is generally a regional aquiclude and is the confining layer for the lower
highly productive confined aquifer with transmissivity less than 5m2/day (the average T
value is 5.7m2/day). For modeling purpose this unit is assumed to have a constant
thickness of 50m.
Upper Sandstones
The upper sandstone outcrops in a small aerial extent at Haseliso, north of Dire Dawa at
the airport and Northwest of the town. This unit is assumed to be found below the
confining layer and alluvial deposits in most part of the lower part of the study area. The
drilled wells in the aquifer show that the thickness of the aquifer is variable.
The
minimum thickness of the aquifer penetrated at the Direjara well field is 36m. However
the estimated average thickness of the unit is 200m (Tesfa michael). The static water
level varies from 9.3m (Sabiyian) to 69.3m (Direjara) with the specific discharge of 0.13
to 68.97lit/sec/m.
Hamanalei limestone
The Hamanalei limestone outcrops in the Dire Dawa groundwater basin together with the
upper sandstone in a lesser aerial extent.
Drilling results show that the limestone
uncomfortably underlies the upper sandstone. The limestone in the study area could not
be independently characterized and the aquifer characterization for the upper sandstone
applies also for the limestone. The lower part of the limestone is a gray-white limestone
inter-bedded with shale overlain with oolitic limestone and the upper part is well-bedded
gray fossiliferous limestone (Seife Mikael Berhe. 1982, Habteab zeray & Jiri Sima,1986).
For modeling purpose only the upper jointed & cavernous part of this unit is assumed
(175m) and below this it is considered as a quiclude (no flow boundary).
39
40
41
4.3. Aquifer system
As it was described before, the aquifer systems are characterized by the upper relatively
low productive unconfined aquifer (which composed of alternation of silt, sand, and
gravels, outcropped and weathered basement, sandstone and limestone) and the lower
relatively high productive confined aquifer composed of fractured sandstone and highly
jointed and cavernous limestone which are found below confining unit at the lower part of
the study area..
This confined aquifer is overlain by tertiary basalts, which are the confining unit in the
area and underline by lower part of the limestone that composed of the massive
limestone intercalated with shale.
For this modeling purpose this lower part of the
limestone is considered as aquicludes.
To build the conceptual model for this particular case the concept of hydro-stratigraphic
units has been applied. Thus concept implies that geologic units of similar hydro-geologic
properties may be combined into single hydro-stratigraphic unit or a geologic formation
may be sub divided into aquifers and confining units.
In fig (4.3) a diagrammatic geologic representation of the model zone is presented, this
figure also shown how complex the zone is. In turn Table (4.1) represents a simplified
scheme with the purpose of modeling. For this purpose it is assumed that the confining
layer is assumed to have an average thickness of 50m and the lower relatively more
productive confined aquifer has an average thickness of 375m. Accordingly the area is
classified into two layer aquifer system such as the upper unconfined aquifer (layer 1),
and the lower more productive aquifer (layer 2).
The exact vertical aquifer thickness data is rarely available for these hydro-geologic units
as the available wells are hardly penetrate to the full thickness of the aquifer or has not
described and organized. However, it is assumed as described in the above for modeling
purpose based on the discussion in the section of hydrogeology.
42
Fig.4.3. Operating principle of water circulation in the study area
Table (4.1) Classification and description of hydro-stratigraphic units
Hydro-
Label
geologic
Estimated
Litho-logic
and
thickness
characteristics
hydrologic
unit
Alluvial
(Layer-1)
Ground surface It
Aquifers
consists
of
poorly
sorted,
elevation minus uncertified mixture of clay, silt, sand,
bottom
of
the gravel,
cobbles,
layer=900m.a.s.l weathered
for each cell.
rock
basement
fragment,
rocks
and
highly weathered sandstone and
limestone.
Tertiary
(Coning Unit)
50m
Tertiary volcanic rocks in the region
Volcanic
are mainly stratiod and Alaji basalts
rocks
outcrops that occupy elevated areas
at the north and northeastern part of
the basin.
Upper
(Layer-2)
Sandstone
(The
and
aquifer
Limestone
area)
375m
of
This unit contains the fractured and
main
permeable
the
Limestone.
sandstone
and
43
4.4. Boundary Conditions
One of the components of a mathematical model is the boundary conditions. Boundary
conditions are mathematical statements specifying the dependent variable or the
derivative of the dependent variable (Anderson etal, 1992). For groundwater system of
the given area, there are two types of aquifer boundaries which control the groundwater
flow direction (Anderson and Woessner, 1992). These boundaries are:
1) Physical boundaries of groundwater flow system which are formed by physical
presence of an impermeable body of rock or a large body of surface water.
2) Hydrologic boundaries of groundwater flow system which are formed as a result of
hydrologic conditions. These invisible boundaries include groundwater divides and
streamlines.
.
When described mathematically, groundwater flow problems are classified as boundary
value problems. This indicates that the selection of appropriate boundary conditions is
critical to the accurate definition and analysis of the problem. In groundwater models,
which are used for analyzing groundwater flow problems, the specification of the
boundary conditions usually defines the source of water to the system and its ultimate
manner of discharge. Thus, boundary conditions are one of the key aspects in the
proper conceptualization of a groundwater system and representation of that system in a
numerical computer code. Conceptualizing the physical process at the boundaries of a
groundwater system and devising a mathematical counterpart that can be incorporated in
to a model is usually not straightforward.
The boundaries in the model of Dire Dawa are selected following geological features and
structures, geomorphologic features and hydro-geological evidences. Among geological
features, faults are the most important. The fault lines are usually serves as physical
boundaries. In the study area most of the boundaries are surface water divides, which is
assumed to coincide with groundwater divides.
44
The lateral and vertical boundaries of the aquifer system of the groundwater basin are
formed, in most cases, by hydraulic boundaries as groundwater divided and physical
boundaries where the aquifers are in contact with the bed rock. The lateral boundary of
the layer-1 of the aquifer system is the groundwater divides except at the northern parts,
where groundwater of this layer is thought to flow out from the system. The northern
topographically low area is considered the way through which water from layer-1
escapes to the Afar area. The bottom boundary of layer 1 is the confining unit where it
assumed to be underline by tertiary basalts through out the study area. Top boundary of
layer-1 is the ground surface.
The lateral boundary of the confined aquifer (layer-2) of the aquifer system, in most area,
is the physical boundaries (basement complex), where the aquifer pinches out and
surface water divides which is considered to coincide with groundwater divides and
serves as no flow boundary. The northern and southern boundaries of this unit
conceptualized as General Head boundary where the layer is recharged by numerous
south-north trending faults and where water is thought to escapes from the area
respectively. The bottom boundary of this layer is also assumed to be no flow boundary.
In the model area, this aquifer unit underlies the confining unit (Tertiary basalt).
4.5. Geometric Characteristics
As can be seen in the conceptual model and from the assumed boundary conditions the
aquifers aerially extend in the non-uniform manner (fig 4.4). The maximum length from
south to north is 35km and the width is 45km in the widest part.
Layer-1 which consists of poorly sorted uncertified mixture of clay, silt, sand, gravel,
cobbles, rock fragments, weathered basement rock and highly weathered sandstone and
limestone and covers the entire area (fig 4.4a showing 3D view of Layer-1). The aerial
extent of these alluvial deposits in the study area is determined from published and
unpublished geologic maps and mainly from Digital elevation model of the area in
collaboration with fieldwork by the author of this paper.
45
Layer-2which consists of the upper sandstone and the highly jointed and cavernous
upper part of the limestone. This layer is assumed to be found below the confining layer
and alluvial deposits in most part of the lower part of the study area (Fig4.4b showing 3D
view of Layer-2).
46
47
4.6. Hydraulic Properties:
The hydraulic conductivity is the constant of proportionality in Darey's law. It is defined as
the volume of water that will move through a porous medium in unit time under a unit
hydraulic gradient through a unit area measured at right angles to the direction of flow.
The hydraulic conductivity of the material in an aquifer or confining units is a measure of
the ease with which water can move through the material. It is a function of properties of
both the matrix and the fluid. Hydraulic conductivity can have any units of Length/Time,
for example m/d. Water in the regional flow system was assumed to have a uniform
density and viscosity, and thus the hydraulic conductivity only varies as the grain size,
shape, sorting & packing (freez and cherry, 1979).Horizontal hydraulic conductivities
generally were greater than vertical hydraulic conductivities, as a result of the
depositional history of the sediments.
The hydraulic conductivities of the upper low permeability aquifer used in the model for
its first run were obtained or estimated from literature. The hydraulic conductivity,
transmissivity and specific yield and/or storage coefficient of poorly sorted alluvial
aquifers are estimated by different literatures such as Davis and De Wiest, 1966,
Driscoll, 1986, Freeze and Charry, 1979, , Todd, 1980, Fetter, 1994 and others.
Accordingly the hydraulic conductivity for this layer in the study area is estimated to be
10-3m/day to 10 1 m/day.
The specific yield of the first layer of the aquifer is also estimated from literature. It is
defined as the volume of water that an unconfined aquifer releases from storage per unit
surface area of aquifer per unit decline of the water table. The values of the specific
yield range from 0.01 to 0.1 and are much higher than the storativity of confined aquifers.
It is sometimes called effective porosity (P.kruseman, 1990).
The horizontal hydraulic conductivities of the lower high permeability aquifer (layer-2)
used in the model for its first run were obtained from the analysis of pumping test data
conducted by different organization. Results of aquifer tests at Sabiyan wells, Direjara
48
wells, and geologic logs from wells and boreholes indicated horizontal hydraulic
conductivities that ranged from 10 3 to 10 3 m/day. This range is typical of carbonate
rocks and sandstones.
Transmissivity is the ability of the aquifer to transmit water. Its definition as it stands in
the groundwater literatures falls in to one of the following categories.
It is the product of the average hydraulic conductivity ’K’ and the saturated
thickness of the aquifer ‘D’.
It is the rate of flow under a unit hydraulic gradient through a cross-section of unit
width over the whole saturated thickness of the aquifer ( kurseman, 1990).
It is the ratio at which water prevailing density and viscosity is transmitted through
a unit width of an aquifer or confining bed under a unit hydraulic gradient. It is a
function of the property of the fluid, the flow media and thickness of the media.
The storativity or storage coefficient of a saturated aquifer of thickness D is the volume of
water released from storage per unit surface area of the aquifer per unit decline in the
component of hydraulic head normal to that surface. In vertical column of unit area
extending through the confined aquifer, the storativity S equals the volume of water
released from the aquifer when the piezometric surface drops over a unit distance.
(Kurseman,1990). As storativity is the volume of water per volume of aquifer, it is a
dimensionless quantity. Its volume in confined aquifers range from 5 x 10 -5 to 5 x 10-3
indicating that large pressure changes over extensive areas are required to produce
substantial water yields (Todd, 1980).
Generally speaking the magnitude and spatial distribution of hydraulic properties of the
aquifers are not well known for the zone and for the last model runs have been estimated
and calibrated during the process of model calibration.
49
Table4.2. Summary of transimissivity of the different geological formation
Geological
Formation
Transimissivity in m2/day
Aquifer
productivity
Min
Max
Mean
Harmon
ic mean
Median
Alluvial (Qa)
7.8
712.8
103.8
27.5
44.3
Moderate
Basalts (P)
2.4
9.9
6.0
4.3
5.7
Very low
Upper
SS
and
Hamanlei LS (Ka+Jh)
9.0
5512.0
1801.9
88.0
375.3
High
Fig.4.5. Transimissivity values of the different geological formations
Transmissivity of different geological formations
10000
Transmi ssivity, m2/day
1000
Transmissivity
Harmonic mean
100
10
1
Alluvial
Basalt
Upper sandstone +limestone
Formation
50
4.7. Groundwater recharge and zonation
Groundwater recharge can be defined as the entry into the saturated zone of water made
available at the water table surface together with the associated flow away from the
water table within the saturated zone (Freez & cherry, 1997). Recharge, which is water
contributing to groundwater passing through water table, could be direct, indirect or
localized based on the source and mechanism by which water reaches the water table.
The amount and type of recharge depends on land use / land cover, topography, climate,
drainage, geographic location, vegetation, structure, soil conditions and others.
Quantification of the rate of natural groundwater recharge is a basic prerequisite for
efficient groundwater management, and is particularly vital in arid and semi-arid regions
where such resources are often the key to economic development (David & Lerner,
1990). The main sources of recharge to aquifers of the study area are subsurface
recharge from mainly the precipitation in the southern escarpment zone and direct
recharge from precipitation.
The physical setup of the well field surface catchments indicates that the east-west
trending faults on the escarpment intercepts surface runoff and rainfall that drains to the
groundwater. However, the faults oriented in the north direction facilitate the flow of
groundwater from mountain towards the deep confined aquifer (Fig4.3). Groundwater
recharge to the lower aquifer, springs and almost all perennial streams in the study area
are developed from the precipitation taking place in the mountains south of the study
area. And hence no direct recharge takes place on the plain to the lower high
permeability aquifer because of the thick alluvial deposits and confining layer of basaltic
origin (BCEOM & WWDSE, 2004). The recharged water flows laterally towards the north
following the general regional topographical gradient. The step E-W fault systems at
some areas resulted in a lateral discontinuity of these sedimentary rocks and making
them to have lateral contact with the basement complex and also with the tertiary
basalts. This lateral discontinuity of the confined aquifers in the rift valley would reduce
the rate of lateral flow of the groundwater recharge from the mountain areas towards this
51
deep aquifers. However, the faults oriented in the north direction facilitate the flow of
groundwater from the mountain towards the deep confined aquifers (Fig.4.3).
Generally the southern and south western parts of the catchments get better recharges
than the plain area as a result of its higher rainfall amount and ephemeral steams from
the escarpments. In addition to this, faults facilitate recharge to the aquifer. The floor of
the catchments gets recharge from precipitation as direct recharge and as indirect from
ephemeral streams. The occurrence of groundwater of lower aquifers at the well fields is
due to the hydraulic and hydro-geological conditions in the escarpment area. Therefore,
the potential of the well fields is directly influenced by the recharge taking place in the
escarpment area.
Therefore, an initial recharge value of 10.4mm as estimated by Eyilachew
Yitayew(2004), to the area of valley depression(plain), situated in the vicinity of the town
of Dire Dawa extending to the west, east and north of Shinile which constitute about
53%(470km2) of the area; 24mm as estimated by BCEOM in association with WWDSE to
be 4% of the average annual precipitation(608mm) to the area of steeped margin of the
valley depression, extending from the plateau to the southern limit of Dire Dawa town,
that covers about 36%(318km2) of the study area; and
56.3mm as estimated by
Eyilachew Yitayew(2004) using Water balance approach for the plateau margin area,
which demarcates the southern limit of the study area, that covers about 11%(97km q2) of
the area are used. From the above the total recharge to the area is computed to be
48493m3/day.These values are used to start running the model and lately modified
during calibration.
4.8. Groundwater discharge
Groundwater discharge can be defined as the removal of water from the saturated zone
across the water table surface, together with the associated flow toward the water table
within the saturated zone (Freez and cherry, 1997). Groundwater use in the study area
has steadily increased since the last two decades, as the result of population increases
52
which is highly influenced by urbanization, industrial developments and due to growth
trends of the local population in using groundwater for their irrigation demands. The
groundwater discharge from the two aquifer units of the area is mostly through
withdrawals from wells and boreholes, from springs and as evapotranspiration where
groundwater level is near to ground surface and groundwater underflow that might occur
as flow through aquifers across the study area boundary at topographic low area (fig.4.6.
Show the boreholes and wells that extract water from the aquifers).
Because the withdrawal record for the system is incomplete, it was not possible to
determine the actual average annual withdrawal rates for each well. Instead an estimate
of the average annual withdrawal rate in each well was made from the available record,
under the assumption that the average of the known record would approximately equal to
the true record.
The average annual rate of evapotranspiration from water table in the study area is
unknown but is presumed to be insignificant compared to groundwater pumpage from
the system. Therefore, evapotranspiration that might occur from shallow groundwater are
not quantified and are not considered by the model. The considered discharge systems
of the aquifer are springs, pumping wells and boreholes and groundwater out flow at
topographically low area. Some springs occur in the escarpment within the model area.
The output from these springs is drawn out from the model by assigning them as they
are extraction wells for the modeling purposes.
Groundwater withdrawals from wells in the study area are mostly used for domestic,
industries, and irrigation purposed. The pumping system of groundwater from both
aquifer system of the study area is either of the followings:
a) The hand-dug wells, which were dug in the upper low permeability aquifer, are
used mostly for irrigation purposes. The data obtained from Dire Dawa Council
Water Mine and Energy Resource Development Office shows that there are 98
hands dug wells in the study area. The hand-dug wells are assumed to be
53
pumped for five hour a day at 2 lit/sec pumping rate and the wells serves for eight
months of a year during dry seasons. Generally the groundwater abstraction rate
by this method as computed from the above was 2352m3 /day. The groundwater
pumpage by this system was assumed to be directly goes to evapotranspiration
after irrigating the crops.
b) The shallow wells, which were drilled in almost all cases in layer-1 of the aquifer
system, are installed with hand pumps and used as water supply for rural
community. The data collected from Dire Dawa Council Water, Mine and Energy
Resource Development Office show that there are about 88 functional such wells.
The hand pumps yields on average about 1 lit/sec. and serve for eight hours a day
for the whole year. Based on this it was estimated that the groundwater
abstraction by this method is 3168m3 /day.
c) The deep wells installed with submersible pumps, abstract water mostly from the
lower high permeability aquifer. The water is mainly used for water supply for the
urban community, for industrial purposes and small amounts for irrigation.
Generally, there are more than hundred such wells in the study area with the main
in Sabiyan well fields, Melkajebdu well fields, private wells and Aseliso well fields.
However, at present there are no boreholes in Aseliso well field tapping water
from both the upper aquifer and the lower aquifer. However, the boreholes in the
Aseliso well field would be implemented within near feature and are considered as
outputs under scenario simulation. The amount of daily discharge of these wells
was obtained from the water meter readings of the well owners and where not
installed with water meter, estimated from pumping test discharge. The current
water demand of Dire Dawa town is about 14,000m 3/day. But the Town’s Water
Supply and Sewerage Authority is currently provides only about 10,000 to 11,000
m3/day of water (Dire Dawa Water Supply and Sewerage Authority). The water
supply authority extracts water from Sabbiyan well field. Out of the nine old
Sabbiyan boreholes only three of them (PW-4, PW-6, and PW-9) are functional
and the rest of them are out of use and are no more functional. Currently another
54
8 new boreholes were constructed and out of which 6 (PW-12, PW-13, PW-14,
PW-15, PW-16 and TW-1(2006)) are connected to the existing systems and are
contributing more than 70% of the current supply. The average daily discharge of
the Sabbiyan wells, which is equal to 10,500m3/day, cannot be exactly distributed
to the functioning nine boreholes because of the absence of data. But for
modeling purpose I tried to distribute for each wells after consulting Dire Dawa
Water Supply and Authority officials. Accordingly the total discharge of
groundwater by such wells from the lower aquifer is compute to be 10,500m 3/day
and the percentage contribution of each well is summarized in Table (4.2A.)
below. The average daily discharge from Melka Jebdu Town Water supply
boreholes (BH-81, BH-08, and BH-09) and Melka Jebdu Irrigation Boreholes
(MJIBH) are estimated to be 2332.8m3/day and their daily percentage contribution
is summarized in Table (4.3B). The functioning private boreholes, which are found
in the study area, are inventoried and collected from secondary data and are
found to be 55. These wells are assumed to pump groundwater at 3lit/sec for four
hour a day for all years. The groundwater abstraction by this method as computed
from the above is 2376m3/day. For modeling purpose these 55 boreholes are
assigned by 3 boreholes situated at the center of the town with pumping rate of
2376m3/day altogether. Therefore, the total amount of water discharged from the
lower aquifer in this case summed to be 15208.8m3/day. See Table 4.2.
d) During the field visit and from secondary data, a total of 19 spring data has been
collected. These springs are found on the escarpment (steep sloped), highly
faulted part of the study area. The springs come out at the contact of the
sedimentary rocks and the basement rocks, contact of dyke and sedimentary
rocks and at slope breaks, and along fault zones. Most of the springs with
relatively high yield are distributed on the Hamanalei limestone formation on the
escarpment emerging along joints of the limestone and contact with basement
rocks. In the upper sandstone the occurrence of springs is very rare due to high
permeability of the fractured sandstone and it may drain the groundwater down to
Hamanalei limestone and some proportion is discharge as spring from the
55
limestone. Most of these springs are perennial and are used for irrigation and
domestic water supply purposes. Groundwater discharges from springs are
estimated to be 180 lit/sec (Data obtained from Dire Dawa council Water, Mine
and Energy Resource Development Office). The output from these springs is
drawn out from the model by assigning them, as they are extraction wells.
Accordingly five assumed wells that pump cumulatively 180 lit/sec (=15552
m3/day) were applied to the upper aquifer, which compensate the springs.
Generally, the groundwater outflow includes withdrawals from supply, industries and
irrigation wells, springs, groundwater evapo-transpiration (not considered for balance)
and subsurface outflow from the study area at the topographically low area. This
groundwater outflows that were not quantified in the water balance, such as groundwater
evapo-transpiration, may account for some discrepancy. The magnitude of this
groundwater evapo-transpiration is not quantified. However, they are likely to be small
relative to the total water balance, given the magnitude of supply, industry and irrigation
wells withdrawals relative to the total outflows.
4.9. Groundwater level and movement
Water level in the aquifers fluctuates in response to charges in the rate of groundwater
recharge and discharges, which are partly a function of changes in climate conditions.
The groundwater level in the study area is governed by the rate of recharge to the
aquifer and water abstraction. The static water levels of the boreholes drilled in both
aquifer collected from drilling data which was measured during different periods were
used to confirm the upper alluvial aquifer is unconfined and the lower sandstonelimestone aquifer is confined. The static water level for both aquifers is nearly equal.
Even through there is an automatic water level recording instrument installed in PW-7 of
Sabiyan well field; the data could not be downloaded from the instrument at the time of
visit. Therefore, the trend of groundwater level could not be shown and mapped. The
general groundwater flow system in the basin is to the north direction (WWDSE-2002).
56
The water level elevation data of 166 wells, boreholes and springs collected from drilling
report data compiled by WWDSE was used to develop the water table map shown in
(fig.4.7). Groundwater moves through the aquifer in the direction of lower water level
altitudes. The altitude and water table contours (fig.4.7a and 4.7b) indicate that the
general direction of groundwater flow is from the southern recharge area to the northern
discharge area. Water supply wells, however, intercept groundwater that would have
flowed to natural discharge areas.
Groundwater movement from recharge areas to discharge areas depends on many
factors such as available water for recharge, aquifer characteristics and the mode of
recharge. Recharge of the deep groundwater flow system in the area is thought to be
mostly from the southern escarpment area. The water table map shown on fig (4.7),
which is established based on measurements taken by WWDSE in bore holes of Aseliso
well fields, Melkajebdu and Sabbiyan well fields and springs shows groundwater in the
aquifer system generally flows from the south to the north across the study area and is
discharged primarily by wells and spring flow. In the conceptualized groundwater map,
where there is recharge in the mountains in the south with northerly direction of regional
slope, one can expect a northerly direction of groundwater flow.
57
58
4.10. Conceptual Groundwater balance
A steady-state water balance for Dire Dawa area groundwater basin describes the
aquifer system and provides a conceptual framework for groundwater flow models. The
water balance identifies and quantifies the hydrologic inflow and outflow components of
the aquifer system and provides data that are used in the development and calibration of
the numerical models of the system. In the steady-state water balance, inflows to and out
flows from the aquifer systems are identified and quantified on the average annual basis.
The water balance described here includes the components simulates in a groundwaterflow model; thus the evapotranspiration and the runoff component of stream flow is not
included.
Inflow to the groundwater in the study area is from precipitation and groundwater inflow
for the lower aquifer and hence precipitation is the ultimate source of water to the study
area. Outflows include withdrawal from supply, industries and irrigation wells and springs
and groundwater outflow. Precipitation recharge rate that was estimated by water
balance method by Eyilachew and WWDSE was used for groundwater balance.
Accordingly an initial recharge value of 48493m 3/day (=17699945m3/years) as estimated
by Eyilachew and others have been taken.
The primary outflows from the study area are through withdrawals from shallow and deep
wells and through spring flows. The total groundwater withdrawal from the well is
20728.8m3/day (=7462368m3/ years) and from springs 15552 m3/day (=5598720 m3/
years) and 12202.2 m3/day is thought to be discharged as groundwater outflow.
Groundwater evapotranspiration that might occur from shallow groundwater are not
quantified and are therefore not considered by the model. Estimated average annual
groundwater withdrawal of the study area is summarized in table (4.2) below.
59
60
Table .4.3a).Summary of Daily Groundwater Abstraction Rates from Wells and
Boreholes and springs of the study area and names of the Boreholes and their codes
(Source Water meter reading of the boreholes owners).
SUMMARY OF WELLS AND BOREHOLES FOUND IN THE STUDY AREA
AND THEIR
DISCHARGE CONTRIBUTION FOR TOWN WATER SUPPLY, IRRIGATION
AND INDUSTRIES.
Well
Name of the Boreholes
Dire
Dawa
Code
Number of Average Total Daily
Boreholes
Discharge (m3/d)
9
10500
4
2332.8
Town
Boreholes(Sabiyan
SBPW
pumping wells)
Melka Jebdu Boreholes
MJBH
&MJIBH
Private Boreholes
PRVBH
3
2376.0
Hand dug & Shallow Wells
HDSHW
5
5520.0
Springs
SPBH
5
15552.0
26 Wells
36280.8
Total
61
Table (4.3b). Daily Groundwater Abstraction Rates from Water points of the study area
and Discharge Contribution of each well with geographic locations of the wells.
Average Daily Discharge Contribution of the Dire Dewa town Water Supply Deep
Wells:
Borehole
Geographic
Ground
Each
Coordinate
Elevation
Daily
well
% contribution
Discharge
Code
Easting
Northing
(m)
(m3/d)
SBPW-4
812527
1063543
1163
1155.0
11
SBPW-6
812767
1063462
1174
472.5
4.5
SBPW-9
813111
1063263
1169
472.5
4.5
SBPW-12
818644
1068005
1470.0
14
SBPW-13
812681
1068000
1438.5
13.7
SBPW14
811995
1063764
1501.5
14.3
SBPW-15
812399
1063264
1438.5
13.7
SBPW-16
812586
1062405
1202
955.5
9.1
SBTW1(2006)
808291
1066600
1105
1596.0
15.2
10500
100.0
Total
62
Table (4.3b) Continued
Average Daily Discharge Contribution of the Melka Jebdu town Water Supply Deep
Wells:
Borehole
Geographic
Ground
Each
Coordinate
Elevation
Daily
well
% Contribution
Discharge
Code
Easting
Northing
(m)
(m3/d)
MJBH-81
808534
1064055
1140
173.0
7.4
MJBH-08
805241
1063462
1141
258.8
11.1
MJBH-09
805167
1063299
1131
173.0
7.4
MJIBH
804049
1062101
1728
74.1
2332.8
100.0
Total
Average Daily Discharge Contribution of Boreholes assigned for Springs:
Borehole
Geographic
Ground
Each
Coordinate
Elevation
Daily
well
% Contribution
Discharge
Code
Easting
Northing
SPBH-1
804500
1053400
3110.4
20
SPBH-2
800251
1050406
3110.4
20
SPBH-3
815902
1049763
3110.4
20
SPBH-4
820010
1062550
3110.4
20
SPBH-5
819000
1048400
3110.4
20
15552
100
Total
(m)
(m3/d)
63
Average Daily Discharge Contribution of deep wells assigned for Private
Boreholes:
Borehole
Geographic
Ground
Each
Coordinate
Elevation
Daily
well
% Contribution
Discharge
Code
Easting
Northing
815100
1062500
815125
810270
(m)
(m3/d)
PRVBH1
PRVBH2
PRBVH3
831.6
35
1065000
831.6
35
1061400
712.8
30
2376
100
Total
Average Daily Discharge Contribution of the Boreholes assigned for shallow and
dug Wells:
Borehole
Geographic
Ground
Each
Coordinate
Elevation
Daily
well
% Contribution
Discharge
Code
Easting
Northing
HDSHW-1
810500
1064000
1104
20
HDSHW-2
815762
1062000
1104
20
HDSHW-3
810072
1064800
1104
20
HDSHW-4
805500
1065200
1104
20
HDSHW-5
815906
1055100
1104
20
5520
100.0
Total
(m)
(m3/d)
64
Table.(4.4). Summary of location and spring discharge
ID
UTM
UNTMN
ALTITUDE NAME
DISCHARGE
1
793411
1051018
1607
Lege Oda Gunfeta
25 l/s
2
798838
1048407
1644
Ulul Mojo Spring
20 l/s
3
798698
1048633
1802
Mede Lugo Spring (Ulul Mojo)
7 l/s
4
798792
1049361
1761
Meda Adem Abdi Spring (Ulul Mojo)
5 l/s
5
802293
1049764
1685
Wahil Spring
6 l/s
6
802321
1050025
1673
Wahil Cheleleka Spring
4 l/s
7
803435
1050512
1621
Bishan Gende Boru
4 l/s
Dujuma Spring Diverted for water Supply of
8
804443
1052225
1357
Sheneni Village
1 l/s
9
804305
1052207
1531
Dujuma springs
7 l/s
10
804132
1052121
1538
Dujuma springs
7 l/s
11
807516
1053914
1381
Lege Merti Elbe (Halo Busa Spring)
3 l/s
12
799366
1051072
Koriso
30 l/s
13
799700
1052250
1560
Hulul Shile
30 l/s
14
803522
1050333
1620
Hara Watu
4 l/s
15
800391
1049534
1766
Borte
5 l/s
16
807621
1053559
1390
Lege meda
3 l/s
17
800664
1052906
1484
Gende Bule Spring
3 l/s
18
807741
1054345
1361
Lago Merti Spring
6 l/s
19
802444
1048155
Gende Mesno
10 l/s
65
4.11. Model input Data processing
Because the information used for the model construction was originally coming from
different sources and formats a process of checking, selection and organization had to
be carried out.
One important step in data processing was to remove wells where levels might be
erroneously measured because of problems with the datum. Another important part was
to organize all necessary information in tables. GIS (Geographic Information System)
was also used to get a better general picture of the study area and to decide on model
boundaries.
The images used in the work were LANDSAT TM, all bands. Having the images in the
necessary format the next step was to geo-reference them, this step was performed
using the topographic map of the area with scale 1:50,000 and Global mapper-7. This
Geo-referenced image was covered a large area than that needed and therefore a subarea had to be selected, which is proved to be useful in reducing the computing and
processing times. Another important aspect of this new image was its usefulness in
constructing and validating the conceptual model of the study area. On top of its
segment, polygon and point maps were overlain in order to check the model boundaries
and location and distribution of bore holes in the area. Finally this image was saved as
background for the different variations of the conceptual models during the calibration
process. Fig (4.8) shows color composite land sat TM image of the study area.
66
67
CHAPTER - 5
Numeric simulation of Groundwater flow
5.1. General Concept and Modeling Approach
Numerical groundwater flow modeling helps to have a good understanding of the current
or to predict the long term tendencies of a hydro-geological system and it allows analysis
of the movement of water through hydro-geologic unit that constitute the groundwater
flow system. A numerical groundwater flow model of the study area is developed to help
better understand the aquifer systems of the basin and to determine the long-term
availability of groundwater by simulating groundwater condition at present and predict the
future condition under different hydrologic and pumping scenarios for various
groundwater management alternatives.
It is mandatory to have good initial data on boundary conditions, fluxes and aquifer
hydraulic parameters for a model to give simulation output that approaches the real
situation. In other words, models can only be good if the input data is good enough.
Especially, input parameters that have the most control on the model output have to be
carefully investigated and correctly estimated. In this study, a shortage of standard
hydro-geological data has been encountered in most parts of the area to have good
estimates of these parameters but collection and assemblage of relevant hydrogeological data in the conceptual model has been made.
There is no water level monitoring data for any wells and boreholes in the area and there
is also no measured daily discharge for any wells and boreholes. The existing records
obtained from Dire Dawa town Water Supply and Sewerage Authority of water levels in
wells is not consistent, therefore, transient simulation calibration was not possible.
Because a reliable head distribution is necessary for a correct model calibration, only
68
those wells and boreholes in which water levels are correctly measured and recorded are
used in the model.
A computer program, MODFLOW 96 +interface to MTD 96 and later, by Mc Donald and
Harbaugh (1988) was used to simulate groundwater flow within the study area aquifer
system. The program uses a finite difference scheme to integrate the equation for three
dimensional/ quasi-three dimensional, saturated groundwater flows under equilibrium
(i.e. steady state) condition. MODFLOW is a versatile finite-difference groundwater
modeling program used to construct numerical flow models of the study area. A
MODFLOW model consists primarily of a set of input files that contains information on
the physical properties of the modeled system such as the geometry, boundary
conditions, internal properties (such as the distribution of hydraulic conductivity and
storage coefficient), and sources and sinks such as groundwater recharge, springs, and
pumping wells. The underlying concept of the approach used was that an understanding
of related basic principles and an accurate description of the specific system under study
will enable an accurate quantitative understanding of the cause and effect relationship.
This quantitative understanding of the relationships allows one to understand the
response of the system under consideration to any proposed scenario or to make
predictions for any defined set of conditions. Once these files are created, the model
program is run to solve a set of equation that describes the distribution of head at
discrete points with in the system and the flow in response to that head distribution.
Aquifer system of Dire Dawa area is modeled following quasi-three-dimensional model
which simulates a sequence of aquifers with intervening confining layers. Like twodimensional aerial models of leaky confined aquifers, confining Layers are not explicitly
represented in a quasi-three-dimensional model, nor are heads in the confining beds
calculated. The effect of confining bed is simulated by means of leakage term (Lij)
representing vertical flow between two aquifers. The leakage term is a function of the
leakance and the head difference across the confining bed. In a quasi three-dimensional
model the head in the unit overlaying the top confining bed, usually an unconfined
aquifer can be calculated directly by the model (Marry p. Anderson & William
69
W.Woessner, 1992). Ignoring horizontal flow in the confining beds causes less than a
5%difference in heads in the modeled layers when the contrast in hydraulic conductivity
between the aquifer and confining beds is at least two order of magnitude (Neuman and
wither spoon. 1969), (fig.5.1 comparing a full three-dimensional and quasi-threedimensional model).
Fig.(5.1)Showing quasi-three-dimensional and full-three-dimensional models.
5.2. Well distribution
A modeled area has been characterized by rough topography and by the absence of any
rivers that can be used as water supply. Due to the fact that the topography is rough in
the area and to low values of precipitation during the year, not all zones are suitable for
an agricultural development at large scale. For those farms if available, the only source
of water is groundwater. The population is also concentrated only in some parts of the
area such as near and/or at Dire Dawa town and melka Jebdu town. These together with
the topography can answer why the wells are found in clusters in the area rather than
being homogeneously distributed. In some part of the modeled area there are no wells at
70
all. Fig (5.2) shows the well distributions in the area and fig (4.6) shows the well
distribution for calibration.
5.3. Governing Equation and Model Code
The movement of groundwater through porous media is described and solved, for two
layers aquifer system under a steady-state flow (as used in this study), on the basis of
the following partial differential equation, which is based on Darcy’s law and the law of
mass conservation (McDonald & Harbaugh, 1988). This equation assumes flow system
viewpoint that allows both vertical and horizontal components of flow throughout the
system and there by allows treatment of flow in three-dimensional profiles (Anderson and
Woessner, 1992).
The Governing partial differential equation used by the MODFLOW to describe the
groundwater flow is:-
∂ Kxx ∂h
∂x
∂x
+
∂ Kyy ∂h
∂y
∂y
+
∂
∂z
Kzz ∂h
∂z
∂h
- W = Ss
∂t
(1)
Where:
Kxx, Kyy, and Kzz
are values of hydraulic conductivity in the x, y and z directions along
Cartesian Coordinate Axes, which are assumed to align with
principal directions of hydraulic conductivity (LT-1),
h
is hydraulic head (L),
W
is a volumetric flux per unit volume and represents
Sinks and/or sources (T-1),
Ss
is the specific storage of the porous material (L-1), and
t
is time (T).
71
This equation describes the distribution of hydraulic head and flow throughout a
continuous region. It is continuous in space and time, and generally cannot be solved
analytically for practical applications involving complex system (Anderson and Woessner,
1992). Practically, the continuous system described in the above equation is replaced by
a set of spatially and temporally discrete points using numerical methods, which form a
set of simultaneous algebraic equations that describe the distribution of hydraulic head at
each point and flow through the system in response to this head distribution. These
simultaneous equations are set up in a matrix form and then solved.
Available data are limited to horizontal properties in aquifers and no relation can be
established regarding the anisotropy of units. Thus, for this study, a hydraulic property
within the layer is assumed isotropic. Consequently Kx and Ky are considered to be equal
at any given location and Kx and Ky are replaced in this discussion by the single term K
to describe horizontal hydraulic conductivity. Since the study is on a Steady-state
condition there is no change in head with time, therefore, this part of the right hand side
of equation (1) becomes zero and it can be re-written as:
∂ (Kxx ∂h ) + ∂ (Kyy ∂h ) + ∂ (Kzz ∂h) - W = 0
∂x
∂x
∂y
∂y
∂z
(2)
∂z
72
73
5.4. Spatial discretization
The numerical method used to approximate governing equation requires that the
modeled domain be divided into discrete volumes, called cells. The three-dimensional
array of cells is known as the model grid. The center of each cell defines the point for
which hydraulic head is determined. The head is taken to represent the average head
within the cell.
The regional groundwater flow system of the Dire Dawa area was represented as an
array of cells arranged in 87 east-west trending rows, 125 north-south trending columns
and 2 layers. The model area corresponding to the study area of 885km2 and it was
selected on the basis of preliminary numerical modeling. Extent of the modeled area is
35km east by 50km north and contains the entire study area. The model uses a uniform
grid size of 400m by 400m and contains 11,062 cells, 2 layers, 87 columns and 125
rows. The regular grid spacing facilitated data in put from DEM and surfer files. The
irregular shape and the locally bounded nature of the two aquifers of the study area
reduced the number active cells in the model to 8,011, with 5491 active cells in layer-1
and 2520 active cells in layer-2. Layer-2 has fewer active cells than layer-1, because
some of the cells, along the southern edge of the basin which is affected by west-east
trending faults, are in area of basement complex that are higher in elevation than that of
the top of layer-2. The aquifers are discretized vertically into two layers (layer-1 and
layer-2). The relationship between the hydro-geologic units and equivalent layers used in
the groundwater flow model is shown in Table 4.1.
Layer-1 corresponds to the upper part of aquifer where poorly sorted clay, silt, and sand
in the central and northern part of the study area and weathered and fractured basement
complex at the hill sides and weathered sandstone and limestone along the southern
border of the study area and this layer is unconfined aquifer. Layer-2 corresponds to the
lower part of the aquifer where the upper sandstone and Hamanlei limestone are
fractured and cavernous. Thus layer is assumed to be confined in most areas during
conceptual model. The two layers of the aquifer system of the study area have variable
74
thicknesses depending on the geologic type and structures. The height of each cell in the
model was equal to the estimated formation thickness, which was determined based on
the USGS 90-meter resolution shuttle Radar Terrain model (SRTM) of the land surface.
The top altitude of layer-1 represents ground surface elevation above sea level and the
bottom altitude of this layer is 900m above sea level through out the study area, which is
conceptualized from the bottom of poorly sorted alluvial sediments as observed from
borehole logs and geophysical survey result that were conducted by different
Organizations. Layer-2 represents the confined, lower high permeability aquifer underlies
the confining layer in most areas. The top of layer-2, which is the bottom of the confining
layer, is 850m above sea level and the bottom of layer-2 is the altitude of the lower part
of the Hamaneli limestone unit which is equal to 475m.a.s.l.
5.5. Boundary condition
Boundary conditions in a groundwater flow model define the locations and manner in
which water enters and exits the active model domain. Boundary conditions define the
geographic extent of the flow system as well as the movement of groundwater into and
out of the system, such as flow to or from streams. They are Mathematical statements
specifying the dependent variable (head) or the derivative of the dependent variable
(flux) at the boundaries of the problem domain (Anderson and Woessner, 1992). The
choice in the type and location of model boundaries is important, as this may affect the
simulation result.
Boundary conditions may be of three general types. One type is a specified-flux
boundary, of which no flow boundary is a special case. This no-flow boundary indicates
that there is no exchange of water between the model cell and the area outside the
model. Known or estimated hydrologic fluxes, such as recharge and well discharge, are
represented using specified-flux boundaries. A no-flow boundary condition set by
specifying flux to be zero. A second type is a specified-head boundary for which head is
given. The third type is a head-dependent flux boundary, for which the boundary flux is
the product of a specified factor and the difference between the simulated head at the
75
boundary and specified head of an external source/sink. These boundary conditions
greatly affect the result of simulation and therefore, actual hydro-geologic settings should
be taken into account to assign appropriate boundary conditions.
The boundaries in the model are selected following geologic feature and structures, geomorphological features and hydro-geological evidences. The geographic boundaries of
the study area groundwater model are chosen to correspond as closely as possible with
natural hydrologic boundaries across which groundwater flow can be assumed
negligible, such as groundwater divides, or can be reasonably estimated. Major
topographic divides are often considered no-flow boundaries because topographic
divides are typically coincide with groundwater divides. Groundwater on either side of a
groundwater divides flows away from the divide and not across it, so the divide itself acts
as a no-flow boundary. Topographic divides often coincides with groundwater divides
because up land areas commonly have larger amounts of precipitation and recharge
than surrounding areas, so water table surface develops a coincident high elevation
region from which groundwater flow diverges.
Lateral Boundary
Physically the lateral boundaries of the model area are generally represented as no-flow
boundaries, with the exception of one area in layer-1 and two areas in layer-2, where it is
represented as head dependent flux boundary fig 5.3a and 5.3b. The lateral no-flow
boundaries are located at the surface water divide of the study area by assuming that
groundwater divide coincides with surface water divide for layer-1 and general head
boundary is used for this layer at topographically low area as shown in fig.5.3a. The
existence and location of this boundary is inferred from water table and topographic
maps. Groundwater-table divides define the no-flow boundary. For the second layer of
the model aquifer system, the lateral no-flow boundary is located at the contact of
sedimentary aquifer and the bed rocks at the east and southwestern direction and
general head boundary at the southern and northern direction, where water enter the
area from the southern direction following the north-south orienting faults and leaves to
76
the north direction. At these localities (General Head Boundaries) water can enter or
leave the system depending on the gradient of the water level at the boundaries. These
head dependent flux boundaries of the model are simulated with the General-Head
Boundary (GHB) module of the MODFLOW.
Upper Boundary
The upper boundary of layer-1, of the groundwater model flow system is defined by the
ground surface elevation. The ground surface elevation is calculated for each cell of the
top layer of the model by overlaying a geographically referenced digital coverage of the
ground surface map (DEM) onto a geographically referenced coverage of the model grid.
The location of these boundaries is partially determined by the flux of the water across
this boundary and by head dependent-flux boundaries. The specific-flux boundary is the
aerially applied groundwater recharge and the head dependent-flux boundaries
represents springs. A value of recharge over the water table boundary is estimated from
previous works. The upper boundary of the second layer is the lower boundaries of the
confining unit.
.
Lower Boundary
The lower boundary of the Layer-1 of the aquifer system is defined as the contact
between the base of the intermediate permeability upper aquifer and the top of the
confining unit, which was characterized by the head distribution along this contact. This
head distribution is assumed to have a value that, although variable with time, is the
same everywhere on the lower boundary (uniform spatial distribution) for a given
moment in time. No-flow boundary is used below the modeled area to represent contact
of Layr-2 with lower massive limestone and basement complex.
77
78
79
5.6. Model input parameters
In general, model input parameter consists of recharge, evapotranspiration and other
artificial stresses imposed on the aquifer such as; hydraulic parameters, boundary
conditions and time discretization in the case of transient simulation.
Model input parameters which include aquifer properties, such as hydraulic conductivity,
transmissivity, vertical conductance, specific yield, and storage coefficient, control the
rate at which water moves through the aquifer, the volume of water in storage, and the
rate and aerial extent of water level declines caused by groundwater development.
For this study, the aquifer system properties are initially estimated from well logs,
analysis of pumping test conducting by different Organizations and from different
groundwater literatures. Arial recharge estimation was obtained from Eyilachew Yitayew
(2004) and from hydro-geologic repot by WWDSE, and pumpage was estimated, as
described in the conceptual model, from daily pumping records and estimation of hand
dug wells and shallow drilled well discharges. These aquifer property values can vary
considerably spatially because of the heterogeneity of the aquifer system material. To
reduce the number of parameter values required in the model, the flow regime of each
model layer is divided into zones within the model domain and each zone is
characterized by a uniform set of values. The definition of each zone is based on the
analysis of available geologic and hydro-geologic data.
5.6.1. Initial and prescribed Hydraulic Heads
MODFLOW requires initial hydraulic heads at the beginning of a flow simulation. For
steady-state flow simulation, the initial heads are used as starting values for the iterative
equation solvers. Therefore, to estimate this initial hydraulic heads in the model, DEM
land surface data source is used for each model cell. This land surface elevation is
calculated from 90m resolutions Shuttle Radar Terrain Model (SRTM) image data using
computer software called Global mapper-7 and surfer-8. Accordingly the initial and
80
subscribed hydraulic head for the model was assigned for each cell of the model layer by
the following methods:
a) Areas where land surface elevation is below 1050m, hydraulic head is assigned
by land surface minus 20m.
b) Area where land surface elevation range from 1050m-1150m, hydraulic head is
assigned by land surface minus 35m.
c) Areas where land surface elevation range from 1150m-1350m, hydraulic head is
assigned by the land surface minus 60m.
d) Areas where land surface elevation is above 1350m, initial hydraulic head is
assigned by the land surface minus 10m.
5.6.2. Hydraulic properties
Aquifer properties, such as vertical and horizontal hydraulic conductivity, transmissivity,
vertical conductance, specific yield, and storage coefficient, control the rate at which
water moves through the aquifer, the volume of water in storage, and the rate and aerial
extent of water level declines caused by groundwater development. The scarcity of
information on the subsurface geology in the saturated zone and of aquifer test data that
is uniformly distributed in the study area precludes mapping hydraulic conductivity on a
cell-by-cell basis. A practical alternative is to represent hydraulic conductivity in a set of
discrete sub regions or zones within which conditions are considered uniform.
Aquifer hydraulic properties required for the quasi 3D steady-state simulations were
horizontal and vertical hydraulic conductivity and vertical leakance. In the following
discussion, the term “hydraulic conductivity” with no modifier refers to horizontal
hydraulic conductivity. Aquifer test data at the well fields, geological logs from wells and
boreholes and secondary data were used to characterize hydraulic conductivities. A
generalized approach was used to estimate the spatial distribution of hydraulic
conductivity and values may be adjusted during model calibration to match observed
data.
81
The horizontal hydraulic conductivity for layer-1 was estimated from groundwater
literatures depending on the geologic and hydro-geologic properties of the aquifer unit.
This layer of the model consists of different geologic units (such as poorly sorted clay,
silt, sand, gravel, weathered and fractured basement complex and highly weathered
sandstone and limestone) and the hydraulic conductivity values for this layer was
estimated to range from 10-3m/day for weathered basement to 10-1m/day for weathered
limestone. The horizontal hydraulic conductivity of layer-1 was applied to each activate
model cell by zoning the similar hydraulic areas depending on the surface spatial
distribution of geologic materials.
The Transmissivity of layer-2 (the main aquifer) was estimated from the analysis of the
preexisting aquifer test data of several boreholes conducted by WWDSE, Water well
drilling and Pile foundation, Hydro PLC, OWWCE, and others; from hydro-geologic report
of Dire Dawa Council by WWDSE; and from literatures (see table 4.2 and fig.4.5). Based
on the above, the aquifer Transmissivity values ranges from 2.4m 2/day to 55124m2/day.
The hydraulic conductivity for layer-2 of the model is calculated from pumping test result
through dividing Transmissivity by the total length of the screen. The horizontal hydraulic
conductivity value so estimated for layer-2 of the model was used for the initial run of the
model.
In MODFLOW, the vertical hydraulic conductivities of two vertically adjacent cells are
used to compute a vertical leakance term. Vertical leakance between layer-1 and layer-2
occurs only where there is a hydraulic head difference between the layers.
Theoretically the leakage properties of the confining beds are used to connect aquifer of
layer -1 and aquifer of layer -2 in the plain area due to the presence of confining layer of
very low permeability. But the occurrence of groundwater at the well fields with in the
lower aquifer is assumed to be due to the hydrologic, and the hydro-geologic conditions
in the escarpment area and hence no groundwater movement vertically from alluvial
aquifer to the lower more productive aquifer which means no vertical leakage (Hydrogeologic Report by WWDSE, 1995).
82
5.7. Model simulated stresses.
As explained before, water demand in the study area has increased dramatically during
the last two decades and therefore groundwater consumption as there is no other
alternatives like rivers, lakes etc. The main source of water for domestic, industrial and
agricultural uses is form groundwater. This situation, associated with the prevalence of
arid and semi-arid climate in the region and low value of annual rainfall and recharge,
may lead to substantial change in aquifer water level. This expected aquifer water level
change is due to responses to stresses on the system and such stresses are due to
recharge and discharges.
5.7.1. Recharge
Recharge rates for the steady-state simulation were taken form previously estimated
values by different researchers (Eyilachew Yitayew and others). Accordingly an initial
recharge value of 10.4mm as estimated by Eyilachew(2004), to the area of valley
depression(plain), situated in the vicinity of the town of Dire Dawa extending to the west,
east, and north of Shinile which constitute about 53%(470km 2) of the area; 24mm as
estimated by BCEOM in association with WWDSE to be 4% of the average annual
precipitation(608mm) to the area of steeped margin of the valley depression, extending
from the plateau to the southern limit of Dire Dawa town, that covers about 36%(318km 2)
of the study area; and 56.3mm as estimated by Ato Eyilachew Yitayew (2004) using
Water balance approach for the plateau margin area, which demarcates the southern
limit of the study area, that covers about 11%(97 km 2) of the area are used. From the
above the total recharge to the area is computed to be 48493m 3/day.These values are
used to start running the model and lately modified during calibration.
83
5.6.2. Discharges
In the study area discharge from groundwater system includes groundwater pumpage
from wells and boreholes and from springs. Groundwater withdrawals from pumping
wells and boreholes and from springs are simulated with the well package of MODFLOW
depending on geographic coordinates of the wells. The pumpage from each model layer
is distributed such that much of pumpage is from layer -2 of the model. The following
assumptions are made regarding groundwater withdrawals from study area for the sake
of simplification.
a) All groundwater withdrawal through borehole pumpage is from layer -2. This is
supported by the data obtained from different sources.
b) Pumpage from individual private boreholes are replaced by three assumed wells,
which pump cumulatively 2376m3/day from the lower aquifer.
c) Five assumed wells, which pump cumulatively 15552m3/days of water, were
applied to the upper aquifer, which compensates the springs.
d) Five assumed wells that pump cumulatively 5520m3/days of water were applied to
the upper aquifer, which compensates the hand dug and shallow wells in the area.
Withdrawals from pumping wells and boreholes and from springs were simulated as
specified flows from the aquifer. Because there are limited well-constriction data
available, all wells were assumed to be fully penetrating in the layers.
84
CHAPTER - 6
Calibration and Sensitivity Analysis
6.1. Model Calibration
Calibration of a flow model refers to a demonstration that the model is capable of
producing field measured heads and flows which are the calibration values. Model
calibration is the process where by model parameter values are adjusted and refined to
provide the best match between measured and simulated values of hydraulic heads and
flow (Anderson and woessner, 1992)
A complication in groundwater problems is that the distribution of heads is always
incomplete and flux calculations are not always known accurately. Estimates of flux have
associated errors that are usually larger than errors associated with head measurements
(Anderson et al, 1992). Nevertheless, it is advisable to use estimates of flow as
calibration values in addition to heads in order to increase the likelihood of achieving a
unique calibration.
Dire Dawa area groundwater flow model is calibrated using a trail-and-error method in
adjusting initial estimates of aquifer properties, recharges and boundary condition to get
a best match between simulated hydraulic heads and measured water levels, and
selected water-budget items. Model fit is commonly evaluated by visual comparison of
simulated and measured heads and flows or by listing measured and simulated heads
together with their differences and some type of average of the differences, which is then
used to quantify the average error in the calibration. The objective of calibration is to
minimize this average error which is called calibration criterion.
85
6.2. Data Used for Calibration
Dire Dawa area groundwater flow model is calibrated to steady-state condition of
average head collected at different times. This was done due to the fact that measuring
water levels in some wells [such as private wells, sealed wells, wells installed with hand
pump and even most of the wells does not have observation pipes] during field work was
not possible. In some cases water level measured during pumping test were used, in
other cases where possible head measuring was conducted during field work
Observations for calibration of the study area groundwater flow model consists of water
level measured data from 64 wells. These head observations were not evenly distributed
through out the model domain but were clustered geographically in the populated areas
of the study area (fig 6.1a and 6.1b) and vertically to the lower layer. Head observations
were related to specific cells (horizontally and vertically) based on the well location and
on the depth of the well's open interval. The time of measurements of some of the wells
were uncertain, as a result, head data were examined carefully and anomalous values
due to measurement or location errors, pumping effects and other errors were removed
from the calibration data set.
6.3. Steady-state calibration
The steady-state calibration involved matching the simulated hydraulic heads to
measured water levels from wells in the study area. Steady-state flow conditions exist
when inflow is equal to outflow and aquifer storage does not exist. Prior to the calibration
of the model, there were criteria established to assess the simulated results in relation to
measured data. The first calibration criterion for the simulation was that the simulated
groundwater surface and hydraulic gradient generally match those of the estimated one,
which were done by comparison of the two. A second calibration accounts of matching
more 80% of wells to within  10m of the observed hydraulic heads. To provide an over
all indication of the quality of the calibration summary statistics on the differences
between simulated and measured water level were calculated after model calibration.
The root mean squared error (RMSE), mean absolute error (MAE), and the mean error
86
(ME) are common ways to express the average differences between simulated and
measured water levels (Anderson and woessner, 1992).
Hydraulic heads for steady-state conditions are sensitive to the amount of water that
recharges to and discharges from the groundwater system, the hydraulic conductivity of
the aquifer system, the boundary conditions, and aquifer thickness.
In a calibration procedure there are three accepted steps (Anderson et al, 1992)
A. To first change the values in cells where the highest deviation occurs.
B. To change just one parameter in each run.
C. To determine if any change of that parameter has a positive or negative effect in
other cells.
Accordingly initial estimates of model impute parameters; especially hydraulic
conductivity, recharge, general head boundary, interface conductance, and boundary
conditions were adjusted within reasonable limit to get satisfactory fit. Initially adopted
recharge values and zones were modified within plausible range. Then horizontal
hydraulic conductivity was adjusted manually to get best fit between observed and
calculated heads. The final calibrated hydraulic conductivity values ranged from 0.05m /
day to 11 m / day for layer-1 and from 0.001 m / day to 120 m / day for layer-2. The
lowest value is to the southern part of the study area and the value increases to north
attaining maximum in sabiyan well field (fig.6.5a and fig.6.5b).
87
88
89
6.4. Calibration Results.
Calibrated water levels and observation wells for the calibrated steady-state model are
shown in table 6.1. Based on the first calibration criteria set earlier for the model, the
simulated groundwater surface and hydraulic gradient generally match those of the
estimated one. Comparison between contour maps of measured and simulated heads
(fig 6.3) were done to get some idea on the spatial distribution of error in the calibration.
The simulated water level gradient of the southern, southwestern, and northwestern
parts of the study area deviates from the measured one. In these part of the study area
the difference between the measured and simulated water level contour may be due to
lack of available water level data, due to problem in boundary conceptualization, or due
to interpolation error of the measured gradient. In general the simulated groundwater
surface is similar to the estimated average groundwater surface in comparison to both
hydraulic heads and gradients, which shows fulfillment of the first criterion. Comparison
between contour maps of measured and simulated heads provides a visual, qualitative
measure of the similarity between patterns, there by giving some idea of the spatial
distribution of errors in the calibration. However, contour maps of field data include
errors introduced by contouring and therefore should not be used as the only proof of
calibration. A scatter plot (fig 6.2a) of measured against simulated heads is another way
of showing calibration fit.
The second calibration criterion is matching simulated hydraulic heads at 80% of the
points within 10m of the observed hydraulic heads. Simulated hydraulic heads matched
observed values within 10m difference heads for 81% of the 64 observed wells and 96
% of the observed wells matched to the simulated heads within  20m difference.
90
91
92
93
94
The overall average difference between simulated and measured heads was expressed,
as given in Anderson and Woessner (1992), using the following three statistical
methods:
1- The mean error (ME) is the mean differences between measured heads (hm) and
simulated heads (hs )
ME =
n
1
n

(hm-hs)
i 1`
Where hm is measured head, hs is calculated head and n is number of head
measurements.
The ME of the calibration for all observation measurements considered has about
0.007m.
2- The mean absolute error (MAE) is the mean of the absolute value of the
differences in measured and simulated heads.
MAE =
1
n
n

/ (hm-hs)/
i 1
The MAE calculated for hydraulic heads is 4.89m.
3- The root mean square (RMS) error or the standard deviation is the average of the
squared differences in measured and simulated heads and can be calculated using
the equation.
RMS = [
1
n

(hm-hs)i2] 0.5
The fitted RMS heads for observation point is about 6.7m. Since RMS is a measure of
differences between the measured and simulated data, lower number reflect a better
model fit.
xcv
Table 6.1 comparisons of Simulated and Observed Heads for steady-state Simulation.
Groundwater altitude,
in meters
No.
Observation
Calculated
Observed
name
Heads(S)
Heads(M)
(M-S)
(M-S)2
│M - S│
1
PW-17
1139.061
1126.65
-12.411
154.03
12.411
2
BH-32
1181.752
1179.55
-2.202
4.85
2.202
3
BH-43
1144.329
1133.5
-10.829
117.29
10.829
4
BH-24
1148.919
1133.43
-15.489
239.94
15.489
5
BH-54
1150.506
1147.8
-2.706
7.34
2.706
6
BH-56
1149.672
1148
-1.672
2.79
1.672
7
BH-70
1149.422
1149.2
-0.222
0.048
0.222
8
BH-68
1148.442
1148
-0.442
0.194
0.442
9
BH-74
1140.555
1143
2.445
6
2.445
10
BH-51
1149.461
1147.75
-1.711
2.89
1.711
11
BH-49
1149.412
1148
-1.412
1.99
1.412
12
BH-48
1147.905
1148
0.095
0.01
0.095
13
BH-47
1145.302
1149
3.698
13.69
3.698
14
BH-46
1147.115
1147
-0.115
0.01
0.115
15
BH-45
1144.353
1149
4.647
21.62
4.647
16
BH44
1144.083
1146
1.917
3.69
1.917
17
BH-20
1157.905
1154
-3.905
15.23
3.905
18
BH-06
1156.914
1152
-4.914
24.11
4.914
19
BH-52
1154.674
1150.23
-4.444
19.71
4.444
20
BH-95
1149.349
1138.3
-11.049
122.1
11.049
21
BH-105
1156.765
1149
-7.765
60.37
7.765
22
BH-12
1154.229
1154
-0.229
0.05
0.229
23
BH-101
1162.417
1163
0.583
0.34
0.583
24
BH-99
1160.558
1166.7
6.142
37.7
6.142
25
BH-86
1155.756
1159.5
3.744
13.99
3.744
xcvi
26
BH-21
1167.238
1164
-3.238
10.5
3.238
27
BH-11
1159.851
1168.5
8.649
74.82
8.649
28
BH-10
1160.967
1168.5
7.533
56.7
7.533
29
BH-33
1182.073
1181.2
-0.873
0.76
0.873
30
BH-31
1180.191
1180.4
0.209
0.04
0.209
31
BH-30
1181.29
1180.5
-0.79
0.62
0.79
32
BH-29
1180.191
1181.06
0.869
0.76
0.869
33
BH-41
1181.002
1181.36
0.358
0.13
0.358
34
BH-40
1180.69
1180.44
-0.25
0.06
0.25
35
BH-36
1180.292
1181.29
0.998
1
0.998
36
BH-35
1180.575
1180.55
-0.025
0
0.025
37
BH-26
1178.255
1180.4
2.145
4.62
2.145
38
BH-23
1149.37
1137
-12.37
153.02
12.37
39
BH-22
1160.045
1170.9
10.855
117.94
10.855
40
BH-07
1149.352
1147
-2.352
5.52
2.352
41
BH-112
1167.483
1161
-6.483
42
6.483
42
DW-28
1151.118
1162
10.882
118.37
10.882
43
DW-24
1149.764
1163
13.236
175.3
13.236
44
DW-29
1145.787
1156
10.213
104.24
10.213
45
DW-33
1154.237
1154
-0.237
0.06
0.237
46
BH-83
1138.087
1130
-8.087
65.45
8.087
47
BH-80
1130.165
1130.6
0.435
0.19
0.435
48
DW-14
1156.53
1158
1.47
2.16
1.47
49
BH-85
1158.141
1168
9.859
97.22
9.859
50
BH-64
1148.866
1158
9.134
83.36
9.134
51
BH-71
1138.93
1140
1.07
1.14
1.07
52
BH-100
1162.417
1162
-0.417
0.18
0.417
53
BH82
1155.557
1160
4.443
19.71
4.443
54
BH-98
1144.046
1132
-12.046
145.2
12.046
55
BH-93
1149.295
1148
-1.295
1.69
1.295
xcvii
56
BH-94
1156.147
1147
-9.147
83.72
9.147
57
BH-103
1155.313
1164
8.687
75.52
8.687
48
BH-93
1149.176
1161.8
12.624
159.26
12.624
59
BH-94
1156.147
1146.58
-9.567
91.58
9.567
60
BH15
1149.163
1147
-2.163
4.67
2.163
61
BH-14
1133.171
1150
16.829
283.25
16.829
62
BH-16
1158.266
1163
4.734
22.37
4.734
63
BH-34
1179.13
1178
-1.13
1.28
1.13
64
BH-32
1181.752
1179.5
-2.252
5.06
2.252
4.264 2879.452
312.742
ME=0.007 RMS=6.7 MAE= 4.89
From the result of the above three statistical error analysis methods, the following are
concluded.
The mean error is 0.007m for all water level measurement for both aquifers. This
indicates that the model is positively skewed in which it favors to the observed water
level, which mean that in the overall calibration of head levels, observed heads were
greater than calibrated heads by about 0.007m. The root mean square error for all wells
is 6.7m and the mean absolute error for all wells is 4.89m. The statistical computation
for residual errors is summarized in table 6.1.
6.5. Simulated water budget
The steady-state model water budget which is evaluated with the calibrated model is
shown in table 6.2. The water balance in the model closes to within about 0.04 per cent.
The comparison of steady-state water budget for the numerical simulation shows that,
estimated groundwater balance for conceptual model is more or less similar to simulated
value with some variation which might be caused by boundary delineation for numerical
model.
xcviii
Table 6.2 Model simulated steady-state hydrologic budget
Hydrologic budget component
Cubic meter per
day(m3/day)
Million cubic meter
per year
(MCM/year)
Inflow
Recharge from precipitation
47194.39
16.99
Total inflow
47194.39
16.99
Pumpage and springs
Outflow through General-head boundary
36845.4
10369
13.26
3.73
Total outflow
47214.4
227.5
Outflow
Budget error (inflow-outflow)
Percent discrepancy (%)
20.01
0.04
6.6. Sensitivity analysis
The purpose of sensitivity analysis is to quantify the uncertainty in the calibrated model
caused by uncertainties in the estimates of the aquifer parameters, stresses, and
boundary conditions (Anderson et al, 1992). Money assumptions and estimates are
used in the design and construction of groundwater flow model. To test the response of
the calibrated model to a range of values for the initial hydraulic properties and
stresses, a sensitivity analysis is done. This is done by varying the values of one input
parameter while keeping all others constant for this analysis; it is possible to observe
the relative sensitivity of the model to various input properties. Thus separate model
simulations are made with varied input properties and the changes in simulated
hydraulic head and in components of the water budget are recorded. The results of the
sensitivity analysis for this study were evaluated by calculating the root mean square
deviation (error) between measured and simulated heads in the modeled area. It is
done to determine the relative response of calculated water level and in components of
xcix
the water budget to uniform changes in the simulated values of recharge and, hydraulic
conductivity values. Each variable was separately increased and decreased by 20, 40,
and 60, per cent of it calibration value for both aquifers. Results of the sensitivity
analysis indicated that model calculated water level were most sensitive to variations in
the values specified for hydraulic conductivity, and recharge (Fig.6.4.). These imply that
small variation in hydraulic conductivity and/or recharge from the calibrated values
brings about high changes in the distribution of simulated hydraulic head. The
parameter and range of values used in the sensitivity analysis are shown in table 6.3.
As shown in fig.6.4. a +15% or – 15% change in both hydraulic conductivity and
recharges causes a large deviation of hydraulic head from the calibrated residual.
Generally speaking, if a parameter has a high sensitivity, observation data exist to
effectively estimate the value.
Table 6.3 Results of Sensitivity Analysis Test on Water Levels
No Change in sensitivity parameter from the Respective RMS head change
calibrated value, in %
from the calibrated value, in %
1
Recharge increased by 20,40 & 60
9.09, 9.23, 9.30
2
Recharge decreased by 20, 40 & 60
9.04, 8.97, 8.91
3
Hydraulic conductivity increased by 20, 40 & 8.3, 9.1, 11.5
60
4
Hydraulic conductivity decreased by 20, 40& 7.7, 15.5,15.6
60
c
6.7. Scenario Analysis
The calibrated groundwater flow model can be used to simulate the potential effect of
alternative water management plans on hydraulic head and groundwater movement in
the study area. It can also be used as a tool to evaluate and compare the responses of
an aquifer system to potential future stresses. One of the aims of this working was
intended to test the responses of the hydrologic system to different scenarios. So,
alternative scenarios were developed to test the responses of the hydraulic system to
changes in water uses or hydrologic stresses under steady-state condition.
In general, the results of the scenarios or their accuracy depend on the validity of the
assumptions behind the scenarios. Moreover, errors introduced due to limitations
associated with the model also affect the result of the scenarios and should be taken in
to consideration during interpretation and application of results. In all scenarios, other
ci
model parameters were kept to the steady-state values except the stress for which the
projection was carried out. The resulting changes in water level and fluxes were
interpreted as the response of the system to the changes introduced on it.
The first scenario, which is pumping of DireJara well field to the satisfaction of the needs
for phase I demand of Harar town. In this scenario the demand is estimated to be 242 lit/
sec. which is equal to (20909m3/day) (Data form Harar water supply project office).This
amount of withdrawal is then distributed among the Dire Jara wells. This scenario is to
investigate the effects of water-management practices that could mitigate potential
adverse effects of increased water withdrawals. Then model simulated results of water
table elevation in the scenario is compared with model calculated steady-state result
and the head calculated for this scenario shows a maximum decline of the water level by
7m near Dire Jara well field and a minimum of 8cm at Sabiyan well field. This shows that
pumping water from Dire Jara well field at the rate of 20909m 3/day which is equal to the
amount demanded for the first phase of Harar town water supply doesn’t bring about
any significant effects to Sabiyan well field. Phase two water demand of Harar town is
estimated to be about 500lit/sec, which is equal to 43200m 3/day. Pumping such amount
of water from Dire Jara well field can causes a total decline of current water level of the
field by about 12m. From this it is possible to say that, second phase water demand of
Harar town can affect the water level of Sabiyan well field, and hence not
recommended.
The second scenario simulates a case of decreased recharge to aquifers by 50% that
may results from lower than normal precipitation. It is real that changes in climate
conditions from time to time are affecting precipitation amount in the county adversely
and reducing recharge to groundwater, as the main source of recharge is precipitation.
The heads calculated for this scenario shows a maximum decline of the water level by
6m and a minimum of 0.45m. And decreases in recharge caused decreases of
groundwater discharge through the general head boundary.
cii
6.8. Model Limitation
A numerical model is useful for testing and refining a conceptual model of a groundwater
flow system, developing an understanding of the system, guiding data collection, and
projecting aquifer responses to change in aquifer stresses within specified limits.
However, a model can only approximate the actual system as it is based on simplified
assumptions. Thus the results of model simulation are as accurate as the measured or
estimated data used to construct the simulations. Therefore, it is essential that for this
groundwater model to be interpreted and used properly these limitations should be
understood. The limitations associated with numerical groundwater flow simulation of
the Dire Dawa area are: 
Study area groundwater flow simulation was based on various assumptions
regarding the real natural system. Some of these assumptions were that the system
was represented as a two layer aquifer and a confined unit, the upper layer is
considered unconfined and the lower aquifer, confined. The lower confined layer is
considered perfectly confined and vertical movement of groundwater from the upper
aquifer to the lower aquifer is assumed to be zero. This assumption and
approximation may lead to error where there is lack of understanding of a detailed
geology in most part of the area.

Highly fractured and tectonically active areas can have a widely variable hydraulic
conductivity, although groundwater level and flow in these areas may be simulated
indirectly, by increasing hydraulic conductivity values, the effects of these structures
on the aquifer system may not be appropriately addressed within the model.

Hydro-geologic parameter values used in the model were chosen with in the general
ranges of previously estimated values, and therefore, model accuracy is dependent,
in part, on the accuracy of that estimation. Uncertainty stem largely from the fact that
this estimated hydro-geologic parameters and observed water level data were
concentrated in the central part of the study area where the population is high.
ciii

Lateral discretization of the study area in to a rectangular grid of cells and vertical
discretization in to layers forced an averaging of hydraulic properties. Each cell
represents a homogeneous block or some volumetric average of the aquifer medium.
Discretization errors occurred because the permeable features of the study area
aquifers are joints, fractures, and dissolution features that might be considerably
smaller than the volume of a model due to the averaging of the hydraulic properties;
the model can not simulate local anomalies in potentiometric surface of the study
area aquifers. Aquifer thickness can also be changed at intervals smaller than the
current model resolution (400m x 400m), especially in the current study area, where
structures are extensive. This level of discretization used in the model was too
coarse to incorporate the effects at local scale, like the effect of numerous faults and
lineaments found in the study area and therefore, the model is not suitable for
analysis of site-specific issues. However, without more field data, finer discretization
was not justifiable.

No short or long-term monitored data, that can provide information about the way the
system responses to changes in stresses. Data that are collected during wide time
range was used for model construction with the assumption that no significant
change occurred. This also leads to model uncertainty.
In the numerical groundwater simulation of the study are, all limitations and uncertainties
involved were clearly stated so that the model can not be misused. Therefore, the model
outputs should be interpreted and applied by considering all these associated
limitations. Hence, the results of simulations considered under different scenarios reflect
the error or uncertainty in the model and the outputs are used as general guides that will
help to understand how the system will respond to new stress and should not be
considered as exact predictions. The fact that the fit between simulated and observed
hydraulic heads during calibration was not perfect might be due to errors and uncertainty
introduced in to the model because of these factors.
civ
CHAPTER - 7
Conclusions and Recommendations
7.1. Conclusions
Groundwater is the primary source of drinking water for population of the study area, for
industry, irrigation and other uses. Therefore, the potential of this precious resource
should be properly assessed and managed. The intensive utilization of groundwater
can lead to negative environmental impacts, such as land subsidence, water resource
mining etc. Thus, it is found vital to study and have the knowledge about the spatial and
temporal distribution, occurrence, and groundwater level for the basin, catchments or,
sub-catchments rather than site specific day to day investigation activity. To accomplish
this task groundwater modeling has wide application in developed countries and
recently got good attention in the developing country too.
Dire Dawa area groundwater flow system is found in the eastern part of the country at
lower part of Awash River basin. In this study a homogeneous, an isotropic and two layer aquifer with a confining unit between them are considered.
Aquasi-three dimensional numerical groundwater flow model under steady-state
condition was constructed as a tool to understand the aquifer system and to predict the
response of the system to future changes in stresses. In doing so, conceptual model
was developed based on the geology and hydrogeology of the area. Combination of noflow boundary and general head boundaries were used to best represent boundary
conditions. The numerical groundwater flow model was simulated using MODFLOW,
1996. The study area was represented by 8,011 cells oriented in south-north directions
and each cell with side of 400m x 400m.
cv
Quantification of the rate of natural groundwater recharge was a basic prerequisite for
efficient groundwater management, and is particularly vital in arid and semi-arid region,
like my study area, where such resources are often the key to economic development.
The main sources of recharge to aquifers of the study area are subsurface recharge
mainly from precipitation in the southern escarpment zone and direct recharge from
precipitation. The physical setup of the well fields surface catchments indicate that, the
east-west treading faults on the escarpment intercepts surface runoff and rainfall that
drains to the ground. However, the faults oriented in the north direction facilitate the
flow of groundwater from mountain towards the deep confined aquifer. These shows
that occurrence of groundwater in the lower aquifer of the well fields is due to the
hydraulic and hydro-geologic conditions of the escarpment area. The recharge used in
the model is adopted from the work of Eyilachew Yitayew (2004) and others.
Groundwater discharge from the two aquifer system of the study area is mostly through
withdrawal from wells and boreholes, through springs, evapotranspiration where
groundwater level is near to ground surface (not quantified) and through groundwater
underflow that occurs across the study area boundary at topographic low area.
Because the withdrawals record for the system is incomplete it was not possible to
determine the actual average annual withdrawal rate for each well. Instead an estimate
of the average annual withdrawal rate in each well was made from the available record,
under the assumption that the average of the known record would approximately equal
to the true record.
The model was calibration using contours constructed from heads measured in 64
water points. Recharge, hydraulic conductivity, and boundary conductance were varied
within plausible ranges during model calibration. The calibration was considered
sufficient when observed heads and simulated heads were with in the calibration criteria
set before calibration, which includes visual comparison of simulated heads to calculate
heads and fitting 80% simulated heads to calculated heads within a maximum
difference of 10m. Large differences between calibrated and observed heads at some
places was due to the degree of accuracy of model input parameters, overall limitations
cvi
of the model design like coarser grid size, or due to errors in observed heads. The
summary statistics after steady-state calibration for residual heads between observed
and simulated values were calculated for 64 measured water levels. The mean errors
(ME), the root square mean error (RMS), and mean absolute error (MAE) were
calculated for the residuals and found to be 0.007m, 6.7m, and 4.89m respectively.
Model simulated heads were found sensitive to hydraulic conductivity and recharge. In
general, if a model is more sensitive to one parameter than the others, the degree of
uncertainty of that parameter will have a greater effect on the model results than the
other parameters. So, care has been taken during the calibration of such parameter to
which the model was most sensitive.
The model was used to simulate the response of the aquifer to different scenarios,
which includes increased withdrawals and decreased recharges. The effects of these
scenarios were evaluated with respect to changes on groundwater heads and sub
surface outflows compared to the steady-state simulated values.
A numerical model is useful for testing and refining a conceptual model of a groundwater
flow system, developing an understanding of the system, guiding data collection, and
projecting aquifer responses to changes in aquifer stresses within specified limit.
However, a model can only approximate the actual system and is based on simplified
assumption and on averaged and estimated conditions. Thus, the results of the model
simulations are only as accurate as the measured and estimated data used to construct
the simulation. These limitations starting from aquifer system conceptualization which
was developed from limited data sources, assumption and generalizations of the field
conditions, estimation of aquifer parameters, identification of aquifer system boundaries
and others, affects the accuracy of the model results even if some parameters were
adjusted during calibration.
7.2. Recommendations
cvii
The Dire Dawa area groundwater flow modeling is constructed under many assumption
and simplifications. Therefore, this model could be improved with additional detailed
hydrologic and geologic knowledge of the area. From the situation encountered and the
result of this very simplified model, the following points are recommended.
 Groundwater is among the nations most precious natural resources. Measurements
of the water level in wells provide the most fundamental indicator of the status of this
resource and are critical to meaningful evaluations of the quantity and quality of
groundwater. Therefore, groundwater level monitoring wells should be placed in the
study area. This also helps to carry out transient groundwater flow modeling so that
the system responses to induced stresses can be predicated with greater confidence.
Water level measurements from observation wells are also the principal sources of
information about the hydrologic stresses acting on aquifers and how these stresses
affect groundwater recharges, storages, and discharges.
 The sensitivity analysis has shown that the model is more sensitive to recharge and
hydraulic conductivity. Therefore, environmental protection activities, and artificial
recharges, which enhance groundwater recharge, should be done.
 The existence of highly fractured and recent tectonic areas can have widely varied
hydraulic properties in short distance. As a result, there is a mixture of over and under
simulated water levels that are often adjacent to each other. Moreover, hydro
geologic data, including measuring water levels, are sparse in many areas, making
interpretations using the model results difficult. Therefore, a detailed investigation of
geology, hydrogeology and structures of the area should be carried out.
 The distribution and rate of recharge and the effect of human activity on recharge
area is poorly understood. Therefore, detailed recharge estimation has to be carried
cviii
out and the action of protecting recharge area from probable causes of pollution
should be carried out.
 Pumping water from Direjara well field to the satisfaction of Harar town water supply
phase I demand doesn’t bring about any effects on water level of the area. But
pumping to the satisfaction of phase II demand can cause high water level decline
and hence they should search for another alternative.
 Advective transport model for the study area is recommended to follow the possible
groundwater pollution of the area that might occur from different sources.
 Post audit for this flow model should be done after some years so as to check the
applicability of the model to the area.
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