RAINFALL-GROUNDWATER RECHARGE RELATIONSHIP
IN WADI MUHAYRIGAH BASIN, CENTRAL SAUDI ARABIA
Mohammed Abdullah Al-Saleh
Associate Prof. of Physical Geography,
Dept. of Geography, King Saud University,
P O Box 2456, Riyadh 11451, Saudi Arabia
Tel 01-467-5369
Fax 01-467-5366
‫ج‬
ABSTRACT:
The Arabian Shield contains only shallow alluvial aquifers along wadi systems
sometimes underlain by weathered bedrock. These aquifers are unconfined and of
limited thickness and areal extent. These aquifers are recharged by local rain water.
Water supplies from shallow aquifers are not of large volumes, however, this perennial
resource is locally important for domestic and agricultural uses.
In an attempt to derive a regression model for rough prediction of groundwater
recharge in Muhayrigah basin, the relationship between monthly water level increments
and one month lagged values of monthly rainfall has been assessed. The coefficient of
determination is relatively low (0.48), but the values of F-ratio and t-test for correlation
coefficient have been found to be significant at a better than 0.05 level. Therefore, the
derived regression model may be used for rough prediction of groundwater recharge in
wadi basins having physical conditions similar to the physical environment of Wadi
Muhayrigah basin.
KEY WORDS: recharge, groundwater, Arabian Shield, Saudi Arabia, drylands.
»
INTRODUCTION
Groundwater is the main natural water resource in Saudi Arabia; however, the geology
of the country exerts great control on it. The geological structure of Saudi Arabia
consists of two major geological parts. The western section is composed of ancient
igneous and metamorphic rocks and is known as the Arabian Shield. The eastern part is
sedimentary strata dipping east and northeast. The Arabian Shield contains only shallow
alluvial aquifers along wadi systems sometimes underlain by weathered bedrock. These
aquifers are generally unconfined and of limited thickness and areal extent. Groundwater
in these aquifers is renewable with average annual recharge about 900 million cubic
meters (Abu Rizaiza and Allam 1989).
Conversely, the most prolific aquifers occur within the sedimentary province. Nine of
the sedimentary strata serve as main aquifer, generally confined. Ministry of Agriculture
and Water (1984: p45) reported that "large volumes of water are stored in some
aquifers, but the average annual recharge to these aquifers is negligible. Because the
recharge is at best scant, virtually all groundwater used comes from aquifer storage
accumulated many thousands of years ago. Therefore, wherever,
groundwater
withdrawals are heavy water levels decline, Where aquifers are thin, use of the water
supply could be threatened, and where aquifers are thicker, pumping could become
uneconomical because of the increasing depth of the water level". This fossil
groundwater constitutes a very important resource in the country, but it is nonetheless
true that the supply from these aquifers is not infinite. Therefore, it has to be considered
in the same way as a mining resource. Although the development of this fossil
groundwater started only in the last few decades, the problem of the sharp decline of
groundwater levels has been confirmed (Pike 1983; Ministry Of Agriculture and Water
1984; Ministry of Planning 1990; Al-Saleh 1992).
In tune with natural water balance, shallow aquifers with renewable groundwater have
been developed by traditional methods to sustain irrigated agriculture in the country for
hundreds of years (Al-Turbak and Al-Muttair 1989). Water supplies from shallow
aquifers are not of large volumes, however, this perennial resource is locally important
for domestic and agricultural uses (Ministry Of Agriculture and Water 1984). Although,
recharge is frequently the most important factor in evaluating groundwater resources in
arid and semiarid areas (Wood and Sanford 1995), a review of literature indicates that
very little is known about groundwater recharge in Saudi Arabia especially the eastern
part of the Arabian Shield (Dincer 1974 1980; Abdulrazzak, et al. 1988; Basmaci and
Al-Kabir 1988; Basmaci and Hussein 1988; Al-Muttair and Al-Turbak 1989; Al-Turbak
and Al-Muttair 1989; Sorman and Abdulrazzak 1993; Abdulrazzak and Sorman 1994;
Alyamani and Hussein 1995).
STUDY OBJECTIVE
The estimation of groundwater recharge to a reasonable degree of accuracy is essential
and vitally needed for the proper assessment and management of groundwater resources.
However, this task tends to be difficult in most areas of the developing countries because
of the absence of measurements of
the influencing factors (climatological,
hydrogeological, and geomorphological variables). In areas lacking systematic data base
where groundwater recharge depends on local precipitation, the availability of relatively
long term data of rainfall and groundwater level data is thought to be valuable for
developing a model for approximate estimation. Therefore, the main objective of this
study is to examine the relationship between rainfall and rise in water table in Wadi
Muhayrigah basin in an attempt to derive a regression model for rough prediction of
groundwater recharge.
METHODOLOGY
In order to achieve the study objective, several steps were followed:
1) The uppermost well in Muhayrigah village was selected to monitor water table
response to rainfall. The well is a hand-dug (4x4m) and 21.70 meters deep, sunk into
alluvium (10 meters thick) and weathered granite. Water level measurements were
carried out on monthly basis for the period from Dec. 1984 to Oct. 1996. The readings
were taken at the middle of each month.
2) Unfortunately, the basin lacks rainfall station. Thus, rainfall data of a nearby station
(Al-Quwayiyah station Lat. 24 05N / Long. 45 14E) were obtained from the Ministry of
Agriculture and Water.
3) Simple linear regression was applied on water level rises as a dependent variable and
rainfall data as an independent variable.
STUDY AREA
Wadi Muhayrigah basin is a relatively small drainage basin and is located within the
Shield area of the Al-Quwayiyah region. It occupies the area between 23 55 and 23 58
latitude north and between 45 01 and 45 05 longitude east (Figure 1). Geologically, the
basin is composed of Precambrian crystalline rock, covered partially with unconsolidated
materials. It contains andesite in the upper reach and gneiss and granite in the middle
and lower reaches. The main wadi in the basin is about 8.5km long. It flows generally
eastwards, however, it changes its flow direction in the lower reach towards the north.
This wadi is one of the main areas in the region that have irrigated traditional palm
groves. Agriculture in this wadi depends on groundwater of the shallow alluvial aquifers
along the wadi system underlain by weathered bedrock. Thus, the wadi provides a
typical example of groundwater resources problems in the agricultural settlements in the
shield area of the region.
Figure1
RECHARGE TO SHALLOW AQUIFERS IN SAUDI ARABIA
Alluvium aquifers in the Arabian Shield are recharged by local rain water. However, it
is influenced by the meteorological, geomorphological, and hydrological characteristics
of the region, and therefore, recharge to groundwater through wadi channels is the most
important recharge mechanisms in Saudi Arabia (Abdulrazzak et al. 1988; Al-Saleh
1988; Basmaci and Hussein 1988).
There is very little rainfall in Central Saudi Arabia (with annual mean about 100mm).
Besides, the temporal distribution of rainfall in the country characterizes with a large
number of low observations and infrequent but very significant extreme events. In
addition, high-intensity is characteristic of rainfall in Saudi Arabia. Jones (1980),
reported that about 50% of rainfall in Saudi Arabia falls at intensities in excess of
20mm/hour, and 20-30% falls at intensities exceeding 40mm/hour. Alyamani and Sen
(1993: p.115) pointed out that "in most of the cases the rainfall occurs locally and
sometimes in the form of violent thunderstorm of short duration".
Since rainfall intensity tends to be high and infiltration in wadi channels tends to be
very rapid, runoff recharge is the most common type of recharge in Saudi Arabia
especially in mountainous areas. Although surface runoff may be considered as zero on
the regional level in desert areas, runoff does occur in most parts of Saudi Arabia with
the exception of sand dune areas. However, runoff in the country is usually local,
irregular, and relatively small reflecting rainfall characteristics (Ministry Of Agriculture
and Water 1984). Mountainous areas with a high percentage of bare rock and steep
slopes generate greater runoff than flatter lands.
The threshold value of rainfall producing runoff in desert areas is estimated to range
between 5mm and 20mm/hour (Llyod 1980). In a sprinkling experiment of 18mm/hour
carried out in a sandy area in the Negev, Israel, the runoff coefficient reached 39.5%
(Yair 1990). Therefore, most desert storms (although infrequent) would generate runoff
on the highly permeable alluvial deposits and consequently recharge to aquifers will
take a place.
Previous studies indicate that the quantities of groundwater in the shallow aquifers in
Saudi Arabia are highly dependent on local precipitation and discharge. Dincer (1980:
pp.23-24) stated that "it would be a misconception to think that these intense rains are
extremely rare phenomena- people living in arid countries are quite familiar with them.
The run-off produced by these rains results in impressive flood waves which are
absorbed by highly permeable wadi alluvia, or disappear in sink-holes in karst limestone
with negligible evaporation losses. Often a flood wave completely disappears before
reaching the terminal area of a wadi system consisting of a mud flat (sabkha, playa),
sand-dunes or the sea". Beaumont (1981: p.48) reported that water tables in these
aquifers "respond rapidly to local precipitation conditions". Kollmann (1984: p.230)
stated that "the variations in groundwater table were directly related to the floods which
are a significant factor in groundwater renewal, in that they enrich groundwater reserves
through streamwater loss". Ministry Of Agriculture and Water (1984: p.33) pointed out
that "floods or large flows occasionally occur in the kingdom but are usually local. After
the surface flow travels a few kilometers, it generally disappears into the dry wadi beds.
Al-Saleh (1988: p.216) stated that "groundwater level rises in the monitored wells
corresponded well with increased rainfall". It is apparent from the above statements that
groundwater recharge to shallow aquifers is related to rainwater. However, the
relationship is not quantitatively defined.
RESULTS AND DISCUSSION
In December 1984, the Al-Quwayiyah region received about 35mm of rainfall in two
successive days which resulted in runoff in most parts of the region, and consequently
water level rose sharply in the study area. Most farmers in the region guide flood water
to their farms which are surrounded by embankments. To avoid the influence of artificial
recharge, the upper most well (21.70 meters deep) in the drainage basin was chosen to
monitor water level in the basin.
The frequency and time interval of measurement are usually adjusted to the
circumstances. Because precipitation bearing depressions from both the Mediterranean
Sea and the Indian Ocean irregularly invade the study area, runoff may not occur at all
during some years. Therefore, defining the relationship between rainfall and rise in water
table in Wadi Muhayriqah basin requires measurements over a relatively long period of
time (more than ten years).
On the other hand, there is a delay between runoff event and the consequential rise in
the water table. This time lag varies from place to place depending mainly on the
thickness, permeability and porosity of the unsaturated zone. Fetter (1988: p.99) pointed
out that "the time lag may be only a few hours in the humid regions for very coarse soils
with water table close to the surface. In arid environments, with very infrequent
recharge and great depths to the water table, water may take years to pass through the
unsaturated zone". In the study area the unsaturated zone is not very thick and the
unconsolidated materials of the wadi bed tend to be coarse; thus the response of the
water table to recharge is expected to be fairly rapid.
Before the flood occurred in the 13th of Dec. 1984, the water table at the selected well
was nearly at the lowest level which was 21.15 meters below surface. Therefore, the
water level, following the flood event, was measured weekly in an attempt to determine
the maximum delay of groundwater response to rainfall. Based on these measurements
water levels were 19.90m, 18.45m and 18.55m below surface in the 20th of Dec. 1984,
27th of Dec. 1984 and 3rd of Jan. 1985 respectively. These values indicate that water
level started rising in the first week and continued to rise in the second week. However,
the reading of the third week showed slight decline in water table. This means that the
maximum time lag is less than three weeks. Therefore, one could consider, at least to a
minimal degree, that monthly water level observations are adequate to examine the
relationship between rainfall and rise in water table. Thus, water level measurements
were carried out on monthly basis. Table 1 shows monthly water level increments, the
corresponding rainfall data, and one-month lagged rainfall values. This table reveals that
monthly water level increments are not well matching with the corresponding rainfall
data nor with the one-month lagged rainfall values. For instance, the Al-Quwayiyah
station received zero rainfall in both April and May 1994 whereas water level rise in
May 1994 was three meters. However, one-month lagged values of monthly rainfall have
a relatively better matching than the corresponding monthly rainfall data.
Plotting time series data of observation wells ease visual interpretation and
consequently indicate the general trends of water table fluctuations (Mandel and Shiftan
1981). Therefore, the construction of water level hydrograph is often the preliminary
step for the primary evaluation of water level data. Thus, in this study, water level data
have been plotted against time (Figure 2). This figure demonstrates water level
fluctuation, and the rises in water level tend to occur in the rainy season in the study area
(rainfall is expected in the period extending from October to May). This confirms the
linkage between rainfall and groundwater recharge.
To examine the relationship between rainfall and groundwater recharge, monthly
water level increments have been plotted against one-month lagged values of monthly
rainfall (Figure 3). The scatter diagram reveals low relationship between the variables.
To quantitatively determine the strength of the relationship, regression analysis was
carried out. ‫ث‬
Table 1
Figure2
Figure3
Unfortunately, the closest rainfall station to the basin is Al-Quwayiyah station which is
about 25 km to the east. Thus, rainfall data of this station has been used to carry out the
regression analysis. Undoubtedly, this is a major limitation in this study.
The correlation coefficient (r) and F-ratio are 0.69 and 18.6 respectively. The
calculated t-value for the correlation coefficient is 4.214 which is better than 2.086 value
at the 0.05 significance level indicated by t-tables. In addition, F-ratio is higher than the
critical value which is 4.35 at 0.05 significance level. Thus, the regression model may be
used for rough prediction of groundwater recharge. The predicting equation can be
written in the following form:
R = 1.00463 + 0.0371453 P
t
t-1
where:
R = monthly water level rise in meters.
P = monthly rainfall in millimeters.
t = time in month.
The coefficient of determination (R square) of the above model is about 0.48.
Therefore, another attempt has been made to seek a better relationship between
groundwater recharge and rainfall using annual data. To carry out the statistical analysis
on annual basis, one would use the hydrological year. Therefore, a combination of data
from two successive years (the late three months of one year and the first five or six
months of the following year) has been used because recharge occurs only in the rainy
season. Thus, the data have been tabulated and treated according to this way (Table 2).
However, regression analysis of the annual data has resulted in a lower relationship with
a correlation coefficient of 0.62.
Certainly, use of rainfall data for the drainage basin rather than a remote station data
would improve the relationship between groundwater recharge and rainfall. This is
especially true for an arid region with highly spatialy variable rainfall.
SUMMARY AND CONCLUSION
The Wadi Muhayrigah basin provides a typical example of groundwater resources
problems in the agricultural settlements in the shield area of the Al-Quwayiyah region. It
is one of the main areas in the region that have irrigated traditional palm groves.
Agriculture in this wadi depends on groundwater of the shallow alluvial aquifer along
the wadi system underlaid by weathered bedrock. This aquifer is recharged by local rain
water and recharge through wadi channels is the most important recharge mechanism in
the basin.
To have an idea about groundwater recharge in this basin the relationship between
monthly water level increments and monthly rainfall values has been assessed. The
results reveal that water level rises in the basin are not highly correlated to rainfall
values of the Al-Quwayiyah station (some 25km east of the basin). The low coefficient
of determination (0.48) would ¼
Table 2
be attributed to the spatial variability of rainfall in Central Saudi Arabia. Although the
coefficient of determination is relatively low, the values of F-ratio and t-test for
correlation coefficient have been found to be significant at a better than 0.05 level.
Therefore, the derived regression model may be used for rough prediction of
groundwater recharge in wadi basins having physical conditions similar to the physical
environment of Wadi Muhayrigah basin.
ACKNOWLEDGMENTS
Thanks are due to the Ministry of Agriculture and Water for providing the raw data of
rainfall, to Dr. Nasser Al-Saaran for his constructive comments on the draft of this paper
and to Mr. Salah Turky for producing figures in this paper.
REFRENCES
Abdulrazzak, M.J., et al.: Estimation of natural groundwater recharge under Saudi
Arabian arid climatic conditions. In: Simmers, I. (ed.), Estimation of natural
groundwater recharge, pp. 125-138 (1988)
Abdulrazzak, M.J., and A.U. Sorman: Transmission losses from ephemeral stream in
arid region. Journal of Irrigation and Drainage Engineering 120, 669-675 (1994)
Abu Rizaiza, O. S., and M. N. Allam: Water requirements versus water availability in
Saudi Arabia. Journal of Water Resources Planning and Management 115, 64-74 (1989)
Al-Muttair, F.F., and A.S. Al-Turbak: Estimation of recharge from a reservoir using two
water budget models. Water Resources Bulletin 25, 727-732 (1989)
Al-Saleh, M. A.: The Application of the Systematic Mapping of Geomorphology for
Groundwater Assessment in Wadi Al-Khanagah, Central Saudi Arabia. Ph.D. Thesis,
University of Southampton, Southampton, England (1988)
Al-Saleh, M. A.: Declining groundwater level of the Minjur aquifer, Tebrak area, Saudi
Arabia. The Geographical Journal 158, 215-222 (1992)
Al-Turbak, A.S., and F.F. Al-Muttair: Effect of precipitation variability on recharge in
unconfined aquifers. International Seminar on Climatic Fluctuations and Water
Management, Cairo, Egypt, 11-14 December 1989.
Alyamani, M.S., and M.T. Hussein: Hydrochemical study of groundwater in recharge
area, Wadi Fatimah basin, Saudi Arabia. GeoJournal 37, 81-89 (1995)
Alyamani, M.S., and Z. Sen: Regional variations of monthly rainfall amounts in the
Kingdom of Saudi Arabia. Journal of King Abdulaziz University- Earth Sciences 6, 113133 (1993)
Basmaci, Y., and M. Al-Kabir: Recharge characteristics of aquifers of Jeddah-Makkah
Taif region. In: Simmers, I. (ed.), Estimation of natural groundwater recharge, pp. 367375 (1988)
Basmaci, Y., and J.A.A. Hussein: Groundwater recharge over Western Saudi Arabia, In:
Simmers, I. (ed.), Estimation of natural groundwater recharge, pp. 395-403 (1988)
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and H. Bowen-Jones (eds.), Change and development in the Middle East, pp. 41-72.
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hydrology-investigations with isotope techniques, IAEA, Vienna, pp. 23-30 (1980)
¿
Dincer,T., et al: Study of the infiltration and recharge through the sand dunes in arid
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(1988)
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Zotl (eds.), Quaternary Period in Saudi Arabia, Volume 2, pp. 226-245. Springer-Verlag,
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July 1980.
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‫ة‬
Table 1 Monthly water level rises and monthly rainfall
___________________________________________________________________
monthly water corresponding monthly one-month lagged of
level rises (m)
rainfall (mm)
monthly rainfall (mm)
___________________________________________________________________
2.7
6.2
38.1
0.3
0
6.2
1.1
12.5
35.5
0.3
11.2
12.5
0.35
78.6
11.2
4.05
225.2
78.6
7.7
0
225.2
0.35
0
18.1
0.3
28.7
20.8
3.45
0
28.7
3.95
7.3
65.3
0.45
66.2
20.4
1.2
0
66.2
0.8
12.4
13.6
1.25
0
12.4
0.25
0
0
0.85
0
0
0.7
0
0
1.35
0
3.8
0.2
3.9
1.8
5.3
2.5
3.9
2.55
7.7
6.5
2.35
6.1
7.7
0.75
0
6.1
3.00
0
0
10.7
5.3
91.3
0.6
0
5.3
Table 2 Annual rainfall and water level rises.
_____________________________________________
Years
Rainfall (mm) water rises (m)
_____________________________________________
84/85
105.5
3.0
85/86
365.7
13.5
86/87
77.3
4.1
87/88
72.6
3.95
88/89
90.1
1.65
89/90
83.0
2.05
90/91
4.7
0.25
91/92
14.3
2.9
92/93
31.4
11.15
93/94
1.4
3.0
94/95
104.6
11.3
º
Editor of GeoJournal,
PO Box 17,
3300 AA Dordrecht,
The Netherlands
Dear Professor Herman van der Wusten,
Enclosed please find my research paper entiteled "RAINFALL-GROUNDWATER RECHARGE
RELATIONSHIP IN WADI MUHAYRIGAH BASIN, CENTRAL SAUDI ARABIA". I hope you
find it acceptable to be published by your highly respected and widely known journal.
Thank you very much for your cooperation.
Yours sincerely
Dr. Mohammed Abdullah Al-Saleh
Department of Geography,
College of Arts,
King Saud University,
P O Box 2456,
Riyadh 11451,
Saudi Arabia
Tel 01-467-5369
Fax 01-467-5366