Chapter 1
INTRODUCTION
______________________________________________________________________________
1.1
Background
According to the committee of the US National Research Council defined hydrologic
Science to include (1) the physical and chemical processes in the cycling of continental water at
all scales as well as those biological processes that significantly interact with the hydrologic
cycle, (2) the spatial and temporal characteristics of the global water balance in all compartments
of the Earth system. However, this definition other scholars argue that it is not focused enough
to give hydrology the unique identity that can distinguish it from other water related sciences.
Dooge J.C.I. states that “The business of hydrology is to solve the water balance equation”, and
quantitatively, the hydrological perspective is reflected in the water balance equation. Therefore,
hydrology can be defined as the science that seeks to explain the water balance dynamics for any
defined spatial (from a point to global) and temporal scale (from seconds to years) and their
relationships with the physical and chemical transport of matter through the hydrologic cycle and
with ecology (IAHS News letter, 1991).
Since the scientific study of the components of the hydrologic cycle are being carried out by the
other water related sciences it is left to the hydrologist to integrate the findings of the other
sciences to explain the dynamics of the water balance of an area over any defined time period
and establish their relationships to the physical and biological environments. According to
Williams (Engineering Hydraulics, ed. By H. Rouse, 1950, pp. 229 “Hydrology is peculiar
among the natural sciences in its dependence upon the findings of other allied sciences. These
sciences are meteorology, climatology, physical geography, agronomy and geology and soil
science, hydraulics, oceanography and limnology.
knowledge of them all”
The hydrologist must have a working
Therefore it can be seen that the classification of hydrological studies according to climatic
regions, tropical, temperate, polar, arid, humid), surface characteristics (urban, farmland, forest,
lakes) and geology (Karst) follows naturally from the definition as they all influence the water
balance characteristics of a defined area. The unnatural classification of hydrological studies
1-1
according to observation techniques (isotope hydrology, satellite hydrology), phenomenon (flood
hydrology, drought hydrology) methodology (Stochastic hydrology, deterministic hydrology)
and the major domain in which water moves (surface water hydrology, groundwater hydrology)
can also clearly be seen.
1.2
The development of hydrology
Hydrology is one of the newest of the natural sciences although history dates back as early as
1000 B.C. when philosphers like Homer, Plato and Aristotle, speculated on the concept of the
hydrological cycle. Many of these philosophical concepts proved erroneous. The forerunner of
the modern concept of hydrological cycle was Marcus Vitruvious who was contemporary to
Christ and who stated that, the groundwater is far the most part derived from rain and snow by
infiltration through the surface.
In a broader sense, the various periods of development of hydrology according to Chow (1964)
may be classified as follows:
(a) The period of speculation (Ancient) before 1400 A.D.
(b) Period of observation
1400- 1600
(c) Period of measurement 1600-1700
(d) Period of experimentation 1700 - 1800
(e) Period of mordenization 1800 - 1900
(f) Period of empiricism
1900 - 1930
(g) Period of rationalization 1930 - 1950
(h) Period of theorization
1950 - to date
(i) Period of computerization 1970 - to date
Prominent contributors during the 15th Century include Leonard Da Vinci and Palissy; in the
17th Century, Perrault, Mariotte and Halley whose concepts on flow and flow measurement are
valid to this age. Bernoulli, Pitot and Chezy in the 18th Century made new discoveries in the
understanding of hydraulic principles which greatly accelerated the beginning of hydrological
studies on a quantitative basis.
In the field of groundwater, Poisuille (1856), Dupuit (1863) and Thiem (1906) for the first time
applied the knowledge of geology to the solution of hydrological problems. In the field of
surface water, outstanding contributions were for Humphrey and Abbot (1855), Carter (1869),
and Manning (1889) whose formulae helped in the systematic stream gauging. During the
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1930's, the development of quantitative hydrology was substantial but still immature as it was
largely empirical with the physical basis for determination of most hydrological quantities
having not yet been established. As a result, solutions of practical hydrological problems were
soon found to be unsatisfactory and many governmental agencies and technical societies
increased their effort towards a systematic advancement of the science in hydrology. During the
period of rationalization, Sherman (1932) made a distinct advance by demonstrating the use of
unit hydrograph in the determination of the runoff hydrograph, by translating the rainfall excess
into surface runoff. Horton in 1933 was the first to recognize the ability of the drainage basin to
absorb and detain water and proposed the theory of infiltration capacity. The non-equilibrium
theory introduced by Theis (1939) revolutionalized the whole concept of hydraulics of wells.
With the classical theory of Einstein (1950), bed load formulation for sediment production, the
physics of erosion and transport of solids by fluids were fairly well understood. His works on
suspended solids (1972) and high sediment rate in alluvial rivers (1972) further gave insight to
the role of precipitation falling on solid ground, partly evaporating, partly infiltrating and partly
appearing as surface runoff and it is this last part of the precipitation which contributes mostly to
river flow carrying suspended sediment with it. While discussing the river ecology and man
(1972), he considered sediments of sizes all the way from boulders, down to the most minute
clay particles including gels. According to Einstein, it is imperative that every student of river
ecology familiarise himself thoroughly with the rules and laws of river hydraulics of which
motion of its sediment (be it a river, a lake or an ocean) is an important factor.
Among the contributions made in the recent past, mention should be made of the pioneering
work of Linsley (1949) on the application of principles of hydrology to small and large
watersheds, using the concept of unit hydrograph and that of Horton’s infiltration theory.
Together with Crawford (1966), he developed the classical ‘Stanford Watershed Model Mark
IV’ with as many as 39 hydrological variables, many of which could be used for a rigorous
analysis of a conceptual model. His method of multiple regression analysis by way of co-axial
graphical correlation technique is widely used to correlate annual or seasonal precipitation with
the physiographic parameters of the drainage basin.
Corey and Corey (1965) studied the non-steady drainage of partially saturated soils. They
introduced both theoretical and experimental works on unsteady drainage of similar and
dissimilar media making use of a complicated partial differential equation of the second order,
with moisture-dependent diffusivity and permeability coefficients. The capillary force and
effective permeability at various points would depend upon whether we work on drainage or
1-3
imbibition cycle. The process of drainage and imbibition constitute a hysteresis loop and
involves pressure reversals. The study has direct bearing on the infiltration process in hydrology
and explains the retention and release of moisture during the drainage cycle.
To determine the effect of man-made changes on the drainage basins the use of synthesized
hydrographs are in vogue which facilitates the estimation of streamflow records, calculation of
runoff from ungauged stations and the calculation of extreme flood discharges from either
measured of hypothetical precipitation storms. Donald (1968) presented a greatly simplified
empirical method which proves valuable as an interim procedure for hydrologic analysis. The
important assumption of this method is that the discharge hydrograph comprises of two major
components, that is formed by storm runoff and the other formed by groundwater storage,
usually called the base flow. The concept of a 24-hour unit hydrograph is introduced and the
daily runoff values are synthesized using precipitation excess on a daily basis. As base flow
recession curves may change with seasons the depletion curves may be varied in nature.
Fleming (1974, 1975) discussed an application of systems analysis to water resources problems,
citing the advantages of computer simulation and systems analysis approach in accounting for
the interacting processes affecting hydrologic response of physical watershed and the integration
of this response to assess and evaluate the water resource.
Yevjevich (1972) introduced the application of probability theory and mathematical Statistics to
hydrology. He stresses that since most hydrological processes in nature are governed by the laws
of chance, the use of probability theory and mathematical statistics is unavoidable in the
extraction of information from hydrologic data..
The computer era (from 1950's to date) has revolutionalized the study of hydrology and the
solving of hydrology and water related problems. The computer erra has also enabled modelers
to develop many different kinds of models and information management systems and Decision
support systems (DSS), in order to provide solutions to water related problems. The application
of GIS coupled with hydrological models in hydrology has also helped in the advancement of the
science of hydrology.
One of the major factor of an information management system is the ability to store, analyse and
retrieve data. A GIS is a computerized mapping tool that provides flexibility, accuracy and ease
of updating capabilities over conventional methods. GIS provides a cataloging of data found in
tabular format. The data called attributes, describe the mapped information (Reese and others
1-4
1993). A number of GIS packages are available such as, IDRIS, ARC/INFO, ARC/VIEW, etc.
Geographical Information Systems (GIS), identified by many scientists and managers as a
powerful decision support tool.
Important characteristics of a Decision Support System (DSS) for sustainable water resources
management include flexibility for tackling various 'what-if?' scenarios, the facilitation of
problem identification and solving by analytical tools enabling the end-user to manage, analyse
and present information, and interaction and ease of use to involve the stakeholders into the
management process themselves [Simonovic and Bender 1996]. Integrated water resources
management comprises numerous complex and unstructured management problems including a
geographical component. Resolving ill-structured problems, however, can be achieved by
desegregating them into a series of structured components, each of which tackled with its unique
set of tools [Reitsma 1996], and integrated into a comprehensive framework system. That means,
Geographical Information Systems (GIS), identified by many scientists and managers as a
powerful decision support tool, or physical process models alone do not constitute a decision
support system [Lam 1997]. Using GIS and traditional DSS in an integrated way leads to the
extended concept of SDSS, incorporating the capabilities of both GIS, which accounts for the
spatial dimension of water resources management and is strong in visualisation, and DSS, which
brings user assistance, models, a database, scenario-building and a generic framework into the
system.
According to Wurbs (1998), the concept of Decision Support System became popular during the
1980s in the water management community, as well as in business, engineering and other
professional fields. Wurbs, (1998) defines decision support system as a user-oriented computer
system which supports decision-makers in addressing unstructured problems. The general
concept of a DSS emphasizes:
(a)
(b)
(c)
(d)
solving unstructured problems which require combining the judgement of manager-level
decision-makers with quantitative information.
supporting capabilities to answer “what if” questions quickly and conveniently by
making multiple runs of one or more models
using enhanced user-machine interfaces and
outputting graphically
Decision support systems include a collection of software packages and hardware. The decision
support system might include data management software, watershed runoff, stream hydraulics,
1-5
and reservoir system operation models, a computer platform with peripheral hardware devices
and an automated real time stream flow and rainfall data collection system. Therefore,
hydrological models are often used as components of a decision support .
1.3
The Hydrological cycle
The natural circulation of water near the surface of the earth is illustrated in Figure 1.1. The
driving force of the circulation is derived from the radiant energy received from the sun. The
bulk of the earth' s water is stored on the surface in the oceans. Therefore, it is logical to
consider the hydrologic cycle as beginning with the direct effect of the sun's radiation on the
oceans. The heating of the ocean's surface causes evaporation, the transfer of water from the
liquid to the gaseous states to form part of the atmosphere. Through a combination of favourable
meteorological conditions, the water vapour changes back to the liquid state through
condensation and with favourable atmospheric conditions precipitation (rain, snow etc) is
produced. Precipitation may either return directly to the oceans via the land surface or may be
intercepted by vegetation and the intercepted water may return to the atmosphere by
evaporation. Rainfall reaching the ground may collect to form surface runoff, part of it may
infiltrate into the ground.
Figure 1.1
The Hydrologic Cycle
1-6
The water in the soil then percolates through the unsaturated layers to reach the water table or it
is taken up by vegetation and transpired back into the atmosphere. The land phases of the
hydrologic cycle have an enhanced importance in nature since evaporation is a purifying process:
the salt sea water is transformed into fresh precipitation and therefore, water sources and
storage on the continents consist largely of fresh water except groundwater storage which may
contain dissolved salts. The above statement is sometimes not true especially in industrialized
countries where they are experiencing acid rainfall problems.
1.3.1 Components of the hydrologic cycle
From the introduction one can tell that the hydrological cycle has many components. The
components of the hydrologic cycle are: evaporation and transpiration, precipitation, surface
runoff, infiltration, sub-surface flow, and interception.
1.3.1.1 Evaporation and transpiration
It has been stated earlier that the hydrologic cycle begins with the solar energy. The solar energy
is the principle cause of evaporation and transpiration. With the latent heat of vaporization water
evaporates from water bodies and from any wet surface. Evaporation is influenced by
meteorological variables such as solar energy, air temperature, wind speed, air humidity etc.
Tanspiration is that water vapour transpired by plants during the process of photosynthesis.
Transpiration is influenced by climatic or meteorological factors and is a function of tree species
and soil properties. Evaporation and transpiration is the source of atmospheric water vapour
which is held up in the atmospheric is the form of clouds. The clouds give rise to precipitation
under favourable meteorological conditions.
1.3.1.2 Precipitation
Water is considered as a renewable resource due to the hydrologic cycle. Precipitation which is
a result of precipitating clouds replenishes water in rivers, lakes, soil; groundwater, and in polar
ice regions. Forms of precipitation are: snow, rainfall, hail, sleet, fog etc. The occurrence of
precipitation is influenced by meteorological or climatic factors/variables.
Globally,
precipitation is unevenly distributed in space and time. Details or precipitation analysis are
presented in section 3.6.
1-7
1.3.1.3 Interception
Interception is that part of rainfall and or precipitation that does not reach the earth’s surface. It
is that part of precipitation that get intercepted by trees and any other surfaces before reaching
the ground. In dense forests, a light rain may not reach the ground at all. Therefore, interception
is a function of tree species and their density.
1.3.1.4 Surface Runoff
Surface runoff is that part of rainfall which runs off the earth’s surface into rills then into small
streams and then into large rivers. Surface runoff only occurs when the soil is saturated with
water and the rainfall intensity is higher than the infiltration rate. Surface runoff is influence by
climatic factors and the physical characteristics of the drainage basin. Details of these factors are
presented in section 6.1.
1.3.1.5 Infiltration
Infiltration is defined as the process of water passing from the water air phase in the soil phase.
Infiltration has a direct influence on runoff. Greater infiltration capacity results in smaller surface
runoff, and hence, smaller peak discharges. Infiltration play an important role in the
replenishment of soil moisture and groundwater. Replenishment of soil moisture is important for
plant growth and groundwater recharge. Infiltration is influenced by soil type, land cover and the
initial soil moisture content.
1.3.1.6 Groundwater flow
As explained earlier, infiltration is that process of water passing from the air phase into the soil
phase. The movement of water in the ground strata after the infiltration process is what is called
sub-surface flow. Therefore, during a rain storm, the infiltration water might travel by subsurface flow and contribute to the flow in the river, or it might flow downward to the
groundwater table. Sub-surface flow is very slow in comparison to the surface flow. Therefore,
infiltrating rainfall may take days and even months to appear in the river and thus some rivers
continue to flow even after several months of no rainfall due to groundwater contribution from
the catchment or drainage basin.
1-8
Groundwater is a source of water for different uses such as irrigation, industrial, domestic water
supply for urban and rural areas etc. Groundwater therefore, has a major role to play in the social
economy of a country. An important advantage of groundwater over surface water as a source of
domestic water supply is that it is free from bacteria pollution. This is because, water which
percolates through fine-grained material is usually cleared of bacteria in short distances.
However, polluted water may enter the groundwater around the top of the well casing. Therefore,
sanitation precautions are necessary in order to avoid contamination of groundwater.
1.4
Water balance
According to Dominquez (1997), water balance is an important tool for the design and operation
of various hydraulic structures. It is also important in terms of estimating water availability, both
the present and the future in different regions. The continuity or the mass balance equation is
used to determine the water balance in a region within a specified time interval.The water
balance equation widely used in the field of hydrology is of the form
I(t) –O(t) = S / t)
(1.1)
Where I is the inflow, O is the outflow S/t is the rate of change in storage over a finite time
step of the considered control volume in the system. The equation holds for a specific period of
time and may be applied to any given system provided that the boundaries are well defined.
Other names for the water balance equation are Storage Equation, Continuity Equation and Law
of Conservation of Mass. Examples of water balances are the water balance of the earth surface
or water balance of a drainage basin.
Water balance of the earth surface
The water balance of the earth surface is composed of all the different water balances that can be
distinguished. It is a balance of which all components equal zero. The total inflow is zero,the
total outflow is zero and the change in storage is zero. Figure 1.2 shows the water storage of the
globe. The Figure shows that the oceans receive 79% of the mean annual global precipitation,
and contribute 86% of the global evaporation. The continents receive 21% of the mean global
annual precipitation and contribute 14% of the global evaporation. Also, 7% of the water vapour
in the atmosphere is transported to the continents while 7% of the annual precipitation is returned
to the oceans as runoff from the continents. The estimates of different water balances on the
earth are given on Table 1.1.
1-9
(Principal water fluxes in the world water balance in 1012m3 per year)
Figure 1.2
Quantitative illustration of the water storage of the globe
Table 1.1
Estimates of the different water balances of the earth according to the
international geophysical year(Holy, 1982)
Water Occurrence
Volume
103Gm3
% water
World oceans
1300000
97
Salt lakes/seas
100
0.008
28500
2.14
77.6
Atmospheric water
12
0.001
0.035
Water in organisms
1
0.000
0.003
123
0.009
0.335
Water courses
1
0.000
0.003
Unsaturated zone
65
0.005
0.18
Saturated zone
800
0.60
21.8
36700
1337000
2.77
100
100
Polar ice
Fresh lakes
Total fresh water
Total water
Amount of water
% of fresh water
1-10
Water balance of a drainage basin
The water balance is often applied to a river basin. A river basin (also known as watershed,
catchment or drainage basin) is the area contributing to the discharge at a particular rivercrosssection. The size of the catchment increases is the point selected as outlet moves downstream. If
no water moves across the catchment boundary, the input equals the precipitation P while the
output comprises the evaporation E and the river discharge Q at the outlet of the catchment. The
water balance equation for tha catchment may written as
(P-E)A –Q = S/t
(1.2)
where S is change of storage over the time step/t, and A is the surface area of the catchment
upstream of the where Q has been measured. Figure 1.3 shows the components of the water
balance on a small catchment respectively. The author has used different notation to conform to
the figure given.
Figure 1.3
Components of the water balance on a hillslope or a small catchment.
1-11
(P = precipitation; I = interception; AET = actual evapotranspiration; OF = overland flow; SM
= change in soil moisture; GWS = change in groundwater storage; and GWR = groundwater
runoff).
Water balance of a lysimeter
A lysimeter is a small area or container with or without vegetation for which the terms of the
water balance can be accurately determined. Some lysimeters are weighable which allows a
precise monitoring of the rate of change of storage. The table above contains water balances of
four lysimeters in the dunes near Castricum, The Netherlands, which are averaged over a period
of 25 years (1957 - 1981). The lysimeters have a surface area of 25x25 m2, a depth of 2.25 m and
are covered with different types of vegetation. For the average annual water balance the change
in storage in the lysimeter can be neglected. The 25 year average water balance for the summer
month June shows that the amount of water in the lysimeter decreases, while in the autumn
month of October replenishment of the soil moisture deficit takes place (S/t > 0).
Table 1.2
Water balancess (25 year average) for lysimeters near Castricum, The
Netherlands
Type of land use
P
ANNUAL
WATER
BALANCE
Bare
Shrubs
Deciduous trees
Coniferous trees
841
841
841
841
WATER
BALANCE
MONTH
JUNE
Bare
Shrubs
Deciduous trees
Coniferous trees
46.0
46.0
46.0
46.0
-
E
201
480
522
691
33.4
66.8
76.6
67.0
-
D
640
361
319
150
25.9
6.8
8.9
1.7
=
S/t
0
0
0
0
-13.3
-27.6
-39.5
-22.7
WATER
Bare
91.4
10.9
67.5
13.0
BALANCE
Shrubs
91.4
27.4
23.7
40.3
MONTH
Deciduous trees
91.4
27.4
10.5
53.5
OCTOBER
Coniferous trees
91.4
56.3
6.5
28.6
( P is precipitation, E is evaporation, D is drainage outflow at the bottom and S/t is the change in storage. The
values are presented in mm/year and mm/month)
The mean annual precipitation for the entire world is about 86cm per year and the mean annual
evaporation amounts to about 86cm per year. The amounts of precipitation, runoff
evaporation and other hydrologic quantities are not evenly distributed on earth either
geographically or in time. Figure 1.5 shows the hydrological cycle and water storage of the
globe. It can be seen from figure 1.5 that the atmosphere contains only 0.035% of all fresh
1-12
water. Ice sheets and glaciers contain 75% of all fresh water. Groundwater reservoir storage
amounts to 25% while lakes, rivers and soil moisture storage amounts to 0.39%. The oceans
stores 97% of all global waters. The oceans also receive 77% of the mean annual global
precipitation, and contribute 84% of the global evaporation. The continents receive 23% of the
mean global annual precipitation and contribute 16% of the global evaporation
1.5
Hydrological data
Data collection is the sole responsibility of each nation. Usually data collection in hydrology is
done by the water department in collaboration with the departments of meteorology,
hydrogeology etc. Other information relevant for the execution of water resources projects are
collected by all relevant sectors/ministries engaged in the use of water and land resources. The
accuracy of hydro-meteorological data depends on the density of instrumentation and thus the
willingness of governments to invest in data collection.
Other institutions involved in data collection and analysis and/or in support of governments are:
the World Meteorological Organization (WMO), WHO and UNESCO. The above organizations
provide funds for the purchase of instruments and for training.
The data that is required for the planning, development, operation and management of water
resource schemes are physical and social economic data. It is the physical part of the information
requirement that is presented here in.
1.5.1 Hydrologic data
The following hydrologic information is required for planning, management and operation of
water resources projects in a river basin: Streamflow (water levels, discharge), sediment
discharge, precipitation etc.
1.5.2 Hydrogeologic data
The required hydrogeologic information is: the extent, thickness, capacity, hydraulic
characteristics, dependable yield of the aquifer, number of springs etc.
1-13
1.5.3 Meteorologic data
Meteorologic data is required on variables such as: temperature, wind speed and direction,
evaporation, humidity, air pressure, solar radiation etc. This information is very important in the
estimation of water losses from reservoirs and consumptive use of water.
1.5.4 Water quality data
The quality of water is usually defined in terms of the water's suitability for irrigation, domestic,
industrial, recreation etc. Therefore, data on the following water quality parameters is required
and these are: dissolved oxygen (D.O.), biochemical oxygen demand (B.O.D.), coliform,
temperature, heavy minerals, suspended solids (turbidity), colour, odour, radio activity, etc.
1.6
The role of hydrology in development
Hydrology is used in Engineering mainly in connection with the design and operation of
hydraulic structures. Hydrology is applied to answer questions/problems such as: Is the flow of
this stream sufficient to meet the needs of (a) a city or industry seeking water supply, (b) an
irrigation project, (c) recreation? Would a storage reservoir be required in connection with any of
the proposed uses, and if so, what should be its capacity? In the design of a flood protection
system, a bridge, a culvert or a spillway for a dam, what is the maximum flood that may be
expected to occur with any specified frequency? There are competing and conflicting uses of
water. In the adjustment of the conflicts in water uses and for the proper solution of the many
problems arising in connection with them a complete data on water resources and a full
understanding of the principle of hydrology become a vital necessity.
Since hydrology is a science that deals with the development and control of water resources, it
has its important influence in agriculture, forestry, geography, water management, political
science, economics and sociology; and it has practical applications in structural design, water
supply, wastewater disposal and treatment, irrigation, drainage, hydro power, flood control,
erosion and sediment control, salinity control, pollution abatement, recreational use of water, fish
and wildlife preservation, insect control and coastal works.
In irrigated agriculture, hydrology is used in determining the crop water requirement, estimating
the available water resources etc. Hydrology is used in the planning, development, operation of
1-14
water supply schemes and management of water resources. It is also used in the control of
hazardous events such as floods and droughts. Hydrology is used in the design of almost all
hydraulic structures. In summary, hydrology is used in almost all sectors of the human endeavor
and therefore, is important for the economic development of all nations. Figure 1.4 shows the
interaction between the natural and social hydrological cycles.
Figure 1.4
Combined natural and social hydrological cycles
What can be seen from Figure 1.4 is the interactions of the atmosphere, surface water, ground
water, seas and oceans and the society. The evaporated water from the oceans and seas, surface
water and groundwater and the evapotranspiration from the biosphere is the source of moisture in
the atmosphere which gives rise to precipitation. The precipitation water ends up in surface
water, groundwater, in the oceans and seas and that part which is intercepted goes back to
atmosphere through the evaporation process. Figure 1.4 shows the interactions of our
environment (atmosphere, hydrosphere, lithosphere and biosphere) with society as part of the
environment. Society which is part of the environment plays a major role in the pollution of the
environment through agricultural activities, effluents from industries, effluents from domestic
1-15
water uses etc.
Air, land and water are the three fragile components of the Spaceship Earth. These three
components are highly integrative resources and therefore, must be properly managed in order to
ensure adequate public health, food supplies and transportation. The quality of life is directly
dependent on how well these resources are planned and managed for sustainable development.
Reference
1.
Corey, G.L., Corey, A.T. and Brookes R.H. 1965. Similitude for non-steady drainage
of partial saturated soils. Hydrology paper of Colorado State University, U.S.A. pp. 1-38
2.
Donominquez, R. 1997. Water Balance in Hydrological Basins. Water International,
Volume 22, No.3, pp168.
3.
Einstein, H.A. 1972. Sedimentation (suspended solids). River Ecology and Man.
Academic Press Inc. Nwe York, pp. 309-318.
Fleming, G. and Rowntree, K. 1975. Mannual on application of systems analysis to
problems of irrigation, drainage and flood control. Report of the I.C.I.D. pp. 7-74
Linsley, R.K., Kohler, M.A. and Paulhus, J.L. 1975. Hydrology for Engineers.
McGraw-Hill book company, New York.
Shaw, E.M. 1983. Hydrology in Practice. Van Nostrand Reinhold (U.K.)
Ven Te Chow 1959. Handbook of Applied Hydrology. Mcgraw-Hill book company,
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6.
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New York. Sections 1-14.
Walling, D.E. 1974. Suspended sediment and solute yields from small catchments prior
to urbanization. Fluvial process in instrumental watersheds. Publication No. 6. The
Institute of British Geographers. Pp.169-192.
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