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Methods of estimating groundwater Recharge
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International Journal of Engineering Associates (ISSN: 2320-0804) # 6 / Volume 5 Issue 2
Methods of Estimating Ground water Recharge
Saiful Islam #1, Ram Karan Singh*2, Roohul Abad Khan#3
#1,3
Lecturer,Department of Civil Engineering,King Khalid University,Abha,KSA
1
saful.islam.iitr@gmail.com
*
Associate Professor, Department of Civil Engineering,King Khalid University,Abha,KSA
Abstract— Estimates of groundwater recharge constitute
fundamental input for most approaches used to evaluate and
manage groundwater resources. Most approaches for
quantifying groundwater recharge measure recharge directly or
indirectly over a limited area (point or small-basin scale) and for
short periods of time. Estimation of recharge, by any method is
normally subject to large uncertainties and errors. In this paper,
various methods of estimating ground water recharge are
outlined and critically reviewed with regard to their limitations.
Keywords— Water balance,water table, hydrological cycle,Water
budget method.
I. INTRODUCTION
Ground-water recharge is a fundamental component in the
water balance of any watershed. However, because it is nearly
impossible to measure directly, numerous methods, ranging
widely in complexity and cost, have been used to estimate
recharge (Lerner and others, 1990; Scanlon and others, 2002).
Practicing hydrologists typically make the best estimates of
recharge possible by the use of methods that are relatively
straightforward in their application and require only
commonly available hydrologic data. In the humid, eastern
United States, where most streams are gaining and the water
table is relatively shallow, recharge typically is estimated by
an analysis of stream flow records, ground-water levels, or the
water balance for a watershed. In some cases, base flow has
been used as an approximation of recharge, with the
acknowledgement that it is probably less than the amount
recharging the ground-water system (Daniel, 1996; Holtschlag,
1997; Szilagyi and others, 2003). A common recommendation
is that recharge should be estimated by the use of multiple
methods and the results compared (Nimmo and others, 2003;
Healy and Cooke, 2002). This is a prudent approach, though
good-quality data usually are not available to make estimates
from multiple methods. In east-central Pennsylvania, however,
there are two hydrologic research sites where long-term
monitoring of climate, ground water, surface water, and the
unsaturated zone allows comparison of multiple methods for
estimating ground-water recharge with avail- able data. The
sites are operated by the U.S. Department of Agriculture,
Agricultural Research Service (ARS), as part of their Pasture
Systems and Watershed Management Research Unit Research
Watershed. Not only do these ARS sites afford long-term,
continuous hydrologic records representative of the humidcontinental climate of the northeastern United States, they
include measurements of unsaturated-zone drainage from
gravity-drainage lysimeters (a dataset rarely available) and
streamflow data from gages in nested watersheds. This study
was conducted in cooperation with the ARS as part of the U.S.
© 2015 IJEA. All Rights Reserved
Geological Survey (USGS) Ground-Water Resources Program
(Grannemann, 2001). It was one of several studies designed to
provide an improved understanding of methods for estimating
recharge in the humid, eastern United States
II. METHODS
Number of methods are available in the literature for the
estimation of natural and artificial recharge to the aquifer,
selection of which depends on available data, local geographic
and topographic conditions, spatial and temporal scale
required and reliability of results obtained by different
methods. According to Scanlon et al. (2002) techniques based
on the surface water and unsaturated-zone data provide
estimates of potential recharge, whereas that base on
groundwater data provides estimate of actual recharge.
Owing to uncertainties involved in each approach, he
suggested to use multiple techniques to increase the reliability
of the results
A. Water table fluctuation (WTF) method
In the application of WTF method the basic assumption is that,
the rise in the groundwater level in unconfined aquifer is only
due to recharge water arriving at the water table. Recharge is
calculated as
R = Sy dh/dt = Sy ∆h/∆t
Where,
R
Sy
Dh
Dt
(T)
=
=
=
=
rate of recharge (LT-1)
specific yield, (M0L0T0)
∆h = water table rise, (L)
∆t = time within which rise dh takes place,
The value of R obtained above can be multiplied by areal
extent of aquifer to get recharge in terms of volume per unit
time.
A time lag occurs between the arrival of water and its
redistribution to the other components like base flow,
groundwater evaporation and net sub-surface flow from an
area. WTF method can be applied over longer time intervals
(seasonal or annual) to estimate the change in subsurface
storage. Healy et al., (2002) reported that the WTF method
for estimation of groundwater recharge was applied as early as
the 1920s and since then has been used in numerous studies.
The method is quite simple as no assumptions are made on the
mechanism by which water reaches to groundwater. The
method has some disadvantages also. Water table fluctuation
6
International Journal of Engineering Associates (ISSN: 2320-0804) # 7 / Volume 5 Issue 2
method is applicable to only unconfined aquifers and the
method cannot account for steady rate of recharge. This
means, if the rate of recharge from an area is equal to rate of
drainage, water levels will not change and WTF method will
predict no recharge. Other difficulties arise in calculation of
specific yield values.
C. Darcy’s law
Many researchers have tried this method this method for the
estimation of groundwater recharge. Allison et al., (1990)
employed water table fluctuation method for estimation of
artificial recharge in southern Australia. They observed
groundwater levels that were steadily increasing at 0.1 m/year
following clearing of native vegetation. Assuming a specific
yield of 0.2 this corresponds to an increase in recharge of 20
mm/year. This value was found consistent with the recharge
estimated by other independent methods.
( )
Comprehensive reviews on the groundwater recharge
estimation methods that are based on groundwater level data
were presented by Healy and Cook (2002). They concluded
that WTF method that uses specific yield and variations in
water table level over time might be the most widely used
method for the estimation of groundwater recharge.
B. Water budget method
The water budget methods are those that are those that are
based on water budget equation. The water budget of a basin
can be stated as
P + Qon = ET + Qoff + ∆S
Where,
P
= precipitation (and may also include
irrigation) (mm/day)
Qon and Qoff = water flow onto and off the site (surface
flow, interflow and groundwater flow) (mm/day)
ET = evapotranspiration (mm/day) and
∆S = change in storage (mm/day).
Based on the above water balance equation, Schict and
Walton (1961) formulated the budget equation for recharge
estimation as:
(
)
Where,
R
= recharge
∆Sgw
= change in subsurface storage
Qbf
= base flow
ETgw
= evaporation from groundwater and
= net surface flow from the basin
In above model all other parameters, except R, can be
measured or estimated. This method can be adopted for wide
range of spatial and temporal scales. However, major
limitation of this approach is that the accuracy of the recharge
estimates depends on the accuracy with which other
components of the water balance equation and measured
(Scanlon et al., 2002).
© 2015 IJEA. All Rights Reserved
Darcy’s law states that fluid flux; such as recharge in an
aquifer system can be calculated if both the head gradients and
hydraulic conductivities are known. Darcy’s law is used to
calculated recharge (R) in the saturated zone according to the
following equation:
( )
(
)
( )(
)
Where,
K( )
=
hydraulic conductivity at the ambient water
content,
H
=
total head, and
h
=
metric potential head
z
=
horizontal distance between the two points
where hydraulic head is measured
Application of Darcy’s law requires measurements or
estimates of the vertical total head gradient and the
unsaturated hydraulic conductivity at the ambient soil-water
content. The method has been applied in many studies under
arid and semiarid conditions
In the areas where thick unsaturated zone exists in unform
porous media the value of metric potential head can be
assumed to be 1.The unit-gradient assumption removes the
need to measures the metric pressure gradient and sets
recharge equal to the hydraulic conductivity at the ambient
water content.
D. Empirical relationships
Empirical relationships can also be developed between
groundwater recharge and rainfall based on seasonal
groundwater balance studies. Kumar and Seethapathi (2000)
made one such attempt for Upper Ganga Canal command
area. An empirical relationship was suggested for estimation
of the ground water recharge by fitting the estimated values of
rainfall recharge and the corresponding values of rainfall in
the monsoon season through the non-linear regression
techniques. The relation between rainfall and recharge is
shown by the equation as
R = 0.63 (P – 15.28)0.76
Where,
R = recharge (m)
P = precipitation (m)
E. Groundwater models
Recharge measurements in the field still contain an
appreciable amount of uncertainty and much study on the
subject is ongoing (Sanford, 2002). Along with the variety of
approaches used to make measurements in the field,
investigators have used groundwater models in estimating
recharge. Models can also be used to predict distribution of
7
International Journal of Engineering Associates (ISSN: 2320-0804) # 8 / Volume 5 Issue 2
recharge in temporal and spatial scales based on the geologic
properties and rate of recharge.
Groundwater flow and contaminant transport models are
being extensively used in the studies related to groundwater
systems. Groundwater flow models are used to calculate the
rate and direction of movement of groundwater through
aquifers and confining units in the subsurface. These
calculations are referred to as simulations. The simulation of
groundwater flow requires a thorough understanding of the
hydro-geologic characteristics of the site.
The accuracy of model predictions depends upon successful
calibration and verification of the model in determining
groundwater flow directions, and transport of contaminants. In
relation with groundwater models, Sanford (2002) has
highlighted two important issues. As groundwater recharge is
a fundamental component of a most groundwater models,
while reviewing one must assess how recharge is represented
in the groundwater models and how recharge is estimated
using groundwater models. Use of groundwater models is
very fruitful. The analysis proposed by artificial recharge
scheme has been improved by groundwater modeling
exercises.
F. Tracer techniques
Recently, the techniques bsed on the heat or chemical isotopic
tracers are gaining much importance in the estimation of
groundwater recharge. Measuring the concentration of the
environment tracers that indicate groundwater age has been
increasingly popular approach in this field. Number of
articles and research papers about application and theories of
isotopic methods for characterizing groundwater and recharge
are available. In the field of groundwater, isotopic tracers
provide a powerful investigative tool. Coplen (993) reported
that another major technological growth area has been in the
application of isotopic analyses to groundwater hydrology,
wherein isotopic measurements are being used to help
interpret and define groundwater flow paths, ages, recharge
areas, leakage, and interactions with surface water.
Datta (1999) used the signatures of 18O isotopes to investigate
groundwater occurrence and recharge in the National Capital
Territory (NCT) of Delhi. These signatures revealed that
groundwater in well of Delhi area are a mixture of varying
proportions of different water sources and the aquifer in the
area does not constitute a homogeneous system in lateral
extent.
Due to large uncertainties involved in the measurement of
individual parameters of each method, many researchers
(Healy and Cook, 2002, Scanlon et al., 2002) have suggested
that it is highly beneficial to apply multiple methods of
estimation to arrive at somewhat reliable results.
McCartney and Houghton (1998) used three independent
methods for the computation of groundwater recharge on the
© 2015 IJEA. All Rights Reserved
Channel Island of Jersey. These are (a) chloride balance, (b)
stream base flow analysis, and (c) rainfall-recharge-runoff
simulation. All three methods produced reasonably consistent
results, indicating that long-term recharge is 16-19% of
average annual rainfall, and results of modeling indicate that
groundwater abstraction may have exceeded recharge in 5 out
of 28 years
III. CONCLUSION
Ground-water recharge is a fundamental component in the
water balance of any watershed. However, because it is nearly
impossible to measure directly, numerous methods have been
used to estimate recharge, and in some cases, base flow has
been used as an approximation of recharge. A common
recommendation in the literature is that recharge should be
estimated from multiple methods and the results compared,
but in reality, comparing the results may be difficult because
of differences inherent in the methods. While estimating
natural ground water recharge, it is essential to have a good
idea of the different recharge mechanisms and their
importance in the study area. Choice of methods should also
be guided by the objectives of the study, available data and the
possibilities to get supplementary data. Economy, too is an
important factor. However, estimates are normally subject to
large errors. No single comprehensive estimation technique
can yet be identified from the spectrum of those available,
which give reliable results. Hence, it is desirable to apply
more than one method based on independent input data.
References
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Aquifer recharge, in Stewart, B.A., and Howell, T.A., eds.,
Encyclopedia of Water Science: New York, Marcel Dekker,
Inc., p. 1-4
2- Szilagyi, Jozef, Harvey, F.E., and Ayers, J.F., 2003, Regional
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