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Methods of estimating groundwater Recharge

  • Department of Civil COllege of Engineering King Khalid University Abha 61421


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.
Methods of Estimating Ground water Recharge
Saiful Islam #1, Ram Karan Singh*2, Roohul Abad Khan#3
#1,3Lecturer,Department of Civil Engineering,King Khalid University,Abha,KSA
*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.
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 humid-
continental 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.
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
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 
R = rate of recharge (LT-1)
Sy = specific yield, (M0L0T0)
Dh = 
Dt =        
The value of R obtained above can be multiplied by areal
extent of aquifer to get recharge in terms of volume per unit
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
International Journal of Engineering Associates (ISSN: 2320-0804) # 6 / Volume 5 Issue 2
© 2015 IJEA. All Rights Reserved 6
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.
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 
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:
    
 
 
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).
C. Darcy’s law
          
aquifer system can be calculated if both the head gradients and
         
calculated recharge (R) in the saturated zone according to the
following equation:
  
 
 
 
 = hydraulic conductivity at the ambient water
H = total head, and
h = metric potential head
z = horizontal distance between the two points
where hydraulic head is measured
      
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
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
International Journal of Engineering Associates (ISSN: 2320-0804) # 7 / Volume 5 Issue 2
© 2015 IJEA. All Rights Reserved 7
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
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
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
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
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.
1- Nimmo, J.R., Stonestrom, David, and Healy, R.W., 2003,
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
estimation of base recharge to ground water using water balance
and a base-flow index: Ground Water, v. 41, no. 4, p. 504-513
3- Grannemann, N.G., 2001, U.S. Geological Survey groundwater
resources program, 2001: U.S. Geological Survey Fact Sheet
056-01, 2 p
4- Healy, R.W., and Cooke, P.G., 2002, Using groundwater levels
to estimate recharge: Hydrogeology Journal, v. 10, p. 91- 109
5- Scanlon, B.R., Healy, R.W. and Cook, P.G. (2002). Choosing
appropriate techniques for quantifying groundwater recharge.
Hydrology Journal, 10: 18-39.
6- Sanford, W. (2002). Recharge and groundwater models: an
overview. Hydrology Journal 10: 110-120 pp.
7- Kumar, C.P. and Seethapathi, P.V. (2000). Assessment of
natural ground water recharge in upper Ganga canal command
8- Datta, P.S. (1999). Groundwater Situation in New Delhi: Red
Alert, Nuclear Research Laboratory, IARI, New Delhi-12
9- McCartney, M.P. and Houghton-Carr, H.A. (1998). An
assessment of groundwater recharge on the Channel Island of
Jersey. J-Inst-Water-Environ-Manag. Lavenham, Suffolk,
England: Terrence Dalton Ltd. Dec. 12(6): 445-451.
10- Holtschlag, D.J., 1997, A generalized estimate of ground-water
recharge rates in the Lower Peninsula of Michigan: U.S.
Geological Survey Water-Supply Paper 2437, 37 p
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© 2015 IJEA. All Rights Reserved 8
11- Daniel, C.C., III, 1996, Ground-water recharge to the
regolithfractured crystalline rock aquifer system, Orange
County, North Carolina: U.S. Geological Survey Water-
Resources Investigations Report 96-4220, 59 p
12- Coplen, T.B. (1993). Uses of Environmental Isotopes, in
Regional Ground Water Quality. Chap. 10 (Alley, W.A., Ed.),
Van Nostrand Reinhold, New York: 227-254.
13- Allison, G.B., Cook, P.G., Barnett, S.R., Walker, J.R., Jolly,
I.D., and Hughes, M.W. (1990). Land clearance and river
salinization in the western Murray basin, Australia. J Hydrol. 9:
14- Lerner, D.N., Issar, A.S., and Simmers, Ian, 1990, Groundwater
rechargeA guide to understanding and estimating natural
recharge: International Association of Hydrogeologists,
International Contributions to Hydrogeology, v. 8, 147 p.
International Journal of Engineering Associates (ISSN: 2320-0804) # 9 / Volume 5 Issue 2
© 2015 IJEA. All Rights Reserved 9
... As taxas de recargas tendem a ser controladas pelo clima, vegetação, topografia e geologia e em regiões de clima úmido, os aquíferos são caracterizados por nível freático pouco profundo e a presença de rios efluentes (Scanlon, Healy & Cook 2002). Nessas condições, há a predominância de utilização de dados de vazão dos rios com o método de decomposição de hidrograma, utilização do método de variação do nível d'água em poços de observação e balanço hídrico em bacias hidrográficas (Islam et al. 2015). ...
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... Model neraca air banyak digunakan dalam penelitian berbasis DAS (Zhang et al., 1999;Boughton, 2005;Barron et al., 2013;Wagner et al., 2013). Peneliti menggunakan berbagai pendekatan untuk memperkirakan imbuhan (Simmers, 1987;Tilahun dan Merkel, 2009;Islam et al., 2016;Niazi et al., 2017). Berbagai metode langsung dan tidak langsung serta model numerik telah digunakan untuk mengestimasi imbuhan airtanah (Sanford, 2002;Singh et al., 2019). ...
... Techniques based on the heat or chemical isotopic tracers are gaining much importance in the estimation of groundwater recharge [22]. Groundwater recharge is low in most cases, however, its estimation by classical methods is problematic; isotope techniques are likely more efficient [7]. ...
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Various studies have shown that the use of tracer techniques such as the chloride mass balance (CMB) and stable isotopes' methods are suitable and good practical approaches to estimate groundwater recharge. Estimating groundwater recharge improves the understanding of groundwater availability in making informed strategies for groundwater resources management. Using secondary data, this study estimated groundwater recharge within the Cuvelai-Etosha Basin (CEB) via the CMB and stable isotopes' methods along six flow paths. Chloride content in groundwater at the flow path's endpoint was treated as an integral value of what has been happening from the starting point up to that endpoint. The stable isotopes' method has used the hydro-calculator to compute fractional losses along transects which determined evaporation losses assuming the rest of water retained along flow paths forms part of groundwater recharge. From the CMB method, endpoint recharge rates range between 0.21% and 38.46% of mean annual precipitation. Based on stable isotopes' method, about 50% of the initial recharge reaches the discharge zones in comparison to only 11% that of the CMB method. From the obtained results, there is much significance between the two methods with the stable isotopes' method estimating much higher values whilst the CMB method seems to underestimate, however, the notion of using CMB method to calculate integral recharge instead of point recharge seems more usable. Groundwater recharge rates determined by both methods agree well with the range of values found in previous studies. Study outlined the protection of recharge zones such as the southern rim of the basin for great groundwater management strategies. The calculated recharge to aquifer systems has indicated that there is a need for sustainable groundwater use as demands may exceed the current potential in the near future.
Water scarcity is now a global issue and to tackle the same at the regional level is of utmost importance. While handling the issue of water scarcity at the regional level, it is necessary to identify the probable causes of water scarcity. This study is aimed at studying the dynamics of groundwater recharge due to change in water levels. In this study, firstly spatial distribution is studied from the water level fluctuation field data for the pre and post monsoon period during 2017. Secondly, the temporal distribution of groundwater recharge is determined from the data of Groundwater Surveys and Development Agency’s key observation wells, for the period from 1996 to 2018. From the evaluation of groundwater recharge dynamics, a map for groundwater recharge is generated, using Geoinformatics through spatial analysis. Based on groundwater recharge dynamics, an artificial recharge zone map is generated. The result indicates falling trends in groundwater during summers and also a similar tendency in average levels of groundwater, which is caused due to the tremendous use of unsustainable groundwater. The findings also indicate the continuous withdrawal of groundwater for agricultural and other purposes, the decreasing trend in rainfall, slope, Geology, changes in land use are attributed to the continuous declining trend of groundwater recharge and subsequently groundwater level, as the situation will be irreversible if necessary steps for artificial recharge activities are not taken immediately. Hence, artificial recharge structures are suggested for tackling the critical issue of water scarcity.
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The stress on the freshwater resources of the planet earth has led the United Nations to add a goal regarding clean water in sustainable development goals list in order to address the global availability of clean water. The widespread use of fertilizers and industrial effluents caused the groundwater contamination in the Haripur District, Khyber Pakhtunkhwa, Pakistan. To investigate and assess the vulnerability of groundwater to contamination, geographic information system (GIS)-based DRASTIC model has been employed. The DRASTIC index values lie between 88 and 190. The lower the DI value, the lower will be the susceptibility towards pollution and vice versa. The indices were classified into five zones, i.e., low (< 109), medium (110–129), moderate (130–149), high (150–169), and very high vulnerable zones (> 170) on the basis of equal intervals. The low vulnerable zone covers almost 6% of the study area, i.e., 118 km2. Medium vulnerable zone encompasses an area of approximately 23%, i.e., 506 km2. The moderate vulnerable zone is the largest in the district covering almost 965 km2, approximately 45% of the study area. The high and very high vulnerable zone encompasses almost 23% and 3% of the study area which means 506 km2 and 66 km2 respectively. A spatial distribution map was generated for nitrate concentration to validate the DRASTIC indices. The results demonstrate a fair relation between groundwater susceptibility and spatial nitrate distribution. This index map will provide a baseline study for this area to develop the safe zones for groundwater exploitation and controlling the current state of deterioration of environmental norms. The areas of high vulnerability are the firsthand task to improve the current situation of crisis especially in the southern parts such as the Hattar industrial area and its surroundings drained by those effluents. These further need specific tasks to restore and reclaim the polluted ecosystem by using proper technological solutions for disposal of these effluents.
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Gunung Agung is a stratovolcano type of volcano which has a height of 3,142 masl and is located in Karangasem Regency, Bali Province. At the end of 2017, Mount Agung's volcanic activity increased until it finally erupted several times in October to December. The government has prepared refuge pockets at the foot of Mount Agung, in areas that are not directly affected by eruption. There are 19 drilling plan points that will be carried out to meet the raw water needs at the evacuation site. This paper presents the groundwater recharge potential including the distribution of water sources, Hydrogeological conditions and the magnitude of groundwater recharge potential at hillside of Mount Agung and the surrounding area. The method used in this study is a field survey, calculation of potential recharge, analysis and evaluation of hydrogeological conditions, distribution of water sources and calculation of potential groundwater recharge. Groundwater at the foot of Mount Agung has the potential to be utilized and developed mainly to cover raw water needs in several refugee locations, namely in the Districts of Sidemen, Abang and Karangasem. The result of the analysis is that the largest groundwater potential is in Kubu Sub-District, namely 97,560,207 m3 / year, with a position that is relatively susceptible to primary hazards and secondary to Mount Agung eruption. For locations that are relatively safe and reachable in the area, they are in Tianyar, Sukadana, Baturinggit, Kubu, and Tulamben Villages, all of which are on the coast of the sea. These results are expected to be used by local governments in an effort to deal with the provision of water from the impact of the eruption of Mount Agung.
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Recharge is a fundamental component of groundwater systems, and in groundwater-modeling exercises recharge is either measured and specified or estimated during model calibration. The most appropriate way to represent recharge in a groundwater model depends upon both physical factors and study objectives. Where the water table is close to the land surface, as in humid climates or regions with low topographic relief, a constant-head boundary condition is used. Conversely, where the water table is relatively deep, as in drier climates or regions with high relief, a specified-flux boundary condition is used. In most modeling applications, mixed-type conditions are more effective, or a combination of the different types can be used. The relative distribution of recharge can be estimated from water-level data only, but flux observations must be incorporated in order to estimate rates of recharge. Flux measurements are based on either Darcian velocities (e.g., stream baseflow) or seepage velocities (e.g., groundwater age). In order to estimate the effective porosity independently, both types of flux measurements must be available. Recharge is often estimated more efficiently when automated inverse techniques are used. Other important applications are the delineation of areas contributing recharge to wells and the estimation of paleorecharge rates using carbon-14.
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Various techniques are available to quantify recharge; however, choosing appropriate techniques is often difficult. Important considerations in choosing a technique include space/time scales, range, and reliability of recharge estimates based on different techniques; other factors may limit the application of particular techniques. The goal of the recharge study is important because it may dictate the required space/time scales of the recharge estimates. Typical study goals include water-resource evaluation, which requires information on recharge over large spatial scales and on decadal time scales; and evaluation of aquifer vulnerability to contamination, which requires detailed information on spatial variability and preferential flow. The range of recharge rates that can be estimated using different approaches should be matched to expected recharge rates at a site. The reliability of recharge estimates using different techniques is variable. Techniques based on surface-water and unsaturated-zone data provide estimates of potential recharge, whereas those based on groundwater data generally provide estimates of actual recharge. Uncertainties in each approach to estimating recharge underscore the need for application of multiple techniques to increase reliability of recharge estimates.
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The clearing of native vegetation in a semi-arid region of southern Australia has led to increases in groundwater recharge of about two orders of magnitude. Although most of the clearing took place early this century, the generally deep water table along with the low rates of recharge means that there is a considerable delay in the response of the aquifer to the increased recharge. The rates of pre- and post-clearing recharge, and the time delay in aquifer response have been estimated using unsaturated zone chloride and matric suction profiles. Predictions of the time lag in aquifer response have been verified using bore hydrographs. The results of these analyses suggest that where the soils are light textured, and the water table is less than 40 m below the soil surface, it is now rising. Where the soils are heavier textured, it is estimated that the water table is rising only where it is less than 10 m below the soil surface. The effect of the increased recharge rates on the salinity of the River Murray, a major water resource, have been predicted using a groundwater model of the region. The predictions suggest that the salinity of the river will increase at about 1 μS cm−1 year−1 over the next 50 years and beyond.
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Naturally occurring long-term mean annual base recharge to ground water in Nebraska was estimated with the help of a water-balance approach and an objective automated technique for base-flow separation involving minimal parameter-optimization requirements. Base recharge is equal to total recharge minus the amount of evapotranspiration coming directly from ground water. The estimation of evapotranspiration in the water-balance equation avoids the need to specify a contributing drainage area for ground water, which in certain cases may be considerably different from the drainage area for surface runoff. Evapotranspiration was calculated by the WREVAP model at the Solar and Meteorological Surface Observation Network (SAMSON) sites. Long-term mean annual base recharge was derived by determining the product of estimated long-term mean annual runoff (the difference between precipitation and evapotranspiration) and the base-flow index (BFI). The BFI was calculated from discharge data obtained from the U.S. Geological Survey's gauging stations in Nebraska. Mapping was achieved by using geographic information systems (GIS) and geostatistics. This approach is best suited for regional-scale applications. It does not require complex hydrogeologic modeling nor detailed knowledge of soil characteristics, vegetation cover, or land-use practices. Long-term mean annual base recharge rates in excess of 110 mm/year resulted in the extreme eastern part of Nebraska. The western portion of the state expressed rates of only 15 to 20 mm annually, while the Sandhills region of north-central Nebraska was estimated to receive twice as much base recharge (40 to 50 mm/year) as areas south of it.
Aquifer recharge is important both for hydrologic understanding and for effective water resource management. Temporal and spatial patterns of unsaturated-zone processes such as infiltration largely determine its magnitude. Many techniques of recharge estimation exist. Water budget methods estimate all terms in the continuity equation except recharge, which is calculated as the residual. Detailed hydrologic models based on water-budget principles can produce recharge estimates at various scales. Empirical methods relate recharge to meteorologic and geographic parameters for a specific location. Surface-water methods include stream-hydrograph analyses to estimate baseflow (groundwater discharge) at lower elevations in a watershed, which is taken to equal the recharge that has occurred at higher elevations. Subsurface methods include analysis of water-table fluctuations following transient recharge events, as well as diverse unsaturated-zone methods. The zero-flux plane method determines the recharge rate from the change in water storage beneath the zero-flux depth, a boundary between water moving upwards due to evapotranspiration and water moving downward due to gravity. Lysimeter methods use buried containers filled with vegetated soil to mimic natural conditions. Water exiting the bottom is considered to be recharge. Darcian methods for estimating flux densities use unsaturated hydraulic conductivities and potential gradients, indicating recharge rates under appropriate conditions. Chemical mass-balance methods use conservative tracers that move with recharging water. Tracer concentrations in deep unsaturated-zone water, together with tracer input rates, indicate recharge rates. Distinct chemical “markers” can indicate travel times, hence, recharge rates. Thermal methods use heat as a tracer. Moving water perturbs temperature profiles, allowing recharge estimation. Geophysical methods estimate recharge based on water-content dependence of gravitational, seismic, and electromagnetic properties of earth materials.
On the Channel Island of Jersey, a third of the total water requirements are obtained from groundwater sources. A reliable estimate of recharge is necessary for sustainable groundwater management. Past resource management has been limited, partly because the rate of recharge is uncertain. This paper describes the application of three independent methods of estimating recharge: (a) chloride balance, (b) stream baseflow analysis, and (c) rainfall-recharge-runoff simulation. All three methods produce reasonably consistent results, indicating that long-term recharge is 16–19% of average annual rainfall, and results of modelling indicate that groundwater abstraction may have exceeded recharge in 5 out of 28 years. These findings have important implications for the management of the island's water resources.
Accurate estimation of groundwater recharge is extremely important for proper management of groundwater systems. Many different approaches exist for estimating recharge. This paper presents a review of methods that are based on groundwater-level data. The water-table fluctuation method may be the most widely used technique for estimating recharge; it requires knowledge of specific yield and changes in water levels over time. Advantages of this approach include its simplicity and an insensitivity to the mechanism by which water moves through the unsaturated zone. Uncertainty in estimates generated by this method relate to the limited accuracy with which specific yield can be determined and to the extent to which assumptions inherent in the method are valid. Other methods that use water levels (mostly based on the Darcy equation) are also described. The theory underlying the methods is explained. Examples from the literature are used to illustrate applications of the different methods.
Groundwater Situation in New Delhi: Red Alert
  • P S Datta
Datta, P.S. (1999). Groundwater Situation in New Delhi: Red Alert, Nuclear Research Laboratory, IARI, New Delhi-12