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Journal of Engineering Science 04(1), 2013 11-22
ESTIMATING GROUNDWATER RECHARGE INTO A SHALLOW UNCONFINED
AQUIFER IN BANGLADESH
Sajal Kumar Adhikary1*, Tanmay Chaki2, Md. Mahidur Rahman1 and Ashim Das Gupta3
1Department of Civil Engineering, Khulna University of Engineering & Technology, Khulna-9203, Bangladesh 2Water Resources Planning Division, Institute of Water Modelling (IWM), Dhaka, Bangladesh
3Water Engineering and Management, Asian Institute of Technology (AIT), Bangkok, Thailand
Received 01 September 2012; Accepted 18 October 2012
ABSTRACT
This paper presents a data conservative approach, in which quantitative groundwater recharge estimation in a
shallow unconfined aquifer is interpreted in details by the analysis of observed precipitation and water level
fluctuations records. Kushtia district in Bangladesh has been taken as a case study area based on the observed
data and information. In the adopted state-of-the-art methodology used for this study, well-known water table
fluctuation technique has been modified so that groundwater recharge in shallow aquifer can be estimated by
using the least available data and information. Observed time series of precipitation and groundwater levels
records at a few monitoring wells in the study area are the only data required to carry out the study. The
approach illustrated in this paper can be useful in any initial level of assessment in groundwater studies. In
addition, results can be applied as input data for developing numerical groundwater model for any groundwater
resource investigation in the study area or similar drainage basins in Bangladesh.
Keywords: Groundwater recharge, Kushtia, Precipitation, Unconfined aquifer, Water table fluctuation
1. INTRODUCTION
Groundwater (GW) recharge is defined as the fraction of total precipitation falling into a drainage basin, which
eventually reaches the water table in the saturation zone of an aquifer (Jukić and Jukić, 2004). It is a fundamental
component of GW systems (Sanford, 2002), because information on GW recharge rates is often necessary for
water resource management, inputs to regional GW models and predictions of climate change impacts (De Silva
and Ruston, 2007). Thus, GW recharge is a critical hydrological parameter, which may need to be estimated at a
variety of spatial and temporal scales depending on the application. Since GW recharge cannot be measured
directly, it is often estimated by using the results of hydrogeologic and geologic investigations, hydro-
meteorological data, observed discharges or GW level hydrographs (Jukić and Jukić, 2004). However, aquifer-
scale recharge estimation is often required for water resource assessment and management, whereas local-scale
recharge is critical to assessment of GW contamination from point sources. Estimation of GW recharge may be
required on temporal scales ranging from days to thousands of years. As aquifers are depleted, recharge
estimation have become more vital in determining appropriate levels of GW withdrawal. In addition, recharge
estimation is becoming more important for contaminant transport, as aquifer management expands from cleanup
of existing contamination to aquifer protection by delineation of areas of high recharge (Scanlon and Cook,
2002). Thus, understanding GW recharge and its accurate estimation is essential for the successful management
of water resources and modelling fluid flow and transport of contaminants within the subsurface (Healy, 2010;
Healy and Cook, 2002). Increasing demand for recharge estimation is forcing the researchers to develop
approaches for building a more thorough understanding of aquifer recharging process and quantifying recharge
rates that reduce uncertainties and increase confidence in recharge estimates (Scanlon and Cook, 2002).
The fraction of precipitation that reaches the phreatic zone in an aquifer depends upon several factors including
soil, topography, vegetation, and climate (Moon et al., 2004). Because of such influencing factors, determining
the GW recharge into aquifers is one of the great challenges in almost all the GW studies (Korkmaz, 1988).
Depending on the available hydrological data and information, numerous techniques for estimating GW recharge
have been discussed intensively in the literatures (Venetics, 1971; Rushton and Ward, 1979; Caro and Eagleson,
1981; Johansson, 1987; Das Gupta and Paudyal, 1988; Korkmaz, 1988; Bekesi and McConchie, 1999; Arnold et
al., 2000; Edmunds et al., 2002; Flint et al., 2002; Lewis and Walker, 2002; Scanlon et al., 2002; Jukić and Jukić,
2004; Moon et al., 2004; Anuraga et al., 2006; Rahman and Roehrig, 2006; Ruston et al., 2006; Sharda et al.,
2006; Thomas and Tellam, 2006; Batelaan and de Smedt, 2007; Coes et al., 2007; De Silva and Ruston, 2007;
Park and Parker, 2008; Martínez et al., 2009; Nziku et al., 2009; Izuka et al., 2010; Jie et al, 2011; Singh, 2011;
Yin et al., 2011). However, most of these approaches often require much time and skill along with lots of
* Corresponding author : [email protected]
KUET @ JES, ISSN 2075-4914 / 04(1), 2013
JES an International Journal
12 Sajal Kumar Adhikary et. al. Estimating Groundwater Recharge into a Shallow Unconfined Aquifer……………….
hydrological data and information, since they apply numerical modeling techniques for estimating GW recharge
into aquifer. Very often, the necessary data and information are not readily available and the cost of using such
models may sometimes be prohibitively very high (Das Gupta and Paudyal, 1988) for a developing country like
Bangladesh. Therefore, simple and straightforward method should be developed for estimating GW recharge
from a few input variables that are relatively easy to collect for most regions. Measurements of GW level
fluctuations along with precipitation events establish a practical means of estimating temporally and spatially
variable GW recharge rates, which have been illustrated by using a number of methods in several literatures
(Veneetics, 1971; Rushton and Ward, 1979; Caro and Eagleson, 1981; Johansson, 1987; Das Gupta and Paudyal,
1988; Korkmaz, 1988; Sophocleous, 1991; Rai and Singh, 1995; Bierkens, 1998; Rai and Manglik, 1999;
Knotters and Bierkens, 2000; Coulibaly et al., 2001; Healy and Cook, 2002; Rai et al., 2006). However, GW
level fluctuation is not only worth of estimating recharge, but also it can provide a practical means of estimating
other hydrological parameters as well (Bauer et al., 2004). In this respect, the water-table fluctuation method can
be a good choice for estimating GW recharge in shallow aquifer, which requires only the knowledge of specific
yield and changes in water levels over time. Advantages of this approach include its simplicity and insensitivity
to the mechanism by which water moves through the unsaturated zone (Healy and Cook, 2002). This paper
presents a data conservative modified water-table fluctuation approach, in which quantitative estimation of GW
recharge in the shallow unconfined aquifer in Kushtia district of Bangladesh is interpreted by analyzing the
precipitation and GW level data. It is expected that the methodology described in this paper can be applied as
preliminary input parameters for developing and simulating a GW model in the numerical environment for any
GW resource investigation scheme in the study area or similar catchments in Bangladesh.
2. METHODOLOGY
2.2 System Conceptualization and Mathematical Formulation
Records of GW level fluctuations in monitoring wells are worth the cost and trouble of collecting only if they are
used as a basis for hydrologic interpretations and for subsequent applications in hydrological investigations.
Although water level records have been important to draw conclusions regarding the occurrence and
development of GW in specific areas under concern, many such records still await interpretations. Similarly, a
wealth of climatologic and other hydrologic data is in need of analysis before taking the final conclusions
(Korkmaz, 1988). In the wet period of a water year, when the precipitation occurs, the first rain wets vegetation
and accordingly other surfaces and then begins to infiltrate into the ground depending on the surface
characteristics.
Figure 1a GW level fluctuations in arithmetic scale caused by the recharge from precipitation
Journal of Engineering Science 04(1), 2013 11-22
13
Figure 1b GW level fluctuations in log-scale caused by the recharge from precipitation
In general, it is possible to calculate the total level oscillation amplitude due to infiltration of the precipitated
water, if the overall recession regime (i.e. the behavior of the aquifer without external recharge) is known
(Korkmaz, 1988). It is generally found that the component forming a GW hydrograph having a recession can be
approximated by a simple exponential relationship (Subramanya, 1994) of the form (Figure 1) as expressed in
Equation (1).
tehh 0
α−= (1)
Where, h0 and h are the water level above discharge level at the beginning of the measurement period and at any
certain time (t), respectively and α is known as coefficient of recession or discharge coefficient.
Like any other exponential formula, on a semi-logarithmic paper when water level above discharge level is
plotted to the log scale and time to the arithmetic scale, the recession curves plot as nearly as straight lines
(Figure 1b). In the logarithmic system with base 10, the formula is converted into the linear form as described in
Equation (2)
t43429.0hloghlog 0 α−= (2)
The shape of the recession curve depends on the water yielding properties of the aquifer material, the
transmissivity and the aquifer geometry (Johansson, 1987).
The recovery of the water level, ∆H under natural hydrological conditions is a mirror image of the recession
curve obtained from GW level fluctuations. The recovery of the water level varies from year to year, depending
on the amount of total precipitation (Rt) in wet period of a water year (Figure 1a). However, GW levels are
influenced by the seasonal cycles (Almedeij and Al-Ruwaih, 2006; Adhikary et al., 2012), which are relevant to
different factors such as recharge from precipitation, evapotranspiration, overland flow and runoff, and discharge
from wells and shows a seasonal pattern of fluctuations (Healy, 2010). The degree of correlation between
fluctuations of GW level and variations in total precipitation (Rt) in wet period of a water year provides an
evidence as to the freeness of the connection between the aquifer recharge and total precipitation (Rt) in wet
period of a water year (Korkmaz, 1988). In this study, the direct estimation of GW recharge in shallow aquifer is
considered using recovery of the GW level (∆H) and total precipitation (Rt) during wet period in a water year
14 Sajal Kumar Adhikary et. al. Estimating Groundwater Recharge into a Shallow Unconfined Aquifer……………….
(Figure 2). The linear regression model developed by the relationship between ∆H and Rt can be expressed by
Equation (3).
tbRaH +=∆ (3)
Where, ∆H is recovery of GW level, and Rt is total precipitation during the wet period in the water year under
consideration, a and b are the regression coefficients of the linear regression model expressed by Equation (3).
This process is diagrammatically presented in Figure 2.
Figure 2 Relationship of total precipitation in wet period and recovery of the aquifer water level
Figure 2 indicates that the precipitation intercept, Re is the intersection of the total precipitation recovery straight
line with zero-total precipitation axis. It represents the amount of surface runoff and evapotranspiration for the
same period. Recovery or recharge from rainfall is a function of the amount of total precipitation (Rt). If the
intercept is Re, the recharge (Rs) can be estimated by the Equation (4), which is expressed as
eRtcRsR −= (4)
Where, Rtc is the computed total precipitation by means of Equation (3) during the wet period in a water year.
However, the major features of this proposed methodology is that it indirectly estimates the water losses such as
evapotranspiration and surface runoff. It can be found by subtracting the recharge amount from the total
precipitation quantity.
2.2 Estimation of Groundwater Recharge
In this paper, a modified data conservative GW fluctuation technique is presented for estimating GW recharge in
a shallow unconfined aquifer of Bangladesh based on the statistical relationship between the precipitation and
water levels. The analysis is based on the original conditions, which are unaffected by heavy pumping. Observed
monthly precipitation records and weekly GW levels are the only data required to carry out the study. Data have
been collected from the central database of Bangladesh Water Development Board (BWDB). For this study,
daily rainfall data at a precipitation data collection station (BWDB rainfall station ID: CL41) and GW level
fluctuation data at a monitoring well (BWDB GW level monitoring station ID: GT5015004) are collected for the
16 years period from 1992 to 2007. However, these two representative monitoring stations located in the study
area have been selected depending on the data availability for a longer period. Shallow aquifer located in the
upper part of the Gorai River catchment (mainly Kushtia district area) in Bangladesh has been selected as a case
study area (Figure 3). The upper part of aquifer in this region is unconfined in nature (Halcrow, 1993; Adhikary,
Journal of Engineering Science 04(1), 2013 11-22
15
2009) and GW is extensively extracted from its shallow depth due to the increased irrigation activities (Halcrow,
1993; Adhikary et al., 2011; Adhikary et al., 2012).
BangladeshKushtia District (Study Area)
N
EW
S
Figure 3 Location of studied aquifer (Kushtia district) in Bangladesh
By analyzing the monthly precipitation data and water level records belonging to the 16 years period from 1992
to 2007, the amount of rainfall causing a rise in water level is determined. These rainfall amounts are called
“total precipitation (Rt)” causing the rise in water level. The water level rise caused by this total precipitation is
termed as “water level rise due to GW recharge (∆H)” and can be found from the plotted graph directly. This
procedure is repeated year after year for the period of total records of 16 years from 1992 to 2007 and the
relationship between these two variables Rt and ∆H is established by using simple statistical technique. However,
∆H is a function of the precipitation amount above a threshold precipitation value, which represents the
precipitation causing surface runoff, evapotranspiration, and subsurface flow or only one of them depending on
the local hydrogeological conditions in the study area (Korkmaz, 1988). These values of water losses can be
obtained by setting ∆H = 0 in the regression equation, which has been schematically presented in Figure 2. The
recharge amount for each water year is then calculated by subtracting the threshold precipitation from the total
precipitation causing GW level rise in that particular year.
3. RESULTS AND DISCUSSION
Shallow aquifer located in the upper part of the Gorai River catchment (mainly Kushtia district area) in
Bangladesh has been selected as a case study area (Figure 3) for this study based on the data availability from
secondary source (i.e. Bangladesh Water Development Board). It covers an area of 1621 square kilometres and is
bounded by Rajshahi, Natore and Pabna districts to the North, Chuadanga and Jhenaidah districts to the South,
Rajbari district to the east and West Bengal and Meherpur district to the west. The upazillas (sub-district) are
Kushtia Sadar, Kumarkhali, Daulatpur, Mirpur, Bheramara and Khoksa. The Ganges, the Gorai, Mathabhanga,
Kaligonga and Kumar are the main rivers flowing through the district. The average maximum and minimum
16 Sajal Kumar Adhikary et. al. Estimating Groundwater Recharge into a Shallow Unconfined Aquifer……………….
temperatures are 37.8 0C and 11.2 0C, respectively with an average annual rainfall of 1,898 mm. The GW levels
naturally fluctuate in response to a sequence of climatic events and to constraints imposed by the hydrogeologic
and topographic characteristics, which is presented in Figure 4.
1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 20072
4
6
8
10
12
14
AJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJO0
200
400
600
800
1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
Water Year
Gro
un
dw
ate
r L
ev
el
(m.P
WD
)P
rec
ipit
ati
on
(m
m)
Water Year
Figure 4 Weekly GWL hydrograph and monthly precipitation hyetograph in the study area for 16 years
Figure 4 reveals that the aquifer is recharged in the wet period of a water year (April to next March in a year) in
Bangladesh (Mirza, 1997) due to the heavy rainfall in that time and discharged out of it in the dry period as there
is not much rainfall available in that time. In addition, evapotranspiration and water withdrawal takes place in
various ways by numerous human interventions, which cause the depletion of the GW level. However, GW
recharge is the largest in the wet period of a water year, especially in the monsoon season, when plants are
dormant and evapotranspiration rates are comparatively less. Besides, in the dry period of a water year, when
evapotranspiration rates along with the GW extraction from underground shallow aquifer exceed the available
moisture from precipitation, GW recharge to the water table is negligible in the shallow unconfined aquifer and
consequently, GW levels decline.
Journal of Engineering Science 04(1), 2013 11-22
17
Figure 5 Fluctuations in water levels caused by recharge from precipitation in the study area
The recovery of the water level (∆H) and the total precipitation (Rt) during the wet period in each water year for
16 years period from 1992 to 2007 are determined, which are presented in Figure 5. A linear regression model
during the whole 16 years period is established and is schematically presented in Figure 6. The results of linear
regression analysis are presented in Table 1. The regression equation is found as represented in Equation (5).
tR∆H ×+= 0.005211.06585- (5)
The average precipitation intercept (Re) of the case study aquifer is found as 204.58 mm (Figure 6), which
represents the amount of surface runoff and evapotranspiration for the same period. This result demonstrates that
2000 2001 2002 20032
4
6
8
10
12
1416
A J A O D F A J A O D F A J A O D F A J A O D F0
100
200
300
400
500
600
700
2000 2001 2002 2003
Water Year
Gro
un
dw
ate
r L
eve
l (m
.PW
D)
∆H = 8.64m ∆H = 6.8m∆H = 6.53m
∆H = 5.94m
Pre
cip
itati
on
(m
m)
Water Year
1633.6 1477.41486.01580.4
2004 2005 2006 20072
4
6
8
10
12
1416
A J A O D F A J A O D F A J A O D F A J A O D0
100
200
300
400
500
600
700
800
2004 2005 2006 2007
Water Year
Gro
un
dw
ate
r L
ev
el (m
.PW
D)
∆H = 6.51m∆H = 8.40m ∆H = 6.27m
∆H = 6.81m
Pre
cip
ita
tio
n (
mm
)
Water Year
1567.1 1668.61788.5 1504.0
1996 1997 1998 19992
4
6
8
10
121416
A J A O D F A J A O D F A J A O D F A J A O D F0
100
200
300
400
500
600
700
1996 1997 1998 1999
Water Year
Gro
un
dw
ate
r L
ev
el
(m.P
WD
)
∆H = 8.45m ∆H = 8.98m∆H = 9.35m
∆H = 6.93m
Pre
cip
itati
on
(m
m)
Water Year
1928.31389.6 1909.9 2015.4
1992 1993 1994 19952
4
6
8
10
12
1416
A J A O D F A J A O D F A J A O D F A J A O D F0
100
200
300
400
500
1992 1993 1994 1995
Water Year
Gro
un
dw
ate
r L
ev
el
(m.P
WD
)
∆H = 6.01m ∆H = 8.9m∆H = 6.45m
∆H = 6.82m
Pre
cip
ita
tio
n (
mm
)
Water Year
1329.81386.2 1709.5 1644.5
18 Sajal Kumar Adhikary et. al. Estimating Groundwater Recharge into a Shallow Unconfined Aquifer……………….
the proposed methodology described in this paper indirectly estimates the water losses such as
evapotranspiration and surface runoff. It can be found by subtracting the recharge amount from the total
precipitation amount. The obtained result is often useful in any initial level of assessment, where model input is
primarily necessary while conducting detailed study using the numerical GW modeling technique.
0 400 800 1200 1600 2000 24000
2
4
6
8
10
12R
eco
ve
ry,
∆H
(m
)
Total precipitation in wet period, Rt (mm)
∆H = -1.06585 + 0.00521 x Rt
Re = 204.58 mm
Figure 6 Relationship of total precipitation in wet period and groundwater recovery
Table 1 Results of linear regression analysis
Type of relation No. of
observations
Correlation
coefficient (r)
Standard error
of estimate Regression Equation
Total precipitation-
GW level recovery 16 0.84 1.49 tR∆H ×+= 0.005211.06585-
However, the computed values (Rtc) of total precipitation by means of the developed regression model for water
years from 1992 to 2007 are summarized in Table 2. The results of the recharge (Rs) computation based on the
available precipitation for the total 16 years period from 1992 to 2007 by using the Equation (4) are presented in
Figure 7. The annual GW recharge in the studied aquifer for the period ranges from 1794 mm in 1999 to 1140
mm in 2003 (Figure 7). The annual average recharge during the 16 years period is estimated as 1413 mm.
However, this is about 74 percent of the average annual precipitation (1898 mm) occurs in the study area.
Estimated GW recharge by using the methodology proposed in this study is finally compared with the previous
studies conducted in the present study area under the studied aquifer (Halcrow, 1993; Adhikary, 2009), which is
presented in Figure 8. However, Healy (2010) describes that since actual recharge rates are unknown, so there
are no standards by which the accuracy of estimated recharge can be evaluated. Figure 8 shows that the estimated
GW recharge values for the whole 16 years period in this study are reasonably closer to those estimated by the
two previous studies (e.g. Halcrow, 1993; Adhikary, 2009). Hence, the obtained result is regarded as quite
reasonable for further applications. The estimated GW recharge based on three different studies presented in
Figure 7 indicates that the modified GW fluctuation approach proposed in this study overestimates in some years
as well as underestimates in other time periods with respect to the study conduced by Halcrow (1993) and
Adhikary (2009). This proves the acceptability of the methodology proposed in this study in estimating GW
recharge in an unconfined aquifer having shallow depth. However, it is noteworthy that these two previous
studies applied the long-term water balance approach and numerical GW modeling technique, respectively for
estimating the GW recharge in the studied aquifer. It is now widely recognized that both methods usually
provide almost accurate results as they take into account lots of hydrological parameters (Moon et al., 2004),
which actually influence the recharging process of the aquifer.
Journal of Engineering Science 04(1), 2013 11-22
19
Table 2 Summary of GW recharge computation for the studied aquifer
Water Year
Annual
Precipitation, Ry
(mm)
Recovery of GW
level, ∆H
(m)
Computed Total
Precipitation, Rtc
(mm)
Recharge, Rs
(mm)
1992 1633.60 6.01 1356.21 1151.63
1993 2301.20 8.90 1912.83 1708.25
1994 1450.60 6.45 1442.58 1238.00
1995 1782.50 6.82 1513.60 1309.02
1996 1719.70 6.93 1534.71 1330.13
1997 2190.50 8.45 1826.46 1621.88
1998 2250.60 8.98 1928.19 1723.61
1999 2267.60 9.35 1999.20 1794.62
2000 1919.10 8.64 1862.93 1658.35
2001 1720.80 6.80 1509.76 1305.18
2002 1868.70 6.53 1457.94 1253.36
2003 1897.60 5.94 1344.69 1140.11
2004 1861.80 6.51 1454.10 1249.52
2005 1996.50 8.40 1816.86 1612.28
2006 1664.00 6.27 1408.03 1203.45
2007 1841.80 6.81 1511.68 1307.10
Annual Rainfall
Evapotranspiration & Surface Runoff
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
0
400
800
1200
1600
2000
2400
2800
Am
ou
nt o
f W
ate
r (m
m)
Year
Recharge
Figure 7 Estimated GW recharge for 16 years (1992-2007) in the study area
20 Sajal Kumar Adhikary et. al. Estimating Groundwater Recharge into a Shallow Unconfined Aquifer……………….
19921993
19941995
19961997
19981999
20002001
20022003
20042005
20062007
0
400
800
1200
1600
2000
2400
GW
Re
ch
arg
e (
mm
)
Year
Present Study
Adhikary (2009)
Halcrow (1993)
Figure 8 Comparison of estimated GW recharge by different studies for studied aquifer
4. CONCLUSIONS
This study presents a modified water table fluctuation technique for estimating GW recharge by using only the
precipitation and water level information for a shallow unconfined aquifer in Bangladesh. The overall goal of
this study has been a synthesis of the results obtained by analyzing the GW level fluctuations and precipitation
records. By following this principle, the study establishes a data conservative approach of estimating GW
recharge in any shallow unconfined aquifer under concern. However, the accuracy of the results totally depends
on the base of water level monitoring data. This study reveals that the shallow GW in the study area is recharged
mainly from the precipitation sources. Besides, the annual average GW recharge in the studied aquifer is found
as 1413 mm for total 16 years period from 1992 to 2007, which is about 74 percent of the average annual
precipitation. However, the study concludes that the proposed data conservative methodology presented in this
study provides appropriate mechanisms for indirect estimation of the water losses such as evapotranspiration and
surface runoff, which will make this method useful in future hydrological computation for GW resource
assessment and management in the field of water resources engineering. Finally, the present study conclusively
proves that the proposed methodology described in this study will open a window of opportunity for any sorts of
initial level assessment of GW resources in a shallow unconfined aquifer in Bangladesh, where shortest possible
data and information are available.
ACKNOWLEDGEMENTS
The authors acknowledge the valuable data supports from the central database of the Bangladesh Water
Development Board (BWDB), Dhaka, Bangladesh. The authors express the special gratitude to the Editors for
Special Issue of Journal of Engineering Science (JES), and anonymous reviewers for their constructive
comments and valuable suggestions to improve the quality of this work.
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