Estimation of Groundwater Recharge Rate
at Chojeong Watershed in Korea
Debnath Jagabandhu*. Kim, Tai Cheol** . Lee, Duk joo***
Groundwater recharge is the process by which water percolates down the soil and reaches the
water table either by natural or artificial method. Groundwater recharge is mostly depends on
the precipitation and computing of the recharge which is important to obtain the optimal safe
discharge or groundwater that should not exceed the recharge. Quantification of the rate of
groundwater recharge is a prerequisite for efficient groundwater resource management. It is
particularly important in regions with large demands for groundwater supplies, where such
resources are the key to economic development. However, the rate of aquifer recharge is one of
the most difficult factors to measure in the elevation of groundwater resources. Estimation of
recharge by any method is normally subject to large uncertainties and errors. In this paper, we
constructed an empirical relationship to estimate the groundwater recharge rate from annual
precipitation. This paper also describes that the determining relative errors of groundwater
recharges between the groundwater balance approach using DAWAST model (Kim, Tai cheol,
1992) and our proposed empirical equation are very small that is below 4%. An attempt has
been made to apply this empirical relationship for computing the groundwater recharge from
annual precipitation at Chojeong watershed area based upon groundwater balance study that
has been carried out of 3 years observations.
Key words: Groundwater, Water Balance, Recharge, Monsoon, Watershed.
* Graduate Student, Division of Agricultural Engineering, Chungnam National Univ.
** Professor, Division of Agricultural Engineering, Chungnam National Univ., Daejeon
*** Researcher, Agricultural Science Institute, Chungnam National Univ., Daejeon
** Corresponding Author, Tel.: +82-42-821-5797
E-mail address: firstname.lastname@example.org
* E-mail : email@example.com
Groundwater recharge may be explained as the process where by the amount of water
present in or flowing through the interstices of the sub-soil increases by natural or artificial
means. Rainfall is the main source for replenishment of moisture in the soil water system and
recharge to groundwater. The amount of water that may be extracted from an aquifer without
causing depletion is preliminary dependent upon the groundwater recharge. Moisture
movement in the unsaturated zone is controlled by suction pressure, moisture content and
hydraulic conductivity relationships. The amount of moisture that will eventually reach the water
table is defined as groundwater recharge. The amount of this recharge depends upon the rate
and duration of rainfall, the subsequent conditions at the upper boundary, the antecedent soil
moisture conditions, the water table depth and the soil type. While estimating groundwater
recharge, it is essential to have a good idea of the different recharge mechanisms and their
importance in the study area. In groundwater resources it is frequently necessary to decide how
much of the recharge to a groundwater unit is required to maintain the base flows in the rivers
of the unit. Groundwater recharge enters the aquifer in a very irregular way both in terms of
seasonal variation and the variation between years. As the water is transferred to the rivers
some of this variation is smoothed out so that the base flow exhibit less severe variation.
Estimating the rate of aquifer replenishment is probably the most difficult one of all
measures in the evaluation of groundwater resources. Estimates are normally and almost
indispensably subject to large error. No single comprehensive estimation technique can yet be
identified from the spectrum of those available, which gives reliable results. Recharge
estimation can be based on a wide variety of models which are designed to represent the actual
physical processes. Many methods are use for estimating the groundwater recharge such as
groundwater balance method, soil water balance method, zero flux plane method, one-
dimensional soil water flow model, inverse modeling technique, and isotope and solute profile
techniques. In these methods, water balance approach, essentially DAWAST model (Kim, Tai
cheol, 1992) study, is a viable method of establishing the groundwater recharge and for
evaluating the methods adopted for the quantification of discharge and recharge from the other
In this paper we designed a empirical relationship for calculating groundwater recharge of
one study area. For this estimation we only need one parameter, which is annual precipitation
II. GROUNDWATER RECHARGE ESTIMATION
Rainfall is the vital source of groundwater recharge in the country. The most commonly used
methods for estimation of groundwater recharge in Korea is water balance method. Based on
the studies undertaken by different scientists and organizations regarding correlation of
groundwater level fluctuation and rainfall, some empirical relationships have been derived for
computation of recharge to groundwater from rainfall. One such relationship pertinent to the
study area (Chojeong watershed) is given below.
Based on the water level fluctuations and rainfall amounts, Chaturvedi in 1936, derived an
empirical relationship to arrive at the recharge as a function of annual precipitation.
R=2.0(P-15) ……………………. (1)
Where, R = Net recharge due to precipitation during the year (inch)
P = Annual precipitation (inch)
This formula was later modified by further work at the U. P. Irrigation research Institute in
India and the modified form of the formula is
R=1.35(P-14) ……………………… (2)
Chaturvedi formula has been widely used for estimations of groundwater recharge due to
rainfall. It may be noted that there is a lower limit of the rainfall below which the recharge due to
rainfall is zero. The percentage of rainfall recharge commences from zero at P = 14 inches,
increases up to 18% at P = 28 inches, and again decreases. The lower limit of rainfall in the
formula may account for the runoff, soil moisture deficit, and interception and evaporation
losses. The above relationship, tentatively proposed for Chojeong watershed area, needs to be
examined and established or suitably altered.
III. GROUNDWATER BALANCE METHOD
Water balance techniques have been extensively used to make quantitative estimates of
water resources. On the basis of the water balance approach, it is possible to make a
quantitative evaluation of water resources and its dynamic behavior under the influence of
man's activities. The study of water balance defined as the systematic presentation of data on
the supply and use of water within a geographic region for a specified period. With water
balance approach, it is possible to evaluate quantitatively individual contribution of sources of
water in the system over different time periods, and to establish the degree of variation in water
regime due to changes in components of the system.
The basic concept of water balance is,
Inflow (I) to the system - Outflow (O) of the system = Change in storage of the system
The general methods of components of computations of water balance include:
(1) Identification of significant components
(2) Evaluating and quantifying individual components, &
(3) Presentation in the form of water balance equation.
Considering the various inflow and outflow components, the groundwater balance equation
is given below and represented in figure 1:
ΔSg =P - Q - ET + ΔSs ………………………………… (3)
Where, ΔSg = Variation of groundwater storage
P = Precipitation
Q = Runoff
ET = Watershed Evapotranspiration
ΔSs = Groundwater inflow (or outflow) from (to) other watersheds.
Fig. 1 Watershed Map of Water Balance
The boundaries of an area usually studied, don't represent stream lines. That is, they are
not perpendicular to the equipotent lines. Hence, the inflow and outflow of groundwater crossing
the area's boundaries must be accounted in the balance equation. One of the factors influencing
the change in water table is specific yield (Sy) of the zone in which the water table fluctuations
occur. It has been recognized that Sy changes as the depth of water table changes. Furthermore,
it should be noted that if the water table drops, part of the water is retained by the soil particles;
if it rises air can be trapped in the interstices that are filled with water. Hence Sy for rising water
is, in general, less than for a falling water table.
All parameters of the water balance equation are computed using independent method
whatever possible. Computations of water balance parameters always involve errors, due to
shortcoming in the techniques used. The water balance equation therefore usually doesn't
balance, even if all its components are computed by independent methods. The discrepancy of
water balance is given as a residual term of the water balance equation and includes the errors
in the determination of the components and the values of the components which are not taken
into account. If it is not possible to obtain the value of a balance parameter by computation, the
parameter may be evaluated as a residual term in the water balance equation.
The water balance may be computed for any time interval. In areas where most of the
rainfall occurs in a part of year, it is desirable to conduct water balance study on part of year
basis that is for monsoon and non-monsoon period. Generally, the period for study in such
situation will be from the time of maximum water table elevation to the time of minimum water
table elevation as the non-monsoon period and from the time of minimum water table to the
time of maximum water table elevation as monsoon period. The monsoon and non-monsoon
periods may be taken as June to September and October to May next year respectively. It is
desirable to use the data of number of years preferably covering one cycle of a dry and wet
The complexity of the computation of the water balance tends to increase as the study area
is increased. This is due to a related increase in the technical difficulty of accurately computing
the numerous important water balance components. To apply equation (3) correctly, it is
essential that both the area and period for which the balance is assessed, be carefully selected.
IV. STUDY AREA
The study area was selected at Chojeong-ri in Korea. The watershed area is 28.3 sq. km., the
watershed slope 8% and main river length 6.75 km. The Chojeong watershed map is shown as
figure 1. The watershed area is located on 36｡ 38＇N latitude and 127｡ 27＇E longitude. This
watershed height is 215m above mean sea level. Climatically, the average annual rainfall is
approximately 1,280mm (512 inch). Around 70% of annual precipitation occurs in monsoon
season (mid June to mid September) according to the calculation of 10 years data. The
temperature varies from in this watershed was from 32｡C to -10｡C. For the data collection of
groundwater balance study, water gauge was installed on Seodang bridge and soil moisture
measurement instrument and automatic weather station were set near Biheung reservoir.
These three data have been collected since March 2001. The water level of stream was
measured in every hour by automatic gauge (WL-14). A set of soil moisture equipments (DIK-
321A) was installed to measure the soil moisture at the depths of 30cm, 50cm and 80cm in
every hour. The stream flow was measured with propeller type flow meter (BFM-001) twice a
month in normal period and several selected water levels in flood period.
Fig. 2 watershed map of Experimental site (Chojeong)
V. DETERMINATION OF GROUNDWATER RECHARGE
Part of the rain water that falls on the ground is infiltrated into soil. This infiltrated water is
utilized partly in filling the soil moisture deficiency and part of it is percolated down reaching the
water table. This reaching water is known as the recharge from the rainfall to the aquifer.
Recharge due to rainfall depends on various hydro-meteorological and topographic factors, soil
characteristics and depth to the water table.
The groundwater balance for the study area of Chojeong watershed was carried out
annually from the year 2001 to 2003. All parameters of groundwater balance equation were
estimated by the using of hydrological and meteorological data and the observed data. The
groundwater recharge of the study period was calculated by substituting these estimates into
the groundwater balance equation. Table 1 presents the groundwater recharge in annual base
and the corresponding recharge rate to the precipitation.
Table 1. Groundwater Recharge from Annual Precipitation
Annual Precipitation, P Groundwater Recharge from
Year Recharge Rate
(mm) (Inch) Precipitation (mm)
2001 525.50 210.20 69.3 0.132
2002 1,400.50 560.20 225.2 0.161
2003 1,297.00 518.80 191.5 0.147
It was observed that as the rainfall increases, the quantity of recharge also increases but the
increase is not linearly proportional. Recharge coefficient (based upon the annual precipitation)
was calculated as Recharge/Precipitation ratio. The recharge rate was found to vary from 0.132
to 0.161 during the study period.
VI. PROPOSED EQUATION FOR QUICK COMPUTING GROUNDWATER
The following proposed empirical relationship was derived by best fitting to the estimated
values of precipitation recharge and corresponding value of rainfall through the linear
RG = 0.0098(PA + 122.877) …………………………… (4)
Where, RG = Groundwater Recharge from Precipitation in Annual Base (mm)
PA = Annual Precipitation (mm)
Table 2. Relative errors with the Proposed Relationship
Annual Groundwater Recharge, RG (mm)
Annual Precipitation Relative
Year PA Groundwater balance Error
RG = 0.0098(PA +
(mm) study 1.367 (%)
2001 525.5 69.3 68.1 1.742
2002 1400.50 225.2 218.8 2.826
2003 1297.00 191.5 198.8 -3.798
Table 3. Relative Errors with Chaturvedi formula
Chaturvedi formula Modified Chaturvedi formula
Annual 0.4 0.5
Groundwater R=2.0(P-15) R=1.35(P-14)
Year balance study Relative Relative
P Groundwater Groundwater
(inch) Error Error
(inch) Recharge, (inch) Recharge, (inch)
2001 210.20 27.72 16.49 40.51 18.91 31.78
2002 560.20 90.08 24.87 72.39 31.55 64.97
2003 518.80 76.60 24.09 68.54 30.33 60.40
Table 2 represents that the relative errors (%) in the computing of the groundwater recharge
from the proposed empirical relationship as compared to the groundwater balance results.
During the study period, the relative errors were found to be less than 4%. On the other hand,
relative errors (%) computed from Chaturvedi formula (equation 1 and 2) were found to be quite
different as shown in Table 3. So, Equation (4) can be conveniently used for quick computation
of groundwater recharge from annual precipitation.
VII. SUMMARY AND CONCLUSION
The papers highlights that the empirical equation is quite related with annual rainfall. Based
upon the annual groundwater balance study for Chojeong watershed area, an empirical
relationship has been suggested for quick assessment of groundwater recharge from
precipitation with reasonable accuracy. Relative errors between the proposed empirical
relationship and groundwater balance study of the groundwater recharge were below 4% based
upon 3 years observations of the study area. The relative errors of the year 2001, 2002, and
2003 were 1.7%, 2.8%, and 3.8%, respectively.
In summary, the estimation of groundwater recharge is normally subject to large errors.
There is no single comprehensive estimation technique that can be yet identified from the
spectrum of those available, which gives most accurate results. Hence, it is desirable to apply
more than one method based on independent input data and to verify with a long period of data.
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