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					                     Estimation of Groundwater Recharge Rate

                            at Chojeong Watershed in Korea



                   Debnath Jagabandhu*. Kim, Tai Cheol** . Lee, Duk joo***



                                          Abstract

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
    Fax: +82-42-825-9889
    E-mail address: dawast@cnu.ac.kr
   * E-mail : deb_engg@yahoo.com
  I. INTRODUCTION

  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

sources.

   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

(PA).
   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.



Chaturvedi Formula

   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.


                                  0.4
                    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


                                        0.5
                      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

year.

   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

RECHARGE


   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

regression technique.


                                             1.367
                 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
                                                            Proposed Relationship
   Year               PA           Groundwater balance                                   Error
                                                              RG = 0.0098(PA +
                     (mm)                  study                         1.367            (%)
                                                                122.877)
   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)
          Precipitation
 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.




REFERENCES

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2. Glenn O. Schwab, Richard K. Frevert, Talcott W. Edminster & Kenneth K. Barnes " Soil

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3. Kumar C. P., 1977. "Estimation of Ground Water Recharge". 1st Journal of Hydraulic

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10. Thornthwaite C. W., 1948. "An Approach towards a Rational Classification of Climate".

   Geogr. Rev.; Vol.124, pp229-241.

				
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