• Journal of Korea Water Resources Association
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• v.38 no.7 s.156
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• pp.517-526
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• 2005

In Korea, the methods of estimating groundwater recharge can categorized into two groups. One is baseflow separation method by means of groundurater recession curve, the other is water level fluctuation method by using the data from groundwater monitoring wells. Baseflow separation method is based on annual recharge and lumped concept, and water-table fluctuation method is largely dependent on monitoring wells rather than water budget in watershed. However, groundwater recharge rate shows the spatial-temporal variability due to climatic condition, land use and hydrogeological heterogeneity, these methods have various limits to deal with these characteristics. For this purpose, the method of estimating daily recharge rate with spatial variability based on distributed rainfall-runoff model is suggested in this study. Instead of representative recharge rate of large watershed, the subdivided recharge rate with heterogeneous characteristics can be computed in daily base. The estimated daily recharge rate is an advanced quantity reflecting the heterogeneity of hydrogeology, climatic condition, land use as well as physical behaviour of water in soil layers. Therefore, the newly suggested method could be expected to enhance existing methods.

• The Journal of Engineering Geology
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• v.25 no.3
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• pp.403-412
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• 2015

We summarize the status of exploitable groundwater reserves in Korea based on reports of the National Basic Groundwater Plan, and review methods for estimating groundwater recharge rates, as recharge is a key factor in the estimation of exploitable groundwater reserves. We also outline the various methods used to assess exploitable groundwater reserves in previous groundwater investigation reports. Regarding advancements in the estimation of exploitable groundwater, we recommend that enhanced estimation methodologies (e.g., the water balance method and the advanced water table fluctuation method) and sustainable groundwater management concepts be adopted in the near future.

• Journal of Korea Water Resources Association
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• v.39 no.4 s.165
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• pp.299-311
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• 2006

In this paper it is outlined the methodology of estimating the parameters of water balance analysis method for calculating recharge, using ground water level rises in monitoring well when values of specific yield of aquifer are not available. This methodology is applied for two monitoring wells of the case study area in northern area of the Jeiu Island. A water balance of soil layer of plant rooting zone is computed on a daily basis in the following manner. Diect runoff is estimated by using SCS method. Potential evapotranspiration calculated with Penman-Monteith equation is multiplied by crop coefficients($K_c$) and water stress coefficient to compute actual evapotranspiration(AET). Daily runoff and AET is subtracted from the rainfall plus the soil water storage of the previous day. Soil water remaining above soil water retention capacity(SWRC) is assumed to be recharge. Parameters such as the SCS curve number, SWRC and Kc are estimated from a linear relationship between water level rise and recharge for rainfall events. The upper threshold value of specific yield($n_m$) at the monitoring well location is derived from the relationship between rainfall and the resulting water level rise. The specific yield($n_c$) and the coefficient of determination ($R^2$) are calculated from a linear relationship between observed water level rise and calculated recharge for the different simulations. A set of parameter values with maximum value of $R^2$ is selected among parameter values with calculated specific yield($n_c$) less than the upper threshold value of specific yield($n_m$). Results applied for two monitoring wells show that the 81% of variance of the observed water level rises are explained by calculated recharge with the estimated parameters. It is shown that the data of groundwater level is useful in estimating the parameter of water balance analysis method for calculating recharge.

• Journal of Korea Water Resources Association
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• v.53 no.7
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• pp.533-543
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• 2020

In order to provide information for proper management of groundwater resources, it is necessary to estimate the rise time of groundwater level by calculating the delay time between the time series of precipitation and groundwater level and to understand the characteristics of groundwater level variation. In this study, total delay time (TDT) and cross correlation coefficient between the moving averaged precipitation generated by using the moving average method to take into account the preceding precipitation and the groundwater level were calculated and analyzed for the nine groundwater level monitoring wells in the Pyoseon watershed in the southeast of Jeju Island. As a result, when the moving averaged precipitation was used, the correlation with the groundwater level was higher in all monitoring wells than in the case of using the raw precipitation, so that it was possible to more clearly estimate the delay time between precipitation and groundwater level. When using the moving averaged precipitation, it had cross correlation coefficients of up to 0.57 ~ 0.58 with the time series data of the groundwater level, and had a relatively high correlation when considering the preceding precipitation of about 24 days on average. The TDT was about 32 days on average, and it was confirmed that the consideration of preceding precipitation plays an important role in estimating the TDT because the days of moving averaged precipitation greatly influences the calculation of the TDT. In addition, through the use of moving averaged precipitation, we found an error in estimating the TDT due to the use of raw precipitation. Through the method of estimating the TDT used in this study and the use of the R code for estimating the TDT presented in the appendix of this paper, it will be possible to estimate the TDT for other regions in the future relatively easily.

• Economic and Environmental Geology
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• v.22 no.4
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• pp.381-393
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• 1989

Hydrogeological modelling was performed to evaluate groundwater flow system in Igsan Area. The study area extends over $790km^2$. The geology consists of Jurassic Daebo granite and gneissose granite and Precambrian metamorphic rocks. The capability of pumping yield is the highest in gneissose granite region among them due to comparatively thick weathered zone with thickness ranging from 10m to 25m. The Colorado State University Finite Difference Model was used for the model simulation. The model was divided into 28 rows and 31 columns with variable grid spacing. The model was calibrated under steady-state and unsteady-state conditions. In the steady-state simulation, the model results were compared with measured water table contours in September 1985 with determining hydraulic conductivities and net recharge rates during rainy season. Unsteady state simulation was done to know the aquifer response due to groundwater abstraction. The non- steady state calibration was conducted to determine the distribution and magnitudes of specific yields and discharge/recharge rates during dry season as matching water level altitudes in May 1986. The calibrated model was used to simulate water level vaiation caused by groundwater withdrawal and natural recharge from 1 October, 1985 until 30 September, 1995. The calibrated model can be used to groundwater development schemes on regional groundwater levels, but it cannot be used to simulate local groundwater level change at a specific site.

• Economic and Environmental Geology
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• v.44 no.3
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• pp.239-246
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• 2011

Groundwater in small islands is used as main water resource but the overuse of groundwater may cause seawater intrusion and temperature decrease in geothermal wells. This study aimed to characterize the hydrogeology of Maeum-ri area in Seokmo Island of Ganghwagun using long-term monitoring at groundwater wells and geothermal wells. In the monitoring period seasonal water level change, consistent drop or increase of water levels are not detected. The groundwater temperature about 10m below ground surface shows year cycle variation having two to five months difference with ambient temperature cycle. The storativity was calculated by tidal method. The storativity estimated by adapting tidal efficiency factor showed some larger values than that by using tidal time lag. The result suggested that the tidal method assuming several assumptions on aquifer condition may produce broad ranges but the calculated ranges at this application are reasonable. The similar shape of groundwater level change and tidal effects was observed at several wells clustered east-south-east direction which may implicate the distribution of vertical fracture system strongly related with groundwater flow channels. The applied methodology and study results will bc valuable to evaluate optimal pumping rate for the preservation of groundwater resources, and to manage geothermal development.

• Journal of Soil and Groundwater Environment
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• v.11 no.5
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• pp.9-19
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• 2006

The accurate estimation of groundwater recharge is important for the proper management of groundwater systems. The widely used techniques of groundwater recharge estimation include water table fluctuation method, baseflow separation method, and annual water balance method. However, these methods can not represent the temporal-spatial variability of recharge resulting from climatic condition, land use, soil storage and hydrogeological heterogeneity because the methods are all based on the lumped concept and local scale problems. Therefore, the objective of this paper is to present an effective method for estimating groundwater recharge with spatial-temporal variability using the SWAT model which can represent the heterogeneity of the watershed. The SWAT model can simulate daily surface runoff, evapotranspiration, soil storage, recharge, and groundwater flow within the watershed. The model was applied to the Musimcheon watershed located in the upstream of Mihocheon watershed. Hydrological components were determined during the period from 2001 to 2004, and the validity of the results was tested by comparing the estimated runoff with the observed runoff at the outlet of the catchment. The results of temporal and spatial variations of groundwater recharge were presented here. This study suggests that variations in recharge can be significantly affected by subbasin slope as well as land use.

• Proceedings of the Korean Society of Soil and Groundwater Environment Conference
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• 2002.09a
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• pp.28-31
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• 2002

The water level fluctuation (WLF) method is a conventional method for quantifying groundwater recharge by multiplying the specific yield to the water level rise. A 2-D unconfined flow model with a time series of the recharge rate is developed. It is used for elucidating the errors of the WLF method which is implicitly based on the tank model where the horizontal flow in the saturated zone is ignored. Simulations show that the recharge estimated by the WLF method is underestimated for the observation well near the discharge boundary. This is due to the fact that the hydraulic stress resulting from the recharge is rapidly dissipating by the horizontal flow near the discharge boundary Simulations also reveal that the recharge was significantly underestimated with increase in the hydraulic conductivity and the recharge duration, and decrease in the specific yield.

• Spatial Information Research
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• v.20 no.3
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• pp.39-50
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• 2012

The objective of this study is the quantitative analysis of climate change effects by performing several statistical analyses with hydrometeorological data sets for past 30 years in Geum river watershed. Temperature, precipitation, relative humidity data sets were collected from eight observation stations for 37 years(1973~2009) in Geum river watershed. River level data was collected from Gongju and Gyuam gauge stations for 36 years(1973~2008) considering rating curve credibility problems and future long-term runoff modeling. Annual and seasonal year-to-year variation of hydrometeorological components were analyzed by calculating the average, standard deviation, skewness, and coefficient of variation. The results show precipitation has the strongest variability. Run test, Turning point test, and Anderson Exact test were performed to check if there is randomness in the data sets. Temperature and precipitation data have randomness and relative humidity and river level data have regularity. Groundwater level data has both aspects(randomness and regularity). Linear regression and Mann-Kendal test were performed for trend test. Temperature is increasing yearly and seasonally and precipitation is increasing in summer. Relative humidity is obviously decreasing. The results of this study can be used for the evaluation of the effects of climate change on water resources and the establishment of future water resources management technique development plan.

• Journal of Soil and Groundwater Environment
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• v.11 no.3
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• pp.52-58
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• 2006

Kim and Lee (2005) suggested Minimum Entropy Deconvolution (MED) to estimate the temporal sequence of the relative recharge. However this study by Kim and Lee (2005) was just related to the verification of the conceptual approach with MED. In this study, we try to characterize the applicability of MED in the case of spatially heterogeneous recharge (distance from recharge area). Simulated results were recorded with some specific sampling points. Estimated results from this study show higher than 0.8 in cross-correlation with the original recharge sequence. In addition, the physical and mathematical meanings of the applied filter length was also investigated. It was revealed that the length of filter is highly related to the spatial distance between recharge area and the monitoring site, and the apparent shape of hydraulic head change.