In this work, the delay time for groundwater recharge was estimated by comparing simulated recharges by means of SWAT(Soil and Water Assessment Tool) model and WTF(Water Table Fluctuation) method. The delay time for groundwater recharge means that the time when the water from rainfall travelled through vadose zone just after getting out of soil zone bottom. As measuring this delay time is almost impossible, we used to compare the estimated values from modeling(SWAT) and analytic method(WTF). The test site is Hancheon watershed which has 8 groundwater measurement stations. The results show that the altitude has a linear relationship with the estimated delay time values. To validate these results, we conducted corelation analysis between transformed groundwater levels and observed ones. The results showed that computed groundwater levels have good corelation(R2=0.97, 0.87, respectively). The estimated delay time would be used for the groundwater behaviour characteristics in vadose zone. As recharge rates vary according to the height, the delay time is thought to be an import variable for the proper groundwater recharge estimation.

In this study, estimation methods for actual evapotranspiration have been studied using the concept of potential and actual evapotranspiration. Among the diverse estimation methods, SWAT-K application is chosen for hydrological modeling. For Jeju island we have characterized annual and monthly evapotranspiration using SWAT-K. In the results, simulated potential evapotranspiration reached to the 91% of small pan evaporation. With respect to the temperature lapse rate(-6 ℃/km) depending on the altitude of Halla mountain, evapotranspiration rate decreased by 7.5% compared to the status when the temperature data from the Jeju weather station were applied to the watershed. As the average of annual rainfall increased, potential evapotranspiration was increased, actual evapotranspiration was, however, decreased.

To investigate groundwater variation characteristics in the Hancheon watershed, Jeju Island, an integrated hydrologic component analysis was carried out. For this purpose, SWAT-MODFLOW which is an integrated surface-groundwater model was applied to the watershed for continuous watershed hydrologic analysis as well as groundwater modeling. First, ephemeral stream characteristics of Hancheon watershed can be clearly simulated which is unlikely to be shown by a general watershed hydrologic model. Second, the temporally varied groundwater recharge can be properly obtained from SWAT and then spatially distributed groundwater recharge can be made by MODFLOW. Finally, the groundwater level variation was simulated with distributed groundwater pumping data. Since accurate recharge as well as abstraction can be reflected into the groundwater modeling, more realistic hydrologic component analysis and groundwater modeling could be possible.