Groundwater Recharge Estimation in Thepkasattri Watershed Phuket, Thailand

Abstract

Groundwater recharge is an important issue for places like Phuket Island, where the fresh water supply is heavily depending on groundwater. Therefore, considering the growing demand y for freshwater and rapidly increasing the population. It is important to investigate the spatial and temporal variation in recharge. In this study groundwater recharge estimation and seasonal groundwater, recharge characterization has been done in Thepkasattri sub-district. Groundwater recharge estimation was done using the chloride mass balance (CMB) and the water table fluctuation method (WTF) method. While seasonal groundwater recharge characterization was done using time- lapse ERT in two locations. The objective of this study is to quantify annual recharge in Thepkasattri and to investigate the seasonal infiltration characteristic through the vadose zone. Based on the CMB method recharge in Thepkasattri varies between 443 to 1439 mm/yr. with mean recharge of 1172 mm/yr. The mean recharge accounts for 47% of the annual rainfall in Thepkasattri. On the other hand, recharge estimated using the WTF method varies from 349 to 552 mm/yr. The average is 418 mm/yr. and this represents 19% of annual precipitation. The recharge estimated using the WTF method falls in the range of 443 to 1439 mm/yr. estimated using the CMB method. There is some uncertainty associated with the WTF method due to lack of water table data that represent the calendar year. Evapotranspiration was estimated using concentration factor (CF) and found to be 53% of annual rainfall. Moreover, spatial variability of recharge is strongly related to the hydraulic property of the unsaturated zone and the land cover of the area. Seasonal groundwater recharge characterization using time-lapse ERT was done in two lines at three dates (March, June, and September in 2018). The location of ERT lines was one on vadose zone composed of alluvial deposit and the second is in weathered granite; the lines were oriented at an angle to the nearby passing stream drainage. Time-lapse ERT in line one shows -100% change in resistivity from dry season to wet season. This change was attributed due to water infiltration through the unsaturated zone and the stream. While in line two -75% of resistive decrease was observed. The change in resistivity was attributed to surficial infiltration through the vadose zone. Based on the relationship established by earlier researchers the decrease in resistivity indicates the degree of water saturation. Therefore, change in resistivity in both locations further indicates the degree of water saturation in the vadose zone. The study further suggests recharge in the study area is episodic that has a direct effect of climatic condition.