Dependency of hydrologic responses and recharge estimates on water-level monitoring locations within a small catchment

Estimation of groundwater recharge is essential for planning sustainable groundwater development. In this study, recharge ratios were estimated using the groundwater hydrographs obtained from monitoring wells located in a small catchment (0.256 km2). The catchment area is a triangular alluvial plain bounded by gentle hills and a large stream. For the recharge estimation, the modified water-table fluctuation (WTF) method was used. Although the study area is very small, the groundwater hydrographs were very different depending on the thickness of the unsaturated zone and the distance of the monitoring well from the main stream. Where the unsaturated zone is thick, the groundwater level changes smoothly in response to rainfall and resulting amplitude is small. In addition, the hydrographs closer to the stream showed quicker responses and larger fluctuations. Resultant recharge estimates showed a very wide variation between 2.5 and 20.1%. Compared with 12.7% estimated by other study mainly based on the water budget analyses, the recharge ratios were underestimated in upgradient area (hills) while they were overestimated in downgradient area (stream). Therefore, for representative and appropriate recharge estimation based on the groundwater hydrographs, it appears that the monitoring location is most important.