Estimating mountain block recharge to downstream alluvial aquifers from standard methods

The purpose of the study is to assess the applicability in estimating the mountain block recharge (MBR) to the downstream alluvial aquifers in the sub-tropical area using baseflow separation and rainfall infiltration methods. The Choushui-Wu River basin, the largest groundwater region in Taiwan was the study area. The high slope mountainous catchment located in the upstream of Choushui-Wu River basin, act as an important conduit in conveying surface and subsurface runoff to the recharge of the downstream alluvial aquifers. Geographic Information Systems (GISs) was applied to facilitate the estimation processes. The estimated MBR using the baseflow separation method was 1.08 × 109 m3/year of which Wu River and Choushui River basins comprised 0.27 × 109 m3/year (25.2%) and 0.81 × 109 m3/year (74.8%), respectively. These results are similar to the previous O18 isotopic study indicating that 22% and 78% were from the Wu River and Choushui River basins, respectively. Moreover, the estimated amount of lateral flow using C14 technique from the upstream of Choushui River basin was 0.83 × 109 m3/year, which is close to the result (0.81 × 109 m3/year) of this study. For comparison, groundwater recharged by rainfall infiltration in the mountainous catchment was derived from the precipitation, evapotranspiration, land use and soil types of the region. The estimated MBR by the rainfall infiltration method is 1.06 × 109 m3/year, which is close to 1.08 × 109 m3/year determined by the baseflow separation method. These results are also similar to the groundwater hydrograph analysis. The proposed methods show simple and efficient computation and do not require complex hydrological modeling and detailed knowledge of soil characteristics. They can reasonably estimate the lateral boundary influx contributing from the MBR and are thus applicable to estimate the MBRs in other sub-tropical regions.