Spatiotemporal shallow and deep groundwater dynamics in a forested mountain catchment with diverse slope gradients, western Japan

Abstract

The sustenance of groundwater resources relies heavily on the process of groundwater recharge, which is projected to become scarce in the future due to climate change and increasing human demand. Estimating groundwater recharge remains an exceptionally challenging task for hydrologists, particularly in catchments characterized by forest-dominant land use and abundant steep hillslopes, where the storage and flow of groundwater are intricately tied to factors such as climate patterns and specific hydrological processes across diverse slopes. Here, we applied the Soil and Water Assessment Tool (SWAT) to evaluate the spatiotemporal variability and magnitudes of shallow groundwater discharge and deep groundwater recharge to improve our understanding of precipitation patterns and terrain impact on groundwater recharge. Our results revealed significant variability in shallow groundwater discharge and deep groundwater recharge across the catchment, with average annual values of 305 mm/year and 165.77 mm/year, respectively. The spatiotemporal variability of these groundwater components was primarily influenced by climatic conditions; however, within the catchment zones, slopes below 20% made significant contributions to the observed variations. The upper mountainous areas influenced by snow contributed significantly to the catchment's groundwater resources, accounting for 42% of the shallow groundwater discharge and 46% of the deep groundwater recharge. In contrast, urbanized lowland areas made much smaller contributions—13% of shallow groundwater discharge and 5% of deep groundwater recharge. Notably, during the driest year, both shallow groundwater discharge and deep groundwater recharge were insufficient to meet the daily water demand of the catchment, indicating the need for alternative water storage methods such as reservoirs. These findings offer insights for decision-making to enhance groundwater resource resilience in the region.