Perennial Groundwater Zone Formation Processes in Thin Organic Soil Layers Overlying Thick Clayed Mineral Soil Layers in a Small Serpentine Headwater Catchment

Abstract

Groundwater zone formation in the soil layers of a headwater catchment is an important factor that controls volumetric and chemical changes in streamflow; it also induces shallow landslides. Previous studies have suggested that the groundwater zone in soil layers generally forms transiently atop low-permeability layers in response to rainfall. This study focused on an unchanneled hollow in a serpentine headwater catchment, where a semi-perennial to perennial groundwater zone was observed in thin organic soil layers (OSLs) overlying thick clay mineral soil layers (CMLs), even during dry periods. We conducted detailed observations in this catchment to clarify the formation processes of the semi-perennial to perennial groundwater zone. The results showed that water is supplied from the CMLs to the OSLs as unsaturated upward flow in areas where the OSLs are dry. This water then accumulates in the downslope hollow, which sustains the groundwater zone in the OSLs during dry periods. The frequent and long-term occurrence of upward flow can be attributed to differences in the hydraulic properties of OSLs and CMLs. This process prevents the OSLs in the hollow from drying, presumably causes volumetric and chemical changes in streamflow, and reduces the stability of OSLs.