Mean transit time and subsurface flow paths in a humid temperate headwater catchment with granitic bedrock

Abstract

Headwater catchments make a considerable contribution to the discharge of whole stream networks and mean transit time (MIT) is a fundamental factor affecting flow and storage in the hydrologic system. We estimated the MIT along with the identification of subsurface water flow processes using time series measurements of stable isotopes and major solutes in the water of a small mountainous catchment with granitic bedrock under a humid temperate climate. The MIT was estimated to be 1.1-1.7 years, 0.5-0.8 years, and 1.5 years for stream water at the catchment outlet and first-order tributaries, soil water, and shallow groundwater, respectively, based on sinusoidal variation in deuterium excess in the hydrologic components and precipitation. The MIT of first-order streams was negatively correlated with the topographic gradient of the sub-catchments. The SiO2 (aq) and Cl- concentrations in first-order streams were used to discriminate dominant subsurface flow contributing to the streams, depending on the topographic gradient. Soil water had a higher seasonal variability in SiO2 (aq) and Cl- concentrations, which were controlled by rainfall infiltration, whereas shallow groundwater had significantly lower seasonal variability, with a depleted isotopic signature, suggesting the contribution of lateral water flows from the upgradient area. The increased seasonal variation in SiO2 (aq) and Cl- concentrations with increasing MTT in first-order streams indicated that the storage capacity and flow paths in the soil layer were determining factors for subsurface water flow in hillslope areas. Seasonal variation in the SiO2 (aq) concentration at the catchment outlet revealed a major contribution of subsurface flow to stream discharge as lateral flow through the soil-bedrock interface and an interflow through the soil layer in the dry and rainy seasons, respectively. Through the integration of the MTT and seasonal variation in hydrogeochemical parameters in soil water, stream water, and groundwater, this study demonstrates that soil depth and the soil-bedrock interface were the main controls on streamflow generation in a headwater catchment with a crystalline bedrock.