Spatial variability in the flowpath of hillslope runoff and streamflow in a meso-scale catchment

Recent studies have reported that not only water travelling through the soil layer but also emerging from fractured/weathered bedrock, contributes to hillslope runoff from steep wet hillslopes. Therefore, discharge is derived from a variety of hillslope sources. However, data are often lacking about how spatial variability in the water movement in bedrock on hillslopes affects hydrological behaviours in individual catchments because most previous studies have focused on single hillslopes in individual catchments. Therefore, we began by examining spatial variability in the bedrock groundwater contribution to hillslope runoff using a data set from Fudoji (central Japan), which has uniform bedrock geology, soil type and land use. We found that most hillslope runoff within the meso-scale catchment (4.27 km(2)) was a mixture of water flowing through the soil layer (subsurface flow) and water emerging from bedrock (groundwater flow). The depths of water sources (flowpath) that contributed to hillslope runoff varied greatly, even though the catchment had uniform bedrock geology, soil type and land use. Furthermore, we examined how bedrock groundwater affected hydrological behaviour in the catchment and found that the streamflow of first-to sixth-order streams were a mixture of water from the soil layer and bedrock groundwater, indicating that the end member of streamflow was the same as hillslope runoff in terms of the depth of flowpath. We also found that the mixing ratio of water from the soil layer and bedrock groundwater in first-order streams exhibited a significant spatial variation, but that the mixing ratio in third-to sixth-order streams did not. This indicates that depth of flowpath is a key component for describing hillslope and catchment hydrological responses. Copyright (C) 2010 John Wiley & Sons, Ltd.