Assessing the hillslope-channel contributions to the catchment sediment balance under climate change

Abstract

Previous research suggests that channels have a significant contribution to the total sediment balance in large catchments. Channels are also very sensitive to changes in land use and management, with implications for channel morphodynamics and the quantity of sediments and nutrients flowing through the system. Here we present a new channel module for the coupled hydrology-soil erosion model SPHY, with the aim to quantify the contribution of hillslope and channel erosional processes to the total sediment balance in large catchments. SPHY simulates all relevant hydrological and soil erosion processes, including infiltration excess surface runoff and soil detachment by raindrop impact and runoff. We developed a novel channel module that simulates river hydraulics and morphodynamics in the channel network, while accounting for sedimentation in reservoirs and check dams. The channel module requires channel characteristics as input, including channel width, channel depth and bed material. The model was applied to a Mediterranean catchment (316 km2) in Southeast Spain, where channel characteristics were obtained from a combination of field measurements and GIS analyses. The model was calibrated using observed discharge, hillslope erosion and check dam sedimentation. Model validation focused on how channel morphodynamics changed in response to reforestation and check dam construction in the period 1956-2001. The model successfully simulates the observed changes in channel morphology, including an overall increase of channel morphodynamics and channel incision downstream of the check dams. Subsequently, the model was applied in a climate change impact assessment, to show how the sediment balance is projected to change under future climate conditions, characterized by an increase of extreme precipitation. The climate change simulations show that rill erosion is projected to increase, while channel erosion decreases. However, channel erosion still remains the main contribution to the total sediment yield in the catchment, which highlights the importance of accounting for channel morphological processes in large-scale erosion assessments.