Applicability of modified SWAT model (SWAT-Twn) on simulation of watershed sediment yields under different land use/cover scenarios in Taiwan

Climate change leads to increasing intensity and frequency of extreme rainfalls, especially in Taiwan with steep slopes and rapid currents. Heavy rainfalls trigger serious erosion and landslides on hillslopes, which increase sand concentration in rivers, and thus affect the water quality of reservoirs and the ecohydrological functions of rivers. We take the Zhuoshui River basin as an example and applied the modified Soil Water Assessment Tool (SWAT) model, SWAT-Twn, to simulate sediment in the basin. In SWAT-Twn, estimation of sediment yield is carried out by integrating the Taiwan Universal Soil Loss Equation (TUSLE) and the landslide simulation. Results of daily streamflow simulation showed that the model performances were above the satisfactory level, while simulations of daily sediment loads showed that the SWAT-Twn model performed better than the official SWAT (SWAT664), in terms of PBIAS of - 46.6 to 16.0% (SWAT-Twn) and - 1.2 to - 107.0% (SWAT664). Two scenarios of land use/cover, scenario 1 with fixed land use/cover and scenario 2 with updated land use/cover in each year, were applied to simulate annual sediment in the river basin for investigating the effects of landslide area variation on sediments. Results of sediment simulation under the two scenarios showed that although updating landslide area may facilitate sediment yield simulation at the subbasin level, the sediment transport equation, Bagnold equation, does not reflect the variation in sediment loads in the watershed. With further modifications, SWAT-Twn is expected to be an effective tool for simulating the impacts of landslide on sediment loads in the watersheds with rainfall-induced landslide.