*4.1. Improvement of SWAT-Twn Model for Watershed Sediment Production*

In the nearly-flat upland watersheds where surface runoff seldom occurs, sediment transported by surface runoff is usually significantly over predicted in a model [65]. Since the Chenyulan watershed is mountainous with hilly slope, modeling the sediment yield and transport responses with considering unique hydrological responses is important. Compared to SWAT 2016, the major modifications in SWAT-TUSLE are C factor and LS factor. The main land use in the Chenyulan watershed is forest, for which C factor is 0.2 by NDVI calculation and much greater than the SWAT default C factor (0.001). Although the decrease in LS factor would cause less sediment yields, the increase in sediment yield by the adjusted C factor was much greater than the decrease in sediment yield by the adjusted LS factor. Thus, it was found that some sub-basins (i.e., sub-basins no. 5, 8, 9, 12, 13, 16–19) where forest is dominated (>70%) generated more sediment yields (Figure 10 and Table 10). Moreover, some agricultural-dominated (>30%) sub-basins (i.e., sub-basins no. 1, 3, 7, 10, 11 15) are mostly located in central and northern parts of the watershed, where slope is less steep. Thus, the adjusted LS factor has a relatively small impact on sediment yields from agricultural-dominated sub-basins. Great change in sediment yield was found in the south part of the watershed, where both forest and grassland are dominated, as the adjusted C factors of pasture and urban are greater than the SWAT default ones. Since the landslide volume equation is the additional improvement in SWAT-Twn from SWAT-TUSLE, the sediment yields by SWAT-Twn were increased by 2%, 40% and 3% in the sub-basins no. 1, 7, and 17, respectively, where the landslide area is greater than 5% of the sub-basin area.

**Figure 10.** Sediment yields at sub-basins simulated by three models.
