*2.6. Sediment Load Estimation*

In order to calibrate the daily sediment load, we estimated the observed daily sediment load by using the sediment rating curve, which describes the relationship between sediment concentration and water discharge. Since the Chenyulan watershed is a small mountainous watershed in Asia, we adopted the sediment rating curve method suggested by [64], who conducted studies in small mountainous watersheds in Japan.

We collected discontinuous data of sediment concentration and corresponding instant streamflow at the Nemoupu gauging station from 2004 to 2015 with a total of 338 data points. Both streamflow and sediment concentration data were first converted into logarithm and fitted with the linear regression model. A good relationship (r = 0.75) between sediment concentration and streamflow was found (Figure 6). Thus, the sediment rating curve was used for estimating the daily mean sediment concentration with the observed daily streamflow data, and the estimated daily sediment load could be further calculated.

**Figure 6.** Sediment rating curve.

#### **3. Results**

In this study, we compared three SWAT models, which are the official SWAT 2016 (version 664), SWAT-TUSLE and SWAT-Twn. The SWAT-TUSLE was modified with TUSLE which calculates C factor based on NDVI and L factors based on the slope. The SWAT-Twn was the integration of SWAT-TUSLE and landslide volume equation. Since we did not modify the streamflow-related equations in SWAT, the streamflow simulations are the same for all these three models. We first calibrated the official SWAT 2016 model for daily streamflow and compared the performance of sediment load simulations from SWAT 2016, SWAT-TUSLE, and SWAT-Twn.
