*3.1. Streamflow Calibration*

The best results from the automated and manual calibration at the outlet of GCEW are shown in Figure 4 and the related values of statistical metrics are in Table 3. Overall, both automatically and manually calibrated hydrographs exhibit a satisfactory performance with a high correlation coefficient (>0.90), NSE (>0.70) and a low RMSE (<8 m3/s). However, total amount of runoff is overestimated by both manual and automated calibration. While the automatically calibrated total volume of runoff is closer to observation, manually calibrated hydrograph fits better with the observed one over the high flow part of the measured hydrograph around the peaking time. As GCEW is self-drained watershed, we assume that large river discharge corresponds to large overland runoff flux. With the exponential relationship between overland runoff flux and transport capacity (Equation (8)), the sediment concentration and sediment flux during high flow periods should be much larger than during low flow periods. In this case, during a single rainfall event, the sediment concentration, flux, and total yield ought to be dominated by high flow period of the runoff event. (The observed sediment concentration and sediment flux graphs shown in Figure 5a,b justified this assumption as the main sediment event only lasted two hours, which corresponds to the high flow duration in the observed hydrograph shown in Figure 4). In this case, although the total volume from the automated calibration is closer to observation than the manually calibrated one, we chose to use the manually calibrated hydro-parameters to drive the sediment model since the high

flow part of the manually calibrated hydrograph is much closer to observation. The values of manually calibrated hydro-parameters are shown in Table 4.

**Figure 4.** Measured (dash line), manually calibrated (solid line) and automatically calibrated (dot line) streamflow at MSGC01 for calibration event. Time series of rainfall intensity is also shown.

**Figure 5.** Measured (dash line) and simulated (solid line) sediment concentration (**a**), and sediment flux (**b**) at MSGC01 for calibration event.


**Table 3.** Performance of the streamflow simulation for calibration event.

