calibration and validation
The total numbers of observed storm-runoff events in watershed XDG for calibration and validation were 47 and 29, respectively. For the calibration dataset, the optimized parameters for the tested methods are given in Table 2. Table 3 presents a comparison of the overall performance of the models using these datasets based on statistical indexes.
4.1.1 The original SCS-CN method
Figure 2 shows the predicted runoff values plotted against the corresponding measurement values for the calibration and validation datasets. The original SCS-CN method under-predicted the large and over-predicted as well as under-predicted some small storm-runoff events (Fig. 2a). The statistical indexes of linear regression in Table 3 also confirm this. A similar pattern either calibration or validation sub-datasets was observed, with regression line slopes of 0.83 and 0.77 and NSE values of -118.64% and -182.60% (Table 3), respectively, which are indicative of the poor performance of the conventional SCS-CN method.
4.1.2 The Huang et al. (2006) and Huang et al. (2007) method
Huang et al.(2006) and Huang et al.(2007) method incorporated the slope and soil moisture condition factors into the standard SCS-CN method, respectively. The prediction of large storm-runoff events was improved with a higher slope of 0.90 using the Huang et al. (2006) method as compared with the original SCS-CN method (0.80) (Fig. 2b). However, it also over-predicted some small storm-runoff events when taken slope factor account into the original SCS-CN method. Therefore, from Table 3 can be inferred that the Huang et al. (2006) method showed little improvement for the full dataset as confirmed by the value of theNSE which decreased from -148.17% (original SCS-CN method) to -177.63%.
Table 3 also shows that the NSE value (-9.8%) of Huang et al.(2007) method during the validation period was much improved as compared to the SCS-CN (-118.64%) and Huang et al.(2006) method (-133.00%), and it was visualized that the over-predicted by the latter two methods of some small storm-runoff events were mitigated. However, the regression slope of Huang et al.(2007) method deviates more from the 1 : 1 line than the original SCS-CN and Huang et al.(2006) method, which suggested the former still underestimated large and overestimated as well as underestimated small runoff events exist (Fig. 2c).