*3.2. Calibration of SPARROW Models and Uncertainty Analysis*

#### 3.2.1. Calibration of SPARROW Models

NWLS calibration of SPARROW models in 2017 was executed, based on 58 and 144 water quality stations in PLB and HRB, respectively. The correlation coefficient (*R* 2 ), the mean square error (MSE), the root mean square error (RMSE), and the NSE (see Table 2) were assessed to evaluate the performances of the models.

**Table 1.** Results of SPARROW models.


For the results of SPARROW models for PLB and HRB, the values of *R* <sup>2</sup> were 0.89 and 0.53, respectively, which verified the acceptability of SPARROW models. MSE were 0.25 and 1.91, respectively. The RMSE for the models were around 0.50 and 1.38, respectively. Although the RMSE of HRB was significantly higher, similar values have been reported by other studies (RMSE = 1.40 [54], RMSE = 0.96 [16]). In addition, the NSE values were similar to the *R* <sup>2</sup> values, which indicated the robustness of the models. Figures 5a and 6a are scatterplots of predicted values versus observed values, in which the majority of the points are located in the vicinity of bisection. The scatterplots of residuals illustrated homoscedasticity (Figures 5b and 6b). The aforementioned analysis implied rational performance of the SPARROW models.

**3. Results** 

*3.1. Calibration and Validation of GWLF Model* 

ble predictive capability of GWLF to estimate streamflow.

The period from 2015 to 2016 was chosen for model calibration, and 2017 was used

for model validation of streamflow. The parameters used during calibration were recession coefficient, seepage coefficient, and SCS curve numbers of different land use types. The optimized parameters are listed in Table S3. The Nash–Sutcliffe efficiency (NSE) is provided to analysis the model performance. Figure 4 shows the results of calibration and validation at seven gauged stations in PLB. The NSE of calibration ranges from 0.67 to

**Figure 4.** Modeled versus Observed values of streamflow and NSE at (**a**) Qiujing, (**b**) Meigang, (**c**) Wanjiabu, (**d**) Hushan, (**e**) Waizhou, (**f**) Dufengkeng, and (**g**) Lijiadu. **Figure 4.** Modeled versus Observed values of streamflow and NSE at (**a**) Qiujing, (**b**) Meigang, (**c**) Wanjiabu, (**d**) Hushan, (**e**) Waizhou, (**f**) Dufengkeng, and (**g**) Lijiadu.
