*2.3. GWLF Model*

GWLF is a combined distributed/lumped parameter model [52], which simulates monthly and annual streamflow, sediment transport, and associated nitrogen and phosphorus fluxes. GWLF was designed to be used in mixed-use watersheds (such as urban, multiple agricultural land uses, and forested land use). Streamflow from each land use category is parameterized using a variation of the Soil Conservation Service (SCS) curve number formulation; erosion is generated using the Universal Soil Loss Equation (USLE). The wastewaters from residences are considered a septic system component in GWLF model. The Excel Solver is used to make calibration in GWLF [53].

#### **3. Results**

#### *3.1. Calibration and Validation of GWLF Model*

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 0.96, while the NSE of validation ranges from 0.51 to 0.88. Such results indicate an acceptable predictive capability of GWLF to estimate streamflow.
