## 2 Data and Methods
### SWAT Model  
The SWAT model is a distributed watershed scale hydrological model developed by the Agricultural Research Service of the United States Department of Agriculture (ARS-USDA) [47; 48; 49] that mainly includes the hydrological-process, soil-erosion, and pollution-load submodels. The key modules involved in the hydrological process submodel of the SWAT model are the surface-runoff, evapotranspiration, soil-water-movement, underground-runoff, and river-confluence modules. The SCS curve number method is generally used to simulate the surface runoff. The simulation of evapotranspiration is  
Figure 1: Digital elevation model (DEM), hydrometric and meteorological stations of the Upper Heihe River Basin (UHRB).  
to calculate the actual evapotranspiration on the basis of potential evapotranspiration, including the evaporation of vegetation canopy retention, vegetation, and soil. Soil water movement is simulated by vertical stratified water balance, and infiltration and interflow are considered. The simulation of groundwater runoff is divided into shallow and deep groundwater runoff, which involves phreatic re-evaporation and irrigation, and is realized on the basis of the water balance equation. The confluence calculation of the runoff flowing into a river channel is based on the variable-storage or Muskingum method [50; 51].  
The following input data are required for building the SWAT model. The digital elevation model (DEM) of the Shuttle Radar Topography Mission (SRTM) with a 30 m resolution was derived from the Geospatial Data Cloud ([[http://www.gscloud.cn/](http://www.gscloud.cn/)]([http://www.gscloud.cn/](http://www.gscloud.cn/)), accessed on 25 April 2022). The soil dataset with a 1 km resolution used in this study is Harmonized World Soil Database version 1.2 (HWSD v1.2) [52], which was obtained from the Food and Agriculture Organization (FAO). Using the QGIS tools, the soil dataset was further reclassified into 16 types (Figure 2). Land-use and land-cover (LULC) data with a resolution of 1 km were downloaded from the Resource and Environment Science and Data Center ([[https://www.resdc.cn/](https://www.resdc.cn/)]([https://www.resdc.cn/](https://www.resdc.cn/)), accessed on 10 April 2022). According to the database provided by the SWAT model, the LULC was reclassified into nine types (Figure 2). The daily meteorological data were obtained from the National Meteorological Science Data Center ([[http://data.cma.cn/](http://data.cma.cn/)]([http://data.cma.cn/](http://data.cma.cn/)), accessed on 20 March 2022), including the maximal and minimal temperature, precipitation, relative humidity, and wind speed of 10 meteorological stations from 1988 to 2000. The daily streamflow data of the Yingluoxia hydrometric station from 1990 to 2000 were obtained from the Environmental and Ecological Science Data Center for West China, National Natural Science Foundation of China.  
The UHRB was divided into 17 sub-basins in this study on the basis of DEM data for runoff simulation. We set 1988-1989 as the warm-up period, and 1990-2000 as the simulation period. Following swat model documentation and SWAT-sensitivity analysis (SA) platform [53; 54] recommendations, 30 parameters related to runoff, simulated in the SWAT model, were selected (Table 1). The Nash-Sutcliffe efficiency coefficient was used to estimate the degree of fitting between the measured and simulated monthly average flow. The sensitivity analysis of the 30 parameters was carried out by using various analytical methods to determine the most sensitive parameters in the SWAT runoff model.  
Figure 2: (**a**) Land-use and (**b**) soil maps of the upper Heihe River basin (UHRB).