SWAT model setup

We used ArcSWAT version 2012.10_4.19 within ArcGIS 10.4.1 to build the IRW SWAT model. Digital elevation model (DEM) was obtained from Shuttle Radar Topography Mission (SRTM) 1 Arc-Second (about 30 m × 30 m) Global Database and downloaded from USGS website (https://earthexplorer.usgs.gov/, 01-28-2018) (Fig 2a). Land use and land cover (LULC) data was from the 2011 NLCD dataset (https://www.mrlc.gov/, 01-31-2018) (Fig 2b), and spatial resolution is 100 m × 100 m. Soil data came from the SSURGO database (https://websoilsurvey.nrcs.usda.gov/, 02-05-2018) (Fig 2c). Climate data obtained from the National Climatic Data Center (NCDC) (https://www.ncdc.noaa.gov/, 02-07-2018) (Fig 2d). Due to missing precipitation and temperature records from NCDC climate data from Jan. 1990 to Dec. 2013, we downloaded alternative Climate Forecast System Reanalysis (CFSR) data from the SWAT official website (https://globalweather.tamu.edu/, 01-31-2018), then filled missing NCDC data using climate data from the closest CFSR grid stations (not shown in Fig 2d). All precipitation data of five climate stations meet the data consistency checks using the double mass curve method [39]. The basic information of thirteen hydrologic stations is listed in Table 1.

^†Note: The column of the simulated upstream area refers to delineate the upstream area by the ArcSWAT program.

The IRW was delineated into 86 subwatersheds with 1023 HRUs under a threshold area of 3000 ha. The multiple land use/soil/slope method was applied to define the HRUs with land use (10%), soil (10%) and slope (5%) threshold. The surface runoff was estimated using the SCS curve number method [40], and the Penman-Monteith equation [41] was applied to calculate the potential evapotranspiration. The streamflow was routed and calculated according to the variable storage routing method [38]. A five-year was used as a warm-up period (1990–1994) to initialize the model input and stabilize the SWAT model. The simulation running period of the SWAT model is from Jan-01-1995 to Dec-31-2013.