## Materials and Methods
### Data Resources  
The topographic data include DEM data (with 30 m precision) of the Central Plains Economic Zone of China, which were obtained from the National Science and Technology Infrastructure Centre--National Earth System Science Data Sharing Infrastructure--Data Center of Lower Yellow River Regions. Landscape data included remote sensing images (with 30 m precision) from Landsat-7 that cover the Yi River watershed area. These were obtained in March 2017 (cloudiness of 0.58%). The images were processed and interpreted using ENVI 5.3 (Harris Geospatial Solutions Inc., Broomfield, CO, USA) software to determine the landscape pattern of the Yi River watershed (Figure 2). For the collection of surface water samples, 42 sampling sites were set on the main stream and major tributaries of the Yi River (Figure 3). The installation of the sampling sites followed the principles that the underlying surface must be similar and the location should be the representative regions of the study area. Samples were collected six times from October 2016 to August 2017: Twice during the drought period (October to December), twice during the ordinary period (March to May), and twice during the high-water period (June to August). At each sampling site, water from the surface layer (0.5 m) was collected and stored in a preprocessed plastic bottle. The situation of the surroundings was recorded. All samples were brought back for laboratory examinations as soon as possible. Before that, they were kept in a cold and dark location. Figure 3: Surface water sampling sites. Figure 2: Main landscape composition types in the Yi River watershed. To determine the water quality indicators, we selected five indicators of surface water quality in this study: Chemical oxygen demand (COD), ammonia nitrogen (NH\({}_{3}\)-N), total phosphorus (TP), dissolved oxygen (DO), and electrical conductivity (EC). We selected these indicators because the water pollutants in the study area mainly include municipal sewage, chemical sewage, and agricultural non-point source pollution. Thus, the surface water quality in the study area can be fully reflected by these indicators. The DO and EC values were measured in situ using a portable instrument (SX713-02, Runsun Instruments Inc., Chengdu, China). COD, NH\({}_{3}\)-N, and TP values were determined using an integrated-triple-parameter water quality tester (Lianhua 5B-6C, Lianhua and Technology Inc., Beijing, China). All laboratory experiments were completed in the College of Environment and Planning, Henan University, China. The experimental procedures to determine COD, NH\({}_{3}\)-N, and TP are described in the following section. The potassium dichromate (K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\)) method was applied to determine the COD. This method involves the use of a strong acid solution where a given mass of K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\) can oxidize reducing substances in the surface water samples. A ferrous indicator solution is added to an excessive amount of K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\) and titrated with an ammonium ferrous sulfate solution. Based on the dosage of ammonium ferrous sulfate, we can calculate the amount of material required to restore the volume of oxygen consumption using the following formula:  
\[\mathrm{COD_{Cr}}\;(\mathrm{O_{2}},\mathrm{mg/L})=[(V_{0}-V_{1})\times C\times 8 \times 1000]/V \tag{1}\]  
where \(C\) is the concentration of the ammonium ferrous sulfate standard solution (mol/L); \(V_{0}\) and \(V_{1}\) are the dosages of the ammonium ferrous sulfate standard solution when titrating the blanks (mL) and the surface water samples (mL), respectively; \(V\) is the volume of the surface water sample (mL); and 8 is the molar mass (g/mol) of oxygen (1/2O). [PERSON]'s reagent photometry was used to determine NH\({}_{3}\)-N, which involves the use of the flocculate precipitate to preprocess surface water samples. We absorbed 0, 0.50, 1.00, 3.00, 5.00, 7.00, and 10.00 mL of surface water samples separately into 50 mL colorimetric tubes after the calibration curve was drawn.