## 2 Materials and Methods  
### Study Area  
Suibin County (131\({}^{\circ}\)08\({}^{\prime}\)121\({}^{\prime\prime}\)-132\({}^{\circ}\)31\({}^{\prime}\)00\({}^{\prime\prime}\) E, 47\({}^{\circ}\)11\({}^{\prime}\)55\({}^{\prime\prime}\)-47\({}^{\circ}\)45\({}^{\prime}\)23\({}^{\prime\prime}\) N) is located in the northeast of Hegang City, Heilongjiang Province, China, in the confluence zone of the Heilongjiang River and Songhua River, covering an area of 3344 km\({}^{2}\). It is an important part of the Sanjiang Plain. There is the Heilongjiang River between Suibin County and Russia in the north, Luobei County in the west, and Huachuan County and Fujin City in the south across the Songhua River. Suibin County belongs to the mid-temperate continental monsoon climate zone, with a large temperature difference between day and night in spring and autumn, a hot and short summer, abundant rainfall, and severe cold and a long winter. The average annual rainfall is 567.9 mm, which is unevenly distributed throughout the year, mostly concentrated from July to September. The average annual evaporation is 650.8 mm, which is the largest in spring and easily causes spring drought. The average annual temperature is 1.8-2.4 \({}^{\circ}\)C, with a maximum of 21.9 \({}^{\circ}\)C in July and a minimum of minus 19.6 \({}^{\circ}\)C in January.  
There are 25 rivers in Suibin County, including 5 rivers in the Heilongjiang River Basin and 20 rivers in the Songhua River Basin. Due to the uneven distribution of annual rainfall, the distribution of annual runoff is also uneven, and the runoff from July to September accounts for more than half of the annual runoff. The terrain in the area is flat, with an average gradient of 1/6000-1/10,000, and the altitude range is between 55 and 77 m. The landforms belong to terraces, high floodplains, and low floodplains. Most of the soil in Suibin County is loamy soil with strong water permeability, and the infiltrability is 0.144-0.47 m/d. Because there is an aquifer layer at a depth of 50-100 cm below the surface, the upper soil has little water retention capacity, and the soil is vulnerable to drought and waterlogging. At present, Suibin County has become one of the largest grain producing counties in Heilongjiang Province, mainly planting wheat, soybeans, rice, and corn.  
The Wanyan River surface waterlogged area, namely the study area, is located in the central part of Suibin County and is one of the major waterlogged areas in the Sanjiang Plain. It is bounded by the watershed between the Heilongjiang River basin and the Wanyan River basin in the north; the watershed between the Songhua River basin, the Wanyan River basin, and the Songhua River dam in the south; and the No. 1 road of Suibin Farm in the west and the Songhua River dam in the east, as shown in Figure 1. The study area is 76.88 km long and 16 km wide with a total area of 1230 km, including 528.8 km\({}^{2}\) of Suibin County, 263.67 km\({}^{2}\) of Suibin Farm, and 437.53 km\({}^{2}\) of 290 Farm.  
The Wanyan River is the main drainage ditch with a length of 92.40 km. The Wanyan River pumping station is located at the end outlet, which was built in 1982 and renovated in 2011, with a design flow of 57.97 m\({}^{3}\)/s. There are also 18 sub-main ditches with a total length of 163.89 km.