Rainstorm flood is one important factor resulting in soil and water loss of watershed. The soil and water loss can cause the increase of sediment concentration in runoff, which will add more surplus flood in flood season and consequently become an important reason for the occurrence of flood disaster. A factor of binding information (BI) was presented based on the information entropy theory. The BI is able to describe the uncertainty degree (i.e., risk value) hiding in system before events happen. According to the BI values of different controllable-observable objectives (COOs), the changing situations of uncertainty degree that events evolve from a COO to another one can be uncovered. So the BI is convenient to test risk changing of soil and water loss and reflect its evolution trend. In addition, the Universal Soil Loss Equation (USLE) has a preferable merit to describe and maintain the details of soil and water loss. Considering these characteristics of the USLE and BI, a notion of risk, coupling risk of soil and water loss, was proposed. The coupling risk is defined as the superposition results multiplied the BI by the factors in USLE. It can not only maintain the details of soil and water loss, but also describe the evolution processes of soil and water loss risk. On the basis of the presented coupling risk, a function relationship between sediment concentration in runoff and coupling risk grade of soil and water loss is established. A maximum rating method is employed to calculate the coefficient of surplus floodwater in flood season based on the independent variable of sediment concentration in runoff. Therefore, the relation between the coefficient of surplus floodwater and the coupling risk grade of soil and water loss is achieved with the intermediate variable of sediment concentration in runoff. A case study related to Jinghe River Basin with representative characteristics of floodwater resources utilization and soil and water loss has been performed. Results demonstrate that: 1) the coupling risk grades of soil and water loss for 1986, 1995 and 2000 at the upper watershed of the controlled section of Zhangjiashan Station are 2.5437, 2.5421 and 2.4722, respectively, which shows a total decrease trend of the coupling risk of soil and water loss in the basin; 2) there is a prominent relationship at the confidence level of 0.01 between sediment concentration in runoff (y) and coupling risk grade of soil and water loss (x) as y=0.0108exp(2.4211x). The square of correlation coefficient R is equal to 0.4037 (R2= 0.4037). The higher the coupling risk grade (x) is, the larger the sediment concentration in runoff (y) is, vice versa; and 3) a prominent function relationship at 0.01 confidence level exists between the coefficient of surplus floodwater in flood season (y) and the coupling risk grade of soil and water loss (x) as y =0.0229exp(0.9937x)R2=0.3878. The bigger the coupling risk (x) is, the larger the coefficient of surplus floodwater (y) is, the larger the amount of surplus floodwater in flood season is, and the greater the flood disaster risk is. The results can provide a reference for analysis on relationship between the risks of watershed soil and water loss and flood disaster.
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