Variable runoff generation layer distributed hydrological model for hilly regions

The variable runoff generation layer concept was proposed based on newly acquired understanding of hillslope hydrological experiments. This concept expands the description of interflow from the soil horizon to the soil-weathering bedrock interface and provides a unified description of the infiltration excess and saturation excess runoff and their conversion mechanism via meticulous depiction of the interflow formation and development process. Based on the concept of the variable generation layer and theory of the kinematic wave model, calculation equations for the infiltration excess (Horton) surface runoff, saturation excess (Dunne) surface runoff, and interflow of a unit grid were derived. The nonlinear reservoir method, 2-d diffusion wave equation, and 1-d diffusion wave equation were applied to separately calculate the groundwater flow, surface runoff routing, and channel flow routing, based on which a variable runoff generation layer distributed hydrological model (VRGL) was established. The VRGL was then applied to the Tunxi watershed, a typical humid watershed in the hilly region. A total of 24 flood events from 2010 to 2019 were studied, and the results indicated that the relative error of both the flood peak and flood volume remained within ±20%, and the Nash-Sutcliffe efficiency value reached approximately 0.84. It was indicated that the accuracy of the VRGL was sufficiently high for flood forecasting in hilly regions.