Improved model of runoff-seepage coupling in unsaturated soil based on fractal theory

Abstract

The Green-Ampt model is widely used in rainfall infiltration analysis of unsaturated soil where fewer parameters are involved. However, the slope surface water is assumed to be constant, and the area above the wetting front is saturated. This has its limitations. In this paper, an improved Green-Ampt model is proposed based on fractal theory considering soil pore characteristics. In the proposed model, the infiltration transition zone is included, the real-time dynamic change of the slope infiltration rate is determined through the runoff-seepage coupling to obtain infiltration profile model at different times. Compared with the existing theoretical results and data from ponded infiltration column tests, the model presented in this paper can accurately calculate the cumulative infiltration of the soil and surface runoff to verify the model’s validity. A set of parametric analyses are conducted to study effect of seepage in unsaturated soil. The results show that The soil’s pore distribution affects the slope’s infiltration profile and the timing of surface runoff generation. The depth of the infiltration zone increases with the increase of the saturated hydraulic conductivity and the soil particle pore size, while it decreases with the increase of the slope angle. At the same time, the runoff-seepage coupling effect cannot be ignored; the generation of surface runoff is delayed with the increase of the slope angle, saturated hydraulic conductivity, and soil particle pore size, which further affects the infiltration rate of the soil. The improved model can effectively analyze the influence of soil pore characteristics on infiltration and is suitable for different soils, especially for silty clay and silty soil.