Rainfall infiltration analysis is key to the prediction of rainfall-induced slope failure. Due to its simplicity and ease to use

the Green-Ampt model is potentially very useful for rainfall infiltration analysis in prediction of shallow landslides. However

the original Green-Ampt model assumes that the ground is level and that the initial distribution of water content does not vary with depth. In this study

the governing equation for infiltration analysis of sloping surfaces with arbitrary distribution of initial water content is derived. The Runge-Kutta method is used to solve the equation numerically. The modified model can be reduced to the previous Green-Ampt model when the initial water content is uniformly distributed. When the initial water content is not uniform

Richards’ equation shows that there is a transition zone between the saturated and unsaturated zone which is not affected yet by the rainfall infiltration. The wetting front calculated from the modified method is in this transition zone and is close to the bottom of the saturated zone. Overall

the distribution of pore water pressure predicted from the modified Green-Ampt model is close to that predicted based on Richards’ function.