Rainfall infiltration in slopes can cause slope failures. A new model was developed to simulate how rainfall, infiltration, and surface runoff interact in slopes. The model shows that when surface and subsurface flow are considered together, more rainwater infiltrates into the slope. This leads to faster changes in pore water pressure and deeper movement of the wetting front into the soil. The model also indicates that under drier conditions, slopes can absorb more rainfall, resulting in significant differences in infiltration rate and pore water pressure compared to models that do not consider the interaction between surface and subsurface flow.