During the simulation of rainfall infiltration

the conventional rainfall boundary cannot accurately reflect the influence of the variation of ponding water depth on the actual rainfall infiltration calculation. To address this issue

the conventional rainfall boundary is improved by incorporating the diffusion wave approximation equation

which allows for the coupling of ponding water depth variations with actual infiltration during heavy rainfall events. To confirm the precision of the improved boundary

two classical experimental cases are tested. The improved boundary is subsequently applied to the simulation of an actual engineering scenario. It is revealed that: (1) the improved rainfall boundary is capable of achieving real-time dynamic conversion between flow and pressure boundaries; (2) when the rainfall boundary functions as the pressure head boundary

the location with the theoretically deepest ponding water depth on the slope surface

as calculated by the improved boundary

manifests at the slope toe. Conversely

when the rainfall boundary serves as the flow boundary during the later stages of rainfall

the ponding water depth becomes negligible

and the location with the theoretical deepest ponding water depth arises at the intersection of the flat slope surface and the steep surface.