How does Horton overland flow form during intense rainfall?

In hydrologic sciences, Horton overland flow occurs when the intensity of rainfall exceeds the infiltration capacity of the soil.1

When rain falls on a land surface, the substrate's permeability determines how much water can be absorbed. If the rainfall rate is greater than the rate at which the soil can infiltrate that water, the excess precipitation cannot enter the ground. This surplus water initially collects in small surface pools and eventually begins to flow over the land surface as overland flow.1

This process is a primary mechanism for generating surface runoff and is a critical step in the transition from dispersed water movement to the formation of concentrated channel flow. Because overland flow is relatively inefficient due to surface resistance, the water tends to incise the landscape, eventually leading to the development of drainage networks.21