Which factor is most associated with increased runoff during rainfall?

Increased runoff during rainfall is closely linked to the saturation of the soil. When the soil is already saturated, it cannot absorb any additional water. As a result, water from rainfall cannot infiltrate the ground and instead flows over the surface, contributing to runoff. This condition often occurs after prolonged precipitation or when the ground is waterlogged, meaning that more rainfall will lead to more water pooling on the surface.

While frozen ground can lead to runoff due to its inability to absorb water, it is not as universally applicable in various climates and situations as soil saturation. Flat and permeable surfaces can facilitate infiltration rather than runoff, and the presence of vegetation generally helps to absorb rainwater, reducing surface runoff by promoting absorption and slowing water movement. Thus, saturation of the soil is the critical factor here, as it directly limits the soil's capacity to absorb additional water during rainfall events.