2026 Complete Practice Test for Hydrological Cycle and Drainage Basins

The key idea is that runoff forms when the amount of rain arriving cannot be absorbed by the soil fast enough. Saturation-excess runoff happens when rainfall input exceeds the soil’s infiltration capacity, so water cannot infiltrate quickly enough and begins to pool and flow over the surface. As the rainfall continues, the surface layer becomes saturated and overland flow develops. This is why describing the condition as rainfall rate exceeding infiltration capacity is the best choice. In contrast, being in a drought or desert reflects little rainfall, and while a saturated soil can lead to runoff, the triggering condition highlighted here is the infiltration-rate limit being surpassed.