What causes overland flow?

• A. Excess rainfall
• B. Infiltration capacity being exceeded
• C. Evaporation
• D. Soil crusting

Answer: (B)

Overland flow happens when water cannot infiltrate the soil fast enough to match the incoming rainfall. The soil has an infiltration capacity—the maximum rate at which it can absorb water. If rainfall intensity or the soil’s conditions (like being saturated, crusted, or compacted) push the required infiltration rate above that capacity, the excess water stays on the surface and runs downslope as overland flow. Evaporation removes water but doesn’t cause runoff, and while very heavy rain can lead to runoff, the direct mechanism is the rainfall rate exceeding the soil’s infiltration capacity.