Surface runoff

Surface runoff is water flowing across land toward streams, lakes, and oceans. It occurs when rainfall or snowmelt exceeds infiltration or when the ground is already saturated.

Infiltration: Infiltration is the entry of water from the land surface into soil or other porous material. Water that infiltrates is temporarily removed from the flow available as surface runoff.
Precipitation: Precipitation is water released from the atmosphere as rain, snow, sleet, or hail. Rainfall and snowmelt supply much of the water that becomes surface runoff.
Runoff coefficient: A runoff coefficient is the fraction of precipitation estimated to leave an area as runoff. It provides a simple way to estimate runoff volume for drainage design.
Flood: A flood is the overflow of water onto land that is normally dry. Large or rapid runoff can raise streams beyond their banks and inundate nearby land.
Infiltration-excess overland flow: Overland flow produced when rainfall arrives faster than the soil can absorb it. This mechanism generates runoff even when the soil is not saturated.
Soil moisture: Soil moisture is the water held in the unsaturated pores of soil. A soil's existing moisture affects how much additional water it can absorb before runoff increases.
Rational method: The rational method estimates peak stormwater discharge from rainfall intensity, drainage area, and a runoff coefficient. Engineers use it to estimate peak runoff from small urban catchments.
Soil erosion: Soil erosion is the detachment and movement of soil by water, wind, or other forces. Flowing runoff can detach and carry soil downslope.
Saturation-excess overland flow: Overland flow produced when water reaches the soil surface after the ground becomes saturated. This mechanism explains runoff from wet areas during or after continued rainfall.
Soil permeability: Soil permeability is the capacity of soil to transmit water through its connected pores. Low permeability limits infiltration and can increase water flowing over the ground.

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