The amount of rainfall or snowmelt that either flows over the soil surface or that drains from the soil and enters a body of water, thereby leaving a watershed. This water is the excess amount of precipitation that is not held in the soil nor is it evaporated or transpired back to the atmosphere. Water that reaches deep groundwater and does not, therefore, directly flow into a surface body of water is usually not considered runoff. Runoff can follow many pathways on its journey to streams, rivers, lakes, and oceans. Water that primarily flows over the soil surface is surface runoff. It travels more quickly to bodies of water than water that flows through the soil, called subsurface flow. As a rule, the greater the proportion of surface to subsurface flow, the greater the chance of flooding. Likewise, the greater the amount of surface runoff, the greater the potential for soil erosion.

See Also

Storm Runoff