Surface runoff is water, from rain, snowmelt, or other sources, that flows over the land surface, and is a major component of the water cycle.
Runoff that occurs on surfaces before reaching a channel is also called overland flow.
A land area which produces runoff draining to a common point is called a watershed.
When runoff flows along the ground, it can pick up soil contaminants such as petroleum, pesticides, or fertilizers that become discharge or overland flow.
Urbanization increases surface runoff, by creating more impervious surfaces such as pavement and buildings do not allow percolation of the water down through the soil to the aquifer.
It is instead forced directly into streams, where erosion and siltation can be major problems, even when flooding is not.
Increased runoff reduces groundwater recharge, thus lowering the water table and making droughts worse, especially for farmers and others who depend on water wells.