This is when the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, melt water, or other sources flows over the land. This is a major component of the water cycle.
Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source. If a nonpoint source contains man-made contaminants, the runoff is called nonpoint source pollution.
A land area which produces runoff that drains to a common point is called a watershed. When runoff flows along the ground, it can pick up soil contaminants including, but not limited to petroleum, pesticides, or fertilisers that become discharge or nonpoint source pollution.