Runoff Defined
Runoff is often defined as the portion of rainfall, snowmelt, and/or irrigation water that runs over the soil surface toward the stream rather than infiltrating into the soil. It is sometimes called surface runoff.
For some purposes however, the definition of runoff also includes water which makes its way relatively quickly to the stream channel just below the surface. This is sometimes called interflow or subsurface runoff, and together with surface runoff makes up the volume of water that hydrologists generally refer to as runoff.