General Runoff Terms

There are a number of terms commonly used to describe runoff processes. Let's briefly introduce some of these terms which will be covered in more detail in later sections of this module.

Basin, drainage basin, or watershed, are all terms used to describe the area contributing to the runoff. The runoff in a basin generally begins at the drainage divide that marks the perimeter of the basin. All runoff within a basin drains to a single point at the outlet of that basin.

Depiction of basic soil water and runoff terms: infiltration, baseflow, interflow, and surface runoff

Baseflow, or base runoff, is the long-term supply of water that keeps at least some water in the stream even during extended dry periods. Baseflow comes from water that percolated down into deep storage.

Infiltration is the downward movement of water through the soil surface. While it is often used interchangeably with percolation, percolation actually refers to movement of water within soil, and infiltration specifically refers just to the process of water entering a soil surface.

Surface runoff is the movement of water across the soil surface toward the stream channel.

Interflow is the relatively rapid movement of water below the soil surface toward the stream channel, typically within 72 hours of when water infiltrates the surface. This process is more likely in areas with deep soil.

Runoff, sometimes called quick response runoff, can refer to surface runoff only, but sometimes it is the combination of surface runoff plus interflow. Thus, it typically represents the movement of water to the stream channel that is not part of baseflow.