Runoff--What's the Difference?

What comes to mind when you hear the word, "runoff"?  Most people think of water running down the street, driveway, or roof.  BUT the term, "runoff,"  represents a whole lot more than that.

Runoff is the total amount of water flowing into a stream. It is the umbrella term for at least six (6) other terms:  surface runoff, direct runoff, overland flow, saturation excess overland flow, interflow, baseflow, and probably a few others. 2, 11, 12  What does all this mean? 


Runoff (RO) is the total amount of water flowing into a stream, or the sum of direct runoff and baseflow.  To determine the amount of annual runoff, subtract the amount of annual evapotranspiration from the annual amount of precipitation.  

Precipitation - Evapotranspiration= Runoff (RO)

RO = DRO + BF    or   RO = OF + SOF + IF + BF


From here, runoff can be divided into two successively smaller subcategories:

 

Direct Runoff (DRO) is the sum of surface runoff and interflow.

DRO = SRO + IF    or   DRO = OF+ SOF + IF

Surface Runoff (SRO) is the sum of overland flow and saturation excess overland flow.

SRO = OF + SOF


Each of the above "runoff" terms includes all or some of the following "flows" shown in the diagram below in blue type.

Types of Flow


(Modified from Berner and Berner, 13)

Overland Flow (OF) is the portion of rain, snow melt, irrigation water, or other water that moves across the land surface and enters a wetland, stream, or other body of water. This generally occurs when  precipitation exceeds the infiltration capacity of the soil. Overland flow usually occurs in urban settings (flows off pavement, roofs, etc.) or where the soils are very fine textured, heavily compacted, or frozen. (Back to Diagram)
Saturation Excess Overland Flow (SOF) is precipitation that cannot be absorbed by the soil because the soil is already saturated with water, so it flows across the land surface to a stream or other body of water.  (Back to Diagram)
Interflow (IF) is the water that travels laterally or horizontally through the zone of aeration (vadose zone) without reaching the water table during or immediately after a precipitation event and discharges directly into a stream or other body of water.  (Back to Diagram)
Baseflow (BF) is the sustained flow (amount of water) in a stream that comes from groundwater discharge or seepage.  It is the sustained flow between storm events.  (Back to Diagram)

(Italicized words defined in the glossary.) 

Sources for page 2, 9, 10, 11

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