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Earth's water: Runoff

When rain or snow falls onto the earth, it just doesn't sit there -- it starts moving according to the laws of gravity. A portion of the precipitation seeps into the ground to replenish Earth's ground water. Most of it flows downhill as runoff. Runoff is extremely important in that not only does it keep rivers and lakes full of water, but it also changes the landscape by the action of erosion. Flowing water has tremendous power -- it can move boulders and carve out canyons (check out the Grand Canyon!).

Some definitions of runoff:

(1) that part of the precipitation, snow melt, or irrigation water that appears in uncontrolled surface streams, rivers, drains or sewers. Runoff may be classified according to speed of appearance after rainfall or melting snow as direct runoff or base runoff, and according to source as surface runoff, storm interflow, or ground-water runoff.

(2) The sum of total discharges described in (1), above, during a specified period of time.

(3) The depth to which a drainage area would be covered if all of the runoff for a given period of time were uniformly distributed over it.

Meteorological factors affecting runoff:

Type of precipitation (rain, snow, sleet, etc.)
Rainfall intensity
Rainfall amount
Rainfall duration
Distribution of rainfall over the drainage basin
Direction of storm movement
Antecedent precipitation and resulting soil moisture
Other meteorological and climatic conditions that affect evapotranspiration, such as temperature, wind, relative humidity, and season.

Physical characteristics affecting runoff:

Land use
Vegetation
Soil type
Drainage area
Basin shape
Elevation
Slope
Topography
Direction of orientation
Drainage network patterns
Ponds, lakes, reservoirs, sinks, etc. in the basin, which prevent or alter runoff from continuing downstream.

Runoff from agricultural land (and even our own yards) can carry excess nutrients, such as nitrogen and phosphorus into streams, lakes, and ground-water supplies. These excess nutrients have the potential to degrade water quality.

This information is courtesy of the Nevada Division of Water Planning.

Rivers Rivers and the landscape River lengths Surf your watershed (EPA)
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The URL for this page is http://ga.water.usgs.gov/edu/runoff.html
Comments? Contact Howard Perlman
Last Modified: May 28, 2003 		Glossary of water terms.
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