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Earth's water: Runoff

Picture of impervious areasWhen 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 watershed (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 watershedS
 • 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 and water quality

A stormwater runoff intake. A significant portion of rainfall in forested watersheds is absorbed into soils (infiltration), is stored as ground water, and is slowly discharged to streams through seeps and springs. Flooding is less significant in these conditions because some of the runoff during a storm is absorbed into the ground, thus lessening the amount of runoff into a stream during the storm.

As watersheds are urbanized, much of the vegetation is replaced by impervious surfaces, thus reducing the area where infiltration to ground water can occur. Thus, more stormwater runoff occurs - runoff that must be collected by extensive drainage systems that combine curbs, storm sewers (as shown in this picture), and ditches to carry stormwater runoff directly to streams. More simply, in a developed watershed, much more water arrives into a stream much more quickly, resulting in an increased likelihood of more frequent and more severe flooding.

Drainage ditches to carry stormwater runoff to storage ponds are often built to hold runoff and collect excess sediment in order to keep it out of streams.

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.

Runoff and the water cycle

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Take Pride in America home page. USA.gov U.S. Department of the Interior | U.S. Geological Survey
URL: http://ga.water.usgs.gov/edu/runoff.html
Page Contact Information: Howard Perlman
Page Last Modified: Wednesday, 13-Aug-2008 07:21:29 EDT