Environmental Management Systems
ENVIRONMENTAL MANAGEMENT SYSTEMS
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. Before runoff can occur, precipitation must satisfy the demands of evaporation, interception, infiltration, surface storage, surface detention and channel detention. Runoff will occur only when the rate of precipitation exceeds the rate at which water may infiltrate into the soil. After the infiltration rate is satisfied, water begins to fill the depressions, small and large, on the soil surface. As the depressions are filled, overland flow begins. The depth of water builds up on the surface until it is sufficient to result in runoff in equilibrium with the rate of precipitation less infiltration and interception. The volume of water involved in the depth build up is surface detention. As the flow moves into defined channels there is a similar build up of water in channel detection. The volume of water in surface and channel detention is returned to runoff as the runoff rate subsides. The water in surface storage eventually goes into infiltration or is evaporated.
Definition 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.
Meteorological factors affecting runoff:
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