ENVIRONMENTAL MANAGEMENT SYSTEMS
Runoff
When rain or snow falls onto the earth, it just doesnt sit there — it starts moving according
to the laws of gravity. A portion of the precipitation seeps into the ground to replenish
Earths 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
