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:

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:

Landuse

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.

Stream flow and Stream Discharge

The term stream flow describes the process of water flowing in the organized channels of a stream or river. Stream discharge represents the volume of water passing through a river channel during a certain period of time. Stream discharge can be expressed mathematically with the following equation:

Q = W x D x V (A*V)

where,

Q equals stream discharge usually measured in cubic meters per second, W equals channel width, D equals channel depth, and V equals velocity of flowing water.

Runoff Hydrographs

Because of streamflow's potential hazard to humans’ mechanical recorders gauge many streams. These instruments record the stream's discharge on a hydrograph. The graph below illustrates a typical hydrograph and