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Catchment
From Wikipedia, the free encyclopedia
A catchment is any device or structure that captures water.
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Catchment area or drainage basin is the term used to describe the area which is drained by a river. It is sometimes called the river basin or watershed (see below). The catchment is the most significant factor determining the amount or likelihood of flooding.
Catchment factors are: topography, shape, size, soil type and land use (paved or roofed areas). Catchment topography and shape determine the time taken for rain to reach the river, Catchment size, soil type and development determine the amount of water to reach the river.
Topography determines the speed with which the runoff will reach a river, clearly rain that falls in steep mountainous areas will reach the river faster than flat or gently sloping areas.
Shape will contribute to the speed with which the runoff reaches a river. A long thin catchment will take longer to drain than a circular catchment.
Size will help determine the amount of water reaching the river, as the larger the catchment the greater the potential for flooding.
Soil type will help determine how much water reaches the river. Certain soil types such as sandy soils are very free draining and rainfall on sandy soil is likely to be absorbed by the ground. However, soils containing clay can be almost impermeable and therefore rainfall on clay soils will run off and contribute to flood volumes. After prolonged rainfall even free draining soils can become saturated, meaning that any further rainfall will reach the river rather than being absorbed by the ground.
Land use will contribute to the volume of water reaching the river, in a similar way to clay soils, only faster. Rainfall on roofs, pavements and roads will be collected by rivers with almost no absorption into the groundwater.
Another common term for catchment is watershed. This link leads to a website showing a flash animation of how rain falling on to the landscape will drain into a river depending on the terrain.
