1.3 The drainage basin as an open system

What is a drainage basin?

An area encompassing areas drained by a river and its tributaries, where hydrological processes

Why are drainage basins also referred to as catchment areas?

Because they ‘catch’ all of the precipitation within the watershed, an imaginary line around the edges of the basin

How do drainage basins lose water?

• evaporation and evapotranspiration to the atmosphere

• surface runoff to the sea

• percolation into groundwater stores

What are the three pathways water can follow when precipitation occurs?

• reach the land surface and infiltrate the topsoil

• become surface runoff

• be evaporated back into the atmosphere

What can delay the pathway taken when precipitation occurs?

• the water could be intercepted by plants or buildings before either evaporating or infiltrating into the surface

• water percolates through rocks underneath the surface to become groundwater, where it may be stored in aquifers for some time

What are basin-wide factors?

Factors such as shape, relief, geology and vegetation that determine what happens to the precipitation where it falls

What is orographic (or relief) rainfall?

Rainfall caused by warm, moist air being forced to rise over high areas, it cooling and condensing to produce clouds and heavy rain.

What is frontal rainfall?

Fronts are formed as part of a low pressure area when warmer moist air from the south-west meets colder polar air from the north or north-west. Rain is formed when warmer air is forced over the denser colder air.

What is convectional rainfall?

Rain that is created during periods of high temperatures, and is often intense and associated with electrical storms and thunder

What is overabstraction and what can it lead to?

It is abstracting too much water from groundwater stores, and can lead to rivers drying up in times of low rainfall

How does deforestation affect the hydrological system?

The removal of the dense forest canopy in rainforests removes the protection of vital topsoil. This has various different impacts, such as rainwater not soaking into the ground but staying on the surface, causing runoff and erosion.

How does urbanisation affect the hydrological system?

New ground surfaces such as concrete increase surface runoff from 10 to 55%, and reduce infiltration from 25 to 10%. This affects the various processes and stores in the cycle, such as groundwater