What is a drainage basin?

A drainage basin is the area of land surrounding a river where all precipitation falling on that land drains into that river. This area is also referred to as the river's catchment.

The watershed

The boundary of a drainage basin is called the watershed. Any precipitation falling beyond the watershed enters a different drainage basin.

The size and shape of a drainage basin influence the processes that occur within it

• High drainage density - Larger drainage basins with steep sides can catch more precipitation and move it quickly into river channels. These basins tend to have a large number of streams.
• Low drainage density - Smaller drainage basins, or those with fewer streams, have a low drainage density.

The drainage basin as an open system

A drainage basin functions as an open system – meaning it has both inputs and outputs, so the total amount of water within the system changes over time. Water enters the system and eventually leaves it.

Inputs

Precipitation is the main input into the drainage basin, including rain, snow, hail, dew, and frost.

Stores

Stores are places where water is held temporarily within the drainage basin.

Types of stores:

• Vegetation storage - Water held on leaves and branches after interception
• Surface storage - Water collected in puddles, lakes, or ponds on the land surface
• Soil storage - Water held within the soil
• Groundwater storage - Water stored in permeable rock below the water table
• Channel storage - Water held within the river or stream channel itself

Flows and processes

Flows are the ways water moves between stores within the system.

Types of flows and processes:

• Interception - Water landing on vegetation or structures (e.g. buildings) before reaching the soil
• Infiltration - Water soaking into the soil from the surface
• Direct runoff - Water flowing over the land surface, either across the whole surface or in small channels, when rainfall exceeds the rate of infiltration
• Saturated overland flow - Water flowing over the surface because the soil is fully saturated and can no longer absorb any more water
• Throughflow - Water moving slowly downhill through the soil
• Percolation - Water seeping downward through the soil into the water table
• Groundwater flow - Water flowing very slowly through permeable rock beneath the water table

Outputs

Outputs are the ways water leaves the drainage basin system.

Types of outputs:

• Evaporation - Water turning into water vapour from surfaces such as rivers, lakes, and soil
• Transpiration - Evaporation from within plant leaves. Plants take up water through their roots and transport it to their leaves, where it evaporates into the atmosphere
• Evapotranspiration - The combined process of evaporation and transpiration together
• Channel flow (river discharge) - Water leaving the drainage basin through a river or stream

Types of precipitation

Precipitation is the main input into a drainage basin and includes all the ways moisture leaves the atmosphere and reaches the ground. While rain is the most common form, it also includes snow, hail, dew, and frost.

Precipitation occurs when air cools, increasing the rate of condensation, which is the process by which water vapour turns into liquid water droplets. There are three main types of precipitation, each caused by a different mechanism of air cooling.

Three types of precipitation

• Frontal precipitation - Warm air is less dense than cool air. When a mass of warm air meets a mass of cool air, the warm air is forced upward above the cooler air. As it rises, it cools, and condensation produces precipitation.
• Orographic precipitation - When warm air meets mountains, it is forced to rise, causing it to cool.
• Convectional precipitation - When the sun heats the ground, moisture at the surface evaporates and rises in a column of warm air. As this air gains altitude, it cools.

Spatial variations in precipitation

• Tropical regions - Convectional precipitation is common, producing effects such as monsoon seasons. This type can also occur during summer months at high latitudes, such as in the Arctic tundra.
• Coastal regions - These often receive more precipitation than inland areas because water evaporated from the oceans is carried onshore by prevailing winds and falls as precipitation.

Physical variables influencing the drainage basin

Several physical factors affect the size of inputs, the speed of flows, the capacity of stores, and the volume of outputs within a drainage basin.

Seasons and climate influence how water moves through the drainage basin

• The size of inputs, flows, and stores varies with the seasons – for example, in the UK, summer is normally drier than winter
• During winter, temperatures may drop below 0°C, causing water to freeze. This reduces the size of flows through the drainage basin while the store of frozen water grows
• When temperatures rise again, flows and outputs can be much larger as ice melts. Higher temperatures also increase evaporation, which can trigger convectional precipitation in the form of short periods of intense rainfall
• Climate affects the type and density of vegetation, which in turn influences how much interception and evapotranspiration takes place

Storms and rainfall intensity

• Intense storms generate more precipitation and greater peak discharges than light rain showers
• The larger input of water causes flows such as runoff, and stores such as groundwater, to increase in size
• Some flows, like infiltration, may not be able to occur fast enough to cope with the volume of input, which increases direct runoff across the surface

Soil type and structure

• Infiltration rates are influenced by soil type, soil structure, and how much water is already in the soil. For example, sandy soils have larger air spaces, allowing faster infiltration compared to clay soils
• Throughflow is faster where there are openings in the soil, such as cracks or animal burrows, which create pathways for water movement
• Soil type can also influence the type of vegetation that grows in a location, creating an indirect effect on interception and transpiration

Vegetation

• Vegetation intercepts precipitation and slows its movement into the river channel. Interception is highest when there is dense vegetation and when deciduous trees have their leaves
• The more vegetation present, the more water is lost through evapotranspiration before it reaches the river channel. This reduces both runoff and peak discharge

Geology

• Water flows slowly through most rocks, but highly permeable rocks such as sandstone, or rocks with many joints such as limestone, allow faster groundwater flow
• Impermeable rocks prevent water from passing through them, which leads to higher rates of direct runoff across the surface

Relief

• The steeper the gradient of the landscape, the faster processes such as direct runoff and throughflow occur
• Surface stores have a shorter residence time in steeper areas because water moves downhill more quickly
• Gradient can also affect the volume of water discharged from a river to the ocean