The Drainage Basin System

Which factors control the amount of discharge in a drainage basin? This all depends on the drainage basin characteristics and what processes are found there.

Physical characteristics

Basin size

This influences the lag time – a large drainage basin will mean that water takes a long time to travel through tributaries or the ground to reach the channel.

Conversely, a small drainage basin means that water has a shorter distance to travel and will result in a shorter lag time.

Basin shape

Circular drainage basins mean that all points on the watershed are equidistant from the channel and this will lead to a shorter lag time and higher peak discharge.

Elongated drainage basins are characterised by longer lag times and lower peak discharge as the water drains from the furthest reaches of the watershed to the channel.

Elevation and slope

A steeply-sided river valley means that gravity assists water in its descent towards the river channel, whereas gently sloping valleys tends to produce longer lag times and lower peak discharges.

Rock type

This has a profound influence on the rate of drainage in a basin.

Permeable rock assistspercolation:

• Porous rock (sandstone, chalk) allows water to percolate through the pore spaces
• Pervious rock (limestone) allows water to travel along joints and bedding planes within the rock

Both types of rock are characterised by a lack of surface drainage and have high rates of infiltration.

Impermeable rock (granite, shale, clay) impedes drainage by restricting percolation. These areas are characterised by high rates of overland flow or surface runoff.

Soil type

Soil type controls the rate of infiltration, soil moisture storage and rate of throughflow.

Sandy soils have high rates of infiltration due to relatively large air spaces or voids between soil particles, whereas clay soils and silts have small pore spaces which allow very little throughflow.

Drainage density

This is the total length of streams in a drainage basin divided by the area of the basin.

Drainage basins characterised by impermeable rock and soils tend to have higher drainage density due to the lack of infiltration and percolation. This means that water enters a channel quickly, leading to an increase in discharge.

Conversely, drainage basins with permeable rock and soil types tend to have low drainage density.

Meteorological factors

These play an important role in the controlling the amount of discharge in a drainage basin.

Rainfall type

The amount and duration determine how saturated the ground is.

Long periods of rainfall often lead to soil reaching field capacity (saturation capacity), which impedes infiltration and leads to high rates of surface runoff.

Snow can act both as a store (it intercepts water) and as a transfer when it melts.

The amount of rainfall which reaches the drainage basin is also influenced by vegetation cover. A drainage basin covered in dense vegetation will experience high rates of interception, root uptake and evapotranspiration - this will reduce the amount of discharge within the basin.

Tropical rainforests are thought to intercept up to 80% of rainfall, whereas arable land intercepts less than 10% of rainfall.

Rainfall intensity

Heavy rainfall may exceed the infiltration capacity of the soil, leading to surface runoff and a rapid increase in discharge.

Antecedent conditions

This refers to the weather conditions in the period preceding a storm event.

Several weeks of prolonged, heavy rainfall will mean that a drainage basin reaches saturation capacity very quickly after a storm event. This can also lead to a rise in the level of the water table, further increasing the likelihood of saturation capacity being exceeded.

Rates of evapotranspiration

Rates of evapotranspiration are not constant throughout the year in mid-latitude locations like the UK.

High temperatures in summer months increase rates of evapotranspiration, reducing discharge.

Low temperatures in the winter reduce evapotranspiration (vegetation also experiences reduced growth rates at this time, meaning that there is less root uptake and so less interception).

Human factors

Urbanisation

Urbanisation reduces infiltration to 0 through its use of impermeable surfaces (tarmac, concrete), and drains and gutters transport water quickly to the river channel.

This means that lag time is reduced and discharge is increased.

Rivers which flow through urban areas are characterised by "flashy" hydrographs and represent a high flood risk.

Deforestation

Deforestation reduces interception, evapotranspiration and the protective canopy layer.

This leads to an increase in rates of infiltration and can result in saturation capacity being reached quickly.

Rainfall will travel by overland flow and can lead to high rates of soil erosion without the binding nature of root systems in place.

Afforestation

This is the planting of trees (often as a soft engineering method for flood management).

Afforestation has the effect of increasing rates of interception and evapotranspiration. This will reduce the amount of discharge in the drainage basin, although it can take many years for trees to mature.

Water extraction

Water extraction for industrial or domestic use reduces the amount of discharge in the drainage basin.

It is unrealistic to examine any of these factors in isolation as they are all linked.

For example, weeks of heavy rain may have fallen in a drainage basin with sandy soils and gentle slopes following a particularly dry summer when the water table fell significantly.

In this case, the ground may not reach saturation capacity for many weeks and an intense storm will not produce torrents of overland flow and flooding.

Similarly, drainage basins tend not to be characterised by only one type of soil and rock (or the presence or absence of an urban area) and so will experience differences in infiltration rates and drainage density throughout the area.

Which factors are more important in controlling the amount of discharge in a drainage basin - physical or human?

The answer to this lies in the location and composition of the drainage basin.

In central London, there is no doubt that human factors have the most significant influence over discharge, whereas in rural Wales it could be argued that physical factors play a more central role.

However, it is estimated that while over 80% of the population in the UK lives in an urban settlement, only 5-10% of the country is actually classed as "urban’". Urban areas comprise a lot of green spaces (parks, gardens, grass verges), making the drainage pattern of basins extremely complex.

It is inevitably going to be a combination of physical and human factors which control how much discharge is in a drainage basin.

This is a must-read article from the BBC, suggesting our image of the UK as a concrete jungle may not quite be reality!

Read The great myth of urban Britain