Deck 33- Factors impacting on drainage basins Flashcards
What physical factors impact on drainage basins?
Climate, relief, soils, geology and vegetation
How does climate impact on flows in the drainage basins?
- Influences the type and quantity of precipitation received
-Impacts on the type of vegetation that can grow
-Impacts on rate of surface evaporation
How does relief impact on flows in the drainage basin?
-Areas of high relief are more likely to experience higher rates of orographic rainfall
-Harder for vegetation to grow on steeper slopes meaning less interception occurs
-Lower rate of infiltration on steeper slopes as downhill movement is promoted
-Water reaches the channel faster in areas with steep slopes as more surface runoff occurs
How do soils impact on flows in the drainage basin?
-Type/quality of soil is important for determining what vegetation can grow impacting on interception rates
-Sandy soils with a high porosity have higher rates of infiltration compared to clay soils with low porosity
-Soils with high infiltration capacities will slow the movement of water towards the channel
How does geology impact on flows in the drainage basin?
-Rocks with lots of microstructures and sedimentary rocks have higher rates of percolation
-Higher percolation rates will slow the movement of water towards the channel as more water is stored in groundwater
How does vegetation impact on flows in the drainage basin?
-Scientific research has shown areas with high rates of transpiration have higher levels of precipitation
-More vegetation results in more interception. Forests intercept more than fields/grassy areas
-Presence of plants reduces the infiltration rate as plants absorb moisture through their roots
-Vegetated areas slow the movement of water and decrease the amount as water is lost by evapotranspiration
What human factors impact on flows within drainage basins?
Cloud seeding, deforestation, afforestation, construction of dams/reservoirs, changing agriculture, irrigation, decrease in industrial abstraction, urbanisation
What is cloud seeding?
Cloud seeding is the process of trying to artificially generate clouds to increase the amount of precipitation entering a drainage basin
What has cloud seeding been used for?
Cloud seeding is commonly used in drought-stricken areas to increase rates of precipitation and alleviate strain on water resources. The process has also been used to clear fog in airports, combat forest fires and even divert rainfall
What are the concerns with cloud seeding?
there are some concerns that rain as cannot be ‘generated’ on a large/global scale the process of cloud seeding is simply moving the issue of drought from one location to another. There are also some health concerns surrounding the use of silver iodide
What is an example of where cloud seeding has been used?
China currently has the largest weather modification programme as it is trying to increase precipitation in its dry northern regions. Between 1999 and 2007 China reported that it created more than 250 billion tons of rain. During the 2008 Beijing Olympics China used cloud seeding to divert precipitation away from their roofless stadium (the Bird’s Nest).
Russia have previously used cloud seeding to prevent radioactive rain from reaching Moscow after Chernobyl in 1986.
How do dams/reservoirs impact on the speed of water movement through drainage basins?
The construction of dams and reservoirs slow down the transfer of water through drainage basins. As the amount of water that is stored on the Earth’s surface increases the evaporation potential also increases. This can result in increased amounts of water leaving a drainage basin. Reservoirs, such as Kielder reservoir (Northumberland) often have a dam that controls the amounts of water that flows into the channel at any given point. This is an important control on discharge and can act as a flood defence by closing the taps after high precipitation events to increase the amount stored upstream and release it steadily into the channel over time.
How does Kielder reservoir also provide water security in North East England?
Reservoirs also serve as an important supply of water for human consumption and the taps can be opened/closed depending on the demand for and availability of water. In dry conditions the taps are often closed to preserve the amount of water stored in the reservoir and help to regulate water supply.
How does deforestation impact on flows in the drainage basin?
Deforestation is the removal of vegetation from an area. It can appear on a small scale and on an industrial scale. The removal of trees leads to a reduction in evapotranspiration and an increase in surface run off. This can lead to an increase in flooding potential and leads to a decline of surface storage. In other words, the drainage basin cycle speeds up.
What are the secondary issues with deforestation when considering flood risk?
The problem of deforestation is further compounded by the new land use, for example often animals such as deer move into the area and graze the ground and compact the soil. The compacted soil reduces the infiltration rate and capacity further leading to further increases in surface run off.
How does afforestation impact on flows within drainage basins?
Afforestation (the establishment of trees and forests in an area where there was no recent tree cover) should have the reverse effect by trapping sediment and slowing the movement of water through the drainage basin.
What are the problems of using afforestation to combat increased flood risk?
it takes time for trees and forests to become established. Recent research carried out in Plynlimon (part of the River Severn catchment) has shown that immediately after planting new, young trees there is an increase in surface run off and sediment loss as a result of further compaction of soils by tractors and planting equipment. Researchers have estimated that it takes 30 years for the process to be reversed when the trees fully grown and established. Once this occurs rates of interception and evapotranspiration will increase reducing surface run off.
How does increasing agriculture impact on flows within drainage basins?
-The conversion of forests to farmland (e.g. the UK) has huge impacts on the rate of interception (in coniferous forests 25-35% of precipitation is intercepted compared to 15-25% in deciduous forests and 10-15% in arable farmland). Forests have higher rates of interception due to the larger plants and canopy structure i.e. layers of vegetation within a forest.
-soils in agricultural land also tend to be more compacted than those in natural forests and woodlands. This compaction is cause by mechanical vehicles, such as tractors and grazing animals trampling over the same area of land. Compacted soils have a low porosity and inhibit rates of infiltration leading to increased surface run off. Waterlogged fields and salinisation (the build up of salt in soils) are common in farmland with poor drainage.
How do farmers aim to try to improve soil quality?
Many farmers aim to improve the quality of drainage of their soils through ploughing which loosens and aerates the soil increasing the rate of infiltration. Another solution is to install drainage systems to divert water away from the fields and problems.
What is irrigation and artificial irrigation?
Irrigation is the supply of water to land or crops to help growth, typically by means of channels. Artificial irrigation is where humans have extracted water (typically from groundwater stores) to add to natural channels to increase crop yields.
What are the problems of artificial irrigation?
Irrigation increases surface stores of water in the channel but can risk depleting sub surface stores and larger surface stores which may have been diverted.
What is an example of artificial irrigation that has caused issues?
The Aral Sea, between Kazakhstan and Uzbekistan, is an example of the damaging impacts of overextraction of water. The Aral Sea began shrinking in the 1960s when Soviet irrigation scheme for the growth of cotton took water from the Syr Darya and Amu Araya rivers, which greatly reduced the amount of water reaching the Aral Sea. By 1994, levels had fallen by 16m, the surface area had declined by 50%, the volume by 75% and the salinity (salt) levels had increased by 300%. The dramatic changes in water level has had major ecological consequences.
How has the UK’s shift towards a post-industrial economy impacted on the amount of water being used by industries?
As the UK has shifted to a post-industrial economy, traditional industries such as the ship building and textile industries have declined and there has been a large increase in tertiary and quaternary industries. These industries are much less demanding on water and has such there have been large reductions in the amount water being abstracted from groundwater stores for industrial purposes.
Why has London’s flood risk increased due to a post-industrial economy being established in the UK?
In London, the water table dropped by 90metres between the mid 1800s and the 1960s due to abstraction for engineering works and breweries.
After deindustrialisation, between the 1960s and early 2000s it is thought that groundwater levels increased by 40metres threatening to flood cellars and basements in houses and cause leakage into tunnels such as those used by the London Underground.
In order to ensure the London Underground does not flood, 27million litres of water are pumped out of the ground daily.
How does urbanisation impact on rates of infiltration?
Urbanisation is the increasing proportion of a population living in urban areas and results in the expansion of urban areas into the surrounding rural areas. As areas become increasingly urbanised, they become covered in impermeable surfaces such as tarmacked roads, pavements and driveways. This limits the amount of infiltration that can take place causing water to build up on the surface instead resulting in increased surface run off and an increase likelihood of flooding.
How does urbanisation impact on interception?
In urban areas there tends to be less vegetation cover, as plants are replaced with buildings. The reduces the amount of evapotranspiration that can take place resulting in a further increase in surface stores and further increases the likelihood of flooding. The high density of buildings means that rain falls on roods and is quickly transferred into drains through gutters and pipes which increase the speed at which water is transferred into the channel and can also flood when blocked.
Which cities in the UK have experienced increased flood risk due to increased urbanisation?
Urbanisation has occurred across the UK and increased the flood risk in many cities e.g. Sheffield (2007, 2018), York (2015, 2021, 2023), Manchester (2015).
