Water and Physical Geography (AQA A Level Geography)

• The quantity and quality of water available to people depend on a number of physical factors
• Climate has a significant influence on the water supply
  • Precipitation
  • Temperature
  • Wind speed and direction

Physical Factors Affecting Water Supply

• The structure and characteristics of the rock underlying a landscape can influence the distribution of groundwater stores
• Layers of rock that contain groundwater are called aquifers
• They need rocks with high porosity or permeability to hold underground water and allow the aquifer to be recharged, e.g.:
• Chalk
• Limestone
• Sandstone
• Gravel

The characteristics and formation of aquifers

• Aquifers can be a reliable source of water because they are:
  • Not subject to evaporation
  • Springs and wells give an even supply throughout the year
  • Not affected by uneven seasonal rainfall
  • Water filtered through bedrock is purer; needs less treatment before drinking compared to reservoir or river water
• But aquifers can be vulnerable to overuse and the improper disposal of chemicals on the surface
• Falling groundwater levels may indicate climate change or over-abstraction
• Over-abstraction occurs when the rate of extraction of water from the aquifer is greater than the recharge rate
• It can lead to a number of serious environmental impacts including:
  • Subsidence
  • Loss of wetlands and reduction in vegetation
  • Drying of soils leading to dehydration of crop roots
  • Lower water table
  • Rivers drying up
  • The saltwater incursion into groundwater

• The Thames Basin aquifer, in the south-east UK, has a good example of an ‘artesian’ aquifer below it:
  • Boreholes are now up to 200m deep
  • There is not high enough hydrostatic pressure to power fountains
  • The main supply of water within London and the Thames basin is from underground chalk aquifers
  • Water is accessed directly through boreholes, supplying 20% of London’s water supply
  • London’s aquifer water levels have dropped in recent years
  • The population of south-east UK continues to increase
  • Demand for water is rising further

  • Risk of over-abstraction leading to depletion of the aquifer

• The size and nature of a river’s drainage basin can influence water supply in the surface storage system
• The larger the basin the larger the catchment area  for inputs into the system via precipitation 
• Impermeable soils and bedrock in the basin support more overland flow and lead to a denser tributary network
• A dense network of tributaries allows falling precipitation to reach the main channel quicker, keeping the river levels higher
• This may be offset by greater rates of evaporation, depending on the climate and season
• Basins that are less dense allow more infiltration into [popover id="DUoOIPfl7osGfJEU" label="groundwater"] stores
• The Thames Basin provides 80% of London’s water supply from tributaries that flow over impermeable clay
• The relief within a drainage basin can also influence the water supply
• Gentle or undulating relief means water passes more slowly through an area and there is greater time for it to infiltrate and contribute to groundwater
• Steep relief generates faster flow and the reverse effect
• Undulating relief is more likely to generate natural lake storage than the other two extremes of very mountainous or very flat relief
• The type and amount of  vegetation in the drainage basin can also influence the water supply
• Vegetation can intercept rainfall, impede surface flow and absorb water through root systems
• This affects the amount of collectable water stored on the surface
• But it can also transpire water, keeping the atmosphere moist and helping rain clouds form increasing the inputs into the basin