water cycle

• Water cycle

  Stores and transfers water globally
• Key things to know about the water cycle

  • The water cycle stores and transfers water
  • Global distribution of water
• Water cycle stores


  Most of the Earth's water is saline (in oceans), freshwater is stored mainly in snow, ice and groundwater
• Water cycle transfers


  Processes involved in moving water between stores (e.g. precipitation transfers water from atmosphere to Earth's surface)
• Main stores in the water cycle

  • Atmosphere
  • Hydrosphere (liquid water)
  • Cryosphere (frozen water)
  • Lithosphere (land)
• 74% of Earth's water is saline, 25% is freshwater, surface and other freshwater comprises only 1.2% of all freshwater
• Groundwater aquifers

  Porous, permeable rocks that store water, the upper level of saturated rock is the water table
• Just over 30% of all freshwater is stored in aquifers
• Ancient (fossil) aquifers in Africa, Middle East and Australia were formed thousands of years ago when the climate was much wetter
• The water cycle essentially describes the Earth's stores and transfers of water
• Climate change can impact the magnitude of water cycle stores
• Cloud formation and precipitation processes can impact the water cycle
• Cryospheric processes like melting of ice sheets and glaciers can impact the water cycle
• The atmospheric circulation model determines cloud formation and precipitation
• The soil water budget describes changes in soil moisture during the course of a year
• Factors affecting the magnitude of water cycle stores include climate, geology, land use, and soil characteristics
• Drainage basin

  The area of land drained by a river and its tributaries
• Drainage basin system

  • An open system with inputs (precipitation) and outputs (evapotranspiration)
  • Includes stores like surface water, soil water, and groundwater, and processes like infiltration, overland flow, and throughflow
• Infiltration capacity is the rate at which water can enter the soil, it is exceeded when the soil is unable to absorb water at the rate it is falling
• Overland flow

  Water that is unable to infiltrate and runs off the surface
• Throughflow


  Water that passes through soils rather than being stored as soil water
• Water balance

  Expresses the relationship between precipitation, evapotranspiration, and runoff
• The total runoff (expressed as a percentage of precipitation) is a measure of the proportion of precipitation that makes its way into streams and rivers
• Factors affecting variations in runoff include precipitation type and intensity, soil type, land use, geology, and infiltration capacity
• Flood hydrograph


  A graph showing the discharge of a river following a particular storm event
• Flood hydrograph

  • Flashy hydrograph with short lag time and high peak vs low flat hydrograph with low peak
  • Affected by drainage basin characteristics like size, drainage density, rock type, land use, and soil moisture
• Discharge

  The volume of water flowing down a river, expressed in cubic metres per second
• Natural variations affecting the water cycle include seasonal changes in precipitation, temperature, and soil moisture
• Human activities affecting the water cycle include land use change, farming practices, and water abstraction
• Draining peatlands can impact both the water cycle and carbon cycle by lowering the water table and enabling decomposition of carbon stores
• River catchment: the River Exe, Devon

  • Highest point at Stanis on Exter
  • Landscape is flat in the south
  • Most of the catchment is underlain by impermeable rocks, predominantly Devonian sandstones
  • Land use is agricultural grassland (70%), woodland (15%) and arable land, with moors and peat bogs (15%) on high ground
• Water balance for the Exe catchment

  1. Precipitation (1291mm)

  2. Evaporation +/- soil water storage (451mm)

  3. Runoff (844mm)
• Runoff accounts for some 65% of the water balance
• Reasons for high runoff

  • Impermeable nature of the bedrock reduces percolation
  • Drainage ditches on Exmoor reduce the amount of soil water storage
• Wimbleball Reservoir


  Reservoir created in 1999 by damming the River Haddo to supply water and regulate flows
• Peatland restoration on Exmoor

  Blocking drainage ditches to increase water content and return peat bogs to saturated, boggy conditions
• Peatland restoration

  Increases water storage, regulates flows, and helps carbon sequestration
• Exmoor Mires Project

  1. Blocking drainage ditches with peat blocks or ground bales

  2. Increasing water content and returning peat bogs to saturated, boggy conditions

  3. Helping to store carbon within the peat
• By 2015, over 1000 ha of peat moorland had been restored and nearly 100km of ditches blocked, raising the water table by 2.65cm
• Flood and drought peaks have been reduced