water cycle

• Global water stores

  • Oceans
  • Glaciers, ice caps and ice sheets
  • Groundwater
  • Surface and other freshwater (permafrost, lakes, swamps, marshes, rivers and living organisms)
• Oceans contain 97% of global water, only 2.5% of stores are freshwater of which 69% is glaciers, ice caps and ice sheets and 30% is groundwater
• Surface and other freshwater only accounts for around 1% of global stores
• Hydrology in Polar Regions


  • 85% of solar radiation is reflected
  • Permafrost creates impermeable surfaces
  • Lakes and rivers freeze
  • Rapid runoff in spring
  • Seasonal release of biogenic gases into atmosphere
  • Orographic and frontal precipitation
• Hydrology in Tropical Rainforests

  • Dense vegetation consuming 75% of precipitation
  • Limited infiltration
  • Deforestation leads to less evapotranspiration and precipitation
  • Very high temperatures
  • Very humid
  • Convectional rainfall
• Components of the water cycle and their residence times

  • Oceans (3600 years)
  • Icecaps (15,000 years)
  • Groundwater (10,000 years)
  • Rivers and Lakes (2 weeks to 10 years)
  • Soil moisture (2-50 weeks)
  • Atmospheric Moisture (10 days)

  
• ITCZ (Inter-tropical Continental Zone)


  The Earth consist of six cells of circulating air, which form the globe's climate control

  
• Hadley Cell


  Air rises at The Doldrums, travels upwards, then sinks as it meets the cooler air of the Ferrel Cell. At this meeting point, precipitation tends to occur. The air then travels southwards, heating up as it does. It will then have heated sufficiently to rise up at the Doldrums, commencing the cycle again.
• Polar Cell


  Cold air sinks near the Arctic Circle, cooling and condensing to form precipitation over northern latitudes. The air then travels southwards, heating until it meets warm air from the Ferrel Cell. The air then rises, causing dry conditions for the land beneath, and then travels northwards, cooling as it does.
• Ferrel Cell


  The middle cell of the ITCZ (tends to be at a mid-latitude location). The air circulation is determined by the Hadley and Polar cells either side, similar to a cog system.
• Drainage basin

  An open subsystem operating within the closed global hydrological cycle, defined as an area of land drained by a river and its tributaries with a boundary (known as the watershed), usually composing of hills and mountains
• On a local scale, the water cycle is an open system (a system of processes of water inputs, outputs and throughputs); on a global scale, the water cycle a closed systems (a system that has no inputs or outputs, only throughputs)
• Components of the water cycle

  • Flows/transfers
  • Inputs
  • Outputs
  • Stores/components
• Precipitation


  Caused by the cooling and condensation of water moisture in the atmosphere, forming clouds that release moisture in the form of rain, snow, hail, sleet, etc.

  
• Factors affecting precipitation

  • Seasonality
  • Variability (secular, periodic, stochastic)
  • Latitude
• Types of rainfall


  • Convectional
  • Frontal/Cyclonic
  • Relief/Orographic

  
• Interception

  The direct intervention of plants' leaves in changing the direction or temporarily stopping precipitation as it falls to the surface
• Infiltration


  The movement of water from the surface into the soil, affected by soil composition, previous precipitation, vegetation, compaction, and relief
• Surface runoff


  Water flows overland, rather than permeating deeper levels of the ground, occurring faster where the gradient of land is greater
• Throughflow

  Water moves through the soil and into streams or rivers, with speed dependent on soil type (clay soils slower, sandy soils faster)
• Percolation


  Water moves from the ground or soil into porous rock or rock fractures (deeper bedrock and aquifers), with rate dependent on fractures and permeability
• Groundwater flow


  The gradual transfer of water through porous rock, under the influence of gravity
• Evaporation

  The direct loss of water moisture from the surface of a body of water, the soil and interception storage to the atmosphere, affected by volume/surface area, vegetation/built environment, and surface colour
• Transpiration

  The biological process where water is lost to the atmosphere through the pores of plants (stomata)
• Water stores in the drainage basin

  • Soil Water
  • Groundwater
  • River Channel
  • Interception
  • Surface Storage
• Water table


  The upper level at which the pore spaces and fractures in the ground become saturated, used to assess drought conditions, health of wetland systems, success of forest restoration programmes

  
• Physical factors influencing the drainage basin

  • Climate
  • Soil Composition
  • Geology
  • Relief
  • Vegetation
  • Size
• Anthropogenic factors influencing the drainage basin

  • Cloud seeding
  • Deforestation
  • Afforestation
  • Dam construction
  • Change in land use
  • Ground water abstraction
• Example

  • Was used in China right before 2008 Beijing games to try and reduce pollution levels
• Deforestation


  • Less vegetation means less interception, less infiltration, more overland flow leading to more flooding, cycle speeds up
• Afforestation


  • More vegetation means interception, less overland flow, more evapotranspiration
• Dam construction

  • Dams reduce downstream river flow and discharge, increase surface stores so more evaporation
• Example


  • Lake Nasser behind Aswan dam in Egypt – 10-16 billion m3 water loss from Nile
• Change in land use


  • Infiltration is 5 times faster under forests compared to grasslands. Converting land to farmland means less interception, increased soil compaction and more surface runoff
• Ground water abstraction


  • When water is taken out faster that the water is recharged, groundwater flow decreases and the water table drops
• Example


  • In China, groundwater irrigates 40% of farmland whilst 70% of drinking water comes from groundwater
• Irrigation


  • Drop in water tables due to high water usage
• Example


  • Aral Sea in Kazakhstan shrank in 1960s due to farmers using the water to grow cotton
• Urbanisation


  • Impermeable surfaces reduce infiltration, increase surface runoff, river discharge increase. Cycle speeds up
• Water Budget


  Precipitation = Discharge + Evaporation ± changes in stores