water cycle

• Global water stores


  • Oceans (96.9%)
  • Icecaps (1.9%)
  • Groundwater (1.1%)
  • Rivers and Lakes (0.01%)
  • Soil moisture (0.01%)
  • Atmospheric Moisture (0.001%)

  
• Only 2.5% of global water stores are freshwater
• 69% of freshwater is in glaciers, ice caps and ice sheets, and 30% is groundwater
• Surface and other freshwater only accounts for around 1% of global stores
• Other surface and freshwater is made up of permafrost, lakes, swamps, marshes, rivers and living organisms
• 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
• The Earth consist of six cells of circulating air, which form the globe's climate control
• Hadley Cell


  1. Air rises at The Doldrums, travels upwards, then sinks as it meets the cooler air of the Ferrel Cell

  2. At this meeting point, precipitation tends to occur

  3. The air then travels southwards, heating up as it does

  4. It will then have heated sufficiently to rise up at the Doldrums, commencing the cycle again
• Polar Cell


  1. Cold air sinks near the Arctic Circle, cooling and condensing to form precipitation over northern latitudes

  2. The air then travels southwards, heating until it meets warm air from the Ferrel Cell

  3. The air then rises, causing dry conditions for the land beneath, and then travels northwards, cooling as it does
• Ferrel Cell


  1. The middle cell of the ITCZ (tends to be at a mid-latitude location)

  2. 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
  • 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; on a global scale, the water cycle a closed system
• Components of the water cycle

  • Flows/transfers
  • Inputs
  • Outputs
  • Stores
• Inputs to the Drainage Basin - 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


  • Direct intervention of plants' leaves in changing the direction or temporarily stopping precipitation as it falls to the surface
  • Any moisture retained by the surface of the leaf (interception store) is generally greatest at the start of storms
  • A plant's interception capacity varies depending on the type of vegetation
• Infiltration


  • The movement of water from the surface into the soil
  • The infiltration capacity is the maximum rate at which water can be absorbed by the soil
  • Factors affecting infiltration include soil composition, previous precipitation, type and amount of vegetation, compaction of soils, and relief of land
• Surface Runoff


  • Water flows overland, rather than permeating deeper levels of the ground
  • Overland flow occurs faster where the gradient of land is greater
  • Surface runoff is the primary transfer of water to river channels, hence heavily influencing their discharge
• Throughflow


  • Water moves through the soil and into streams or rivers
  • Speed of flow is dependent on the type of soil - clay soils have a slower flow rate, sandy soils drain quickly
• Percolation


  • Water moves from the ground or soil into porous rock or rock fractures (deeper bedrock and aquifers)
  • The percolation rate is dependent on the fractures that may be present in the rock and the permeability of the rock
• Groundwater Flow


  • The gradual transfer of water through porous rock, under the influence of gravity
  • Water can sometimes become trapped within these deeper layers of bedrock, creating aquifers and long water stores for the drainage basin
• Evaporation


  • The direct loss of water moisture from the surface of a body of water, the soil and interception storage (on top of leaves) to the atmosphere
  • Factors affecting evaporation rates include volume and surface area of the water body, vegetation cover or built environment surrounding the water, and the colour of the surface beneath the water
• Transpiration


  • A biological process where water is lost to the atmosphere through the pores of plants (stomata)
  • Transpiration rates are affected by seasonality, the type of vegetation, moisture content of the air and the time of day
• Stores in the water cycle

  • 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
• 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