Water cycle:

• Global hydrological cycle

  The circulation of water around the Earth
• Global hydrological cycle

  • It is a closed system of linked processes so there are no external inputs or outputs
  • The amount of global water is finite
  • The state in which the water exists (liquid, vapour or ice) changes over time with changes in climate
• Power sources driving the global hydrological cycle

  • Solar energy (in the form of heat)
  • Gravitational energy (causing rivers to flow downhill and precipitation to fall to the ground)
• Stores

  Reservoirs where water is held
• Main stores in the global hydrological cycle

  • Oceans
  • Glaciers and ice sheets (cryosphere)
  • Surface runoff (rivers, lakes, groundwater)
  • Atmosphere
• The oceans represent by far the largest store, followed by the cryosphere
• Surface runoff consists of rivers and lakes, as well as groundwater
• Of the freshwater stores, the cryosphere is the largest, accounting for 69% of all the global freshwater, followed by groundwater (30%). Less than 1% is stored in the biosphere (vegetation and soil moisture)
• Flows

  The transfers of water from one store to another
• Main flows in the global hydrological cycle

  • Precipitation
  • Evaporation
  • Transpiration
  • Vapour transport
• Fluxes

  The rates of flow between stores
• The greatest fluxes occur over the oceans
• The annual flux between the atmosphere, ocean and land is 413 fluxes in 10 cubed KM cubed
• The global water budget takes into account all the water that is held in the stores and flows of the global hydrological cycle. The most significant feature of the budget is that only 2.5% of it is freshwater; the rest is in the oceans (Figure 52). Even more remarkable is the fact only 1% of all freshwater is 'easily accessible surface freshwater.

  Nearly 70% is locked up in glaciers and ice sheets.
• Residence time

  The average time a molecule of water will spend in one of the stores in the hydrological cycle
• Residence times

  • 10 days in the atmosphere
  • 3600 years in the oceans
  • 15,000 years in an ice cap
• Fossil water and cryosphere

  Water stores that can be thought of as being non-renewable on human timescales
• Accessible surface freshwater is only 1% of all the world's freshwater
• Accessible surface freshwater is the major source of water for human use
• Accessible surface freshwater is a scarce resource needing careful management
• Drainage basin

  An open system within the global hydrological cycle with external inputs and outputs
• The amount of water in the drainage basin varies over time as the inputs vary</b>
• Drainage basins vary in size from that of a small local stream up to a huge river such as the Amazon
• The drainage basins of tributary streams and small rivers sit within the drainage basins of larger rivers
• 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 global water stores are freshwater
• 69% of freshwater is in glaciers, ice caps and ice sheets
• 30% of freshwater is groundwater
• Surface and other freshwater only accounts for around 1% of global water 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 global water cycle


  • Oceans
  • Icecaps
  • Groundwater
  • Rivers and Lakes
  • Soil moisture
  • Atmospheric Moisture

  
• Oceans have a residence time of 3600 years
• Icecaps have a residence time of 15,000 years
• Groundwater has a residence time of 10,000 years
• Rivers and lakes have a residence time of 2 weeks to 10 years
• Soil moisture has a residence time of 2-50 weeks
• Atmospheric moisture has a residence time of 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.