Water cycle

• Why is water important?

  Oceans moderate global temperatures through absorbing, storing and slowly releasing heat.

  Clouds reflect 20% of incoming solar radiation and lower surface temperatures.

  Water vapour absorbs long-wave radiation from Earth, reducing global temperatures by 15 degrees.

  Water makes up 65 to 95% of all living organisms and is crucial to their growth, reproduction and metabolic functioning.

  Used for all chemical reactions in animals.

  Essential economic resource.
• The global water cycle is closed.
• Oceanic water store


  97% of global water (1370000 km cubed times 10 cubed).

  Five oceanic bodies of water ( the Atlantic, Pacific, Indian, Antarctic and Arctic) and other smaller seas covering 72% of the Earth’s surface.

  The Pacific Ocean is the largest.
• Cryospheric water store

  2% of global water (29000 km cubed times 10 cubed)

  Made up of sea ice, icecaps, ice sheets, Alpine glaciers and permafrost.

  Occur in high-altitude and high-latitude areas e.g. Antarctic ice sheet or the Himalayan icecap.
• Groundwater store

  0.7% (9500 km cubed x 10 cubed)
• Lake & rivers water store

  Lakes are 0.01% of global water (125 km cubed x 10 cubed)

  Rivers are 0.0001% of global water (1.7 km cubed x 10 cubed)
• Soil moisture water store

  0.005% of global water (65 km cubed x 10 cubed)
• Atmospheric water store

  0.001% of global water (13 km cubed x 10 cubed)
• Water Cycle Budget

  Annual volume of water movement by precipitation evapotranspiration and runoff between stores.
• Percolation


  is the movement of surface and soil water into underlying permeable rock.
• Infiltration

  Vertical movement of rainwater through the soil due to gravity
• Surface Run-off

  Movement of water across the land surface
• Channel flow


  Movement of water in streams and rivers.
• Condensation


  Water vapour turning into water droplets due to cooling to dew point.
• Evaporation

  Water turning into vapour due to heating. Energy is absorbed as latent heat and later released thus allowing for global heat redistribution.
• Groundwater flow

  Movement of water underground through permeable rocks.
• Interception

  Collection of water by vegetation on leaves, stems and trunks. Capacity influenced by precipitation duration and intensity.
• Overland flow

  Flow of water over the Earth’s surface towards rivers, streams, lakes and oceans.
• Precipitation

  All water that falls to earth as rain, snow, sleet or hail.

  Formed when vapour hits dew point.
• Throughflow

  Movement of water through the soil.
• Transpiration

  Loss of moisture from plants through their stomata,

  Responsible for 10% of all moisture in the atmosphere,

  Influenced by temperature and wind speed.
• Groundwater

  An underground store of water (aquifer) in permeable rocks which is influenced by seasonal rainfall, evaporation and utilisation rates.
• Stem flow and throughfall

  Interception capacity is exceeded and water slowly falls or runs down to the surface.
• Soil moisture

  Short term storage of water in the soil profile - the volume of which may be determined by vegetation cover and temperature.
• Surface Storage

  Storage of water on the surface as puddles
• Percolation


  Downward movement of water deep into the rocks to groundwater store through permeable rocks.
• The water balance

  Precipitation = total runoff + evapotranspiration +/- Storage