CGP The Water Cycle

• Water is Stored in Solid, Liquid and Gas Forms
• The hydrosphere contains 14 sestillion litres of water
• Forms of water

  • Saline water in the oceans
  • Freshwater
  • Frozen in the cryosphere
  • Groundwater
  • Liquid freshwater on the Earth's surface
  • Water vapour in the atmosphere
• Less than 3% of the water in the hydrosphere is freshwater
• Most species, including humans, need freshwater to survive
• Freshwater distribution

  • 69% is frozen in the cryosphere
  • 30% is groundwater
  • 0.3% is liquid freshwater on the Earth's surface
  • 0.04% is stored as water vapour in the atmosphere
• Groundwater may not be cost effective to extract as it is hard to access
• Only a small amount of water on the planet can be used by humans
• Water can change between solid, liquid and gaseous forms


  • For water to boil or melt, it has to gain energy, from the Sun
  • For water to condense or freeze, it has to lose energy
• Global hydrological cycle

  • Water is continuously cycled between different stores
  • It is a closed system with no inputs or outputs of water
• The amount of water present in each store varies over a range of scales from local to global
• The magnitude of each store depends on the amount of water flowing between them
• Different flows occur at a range of spatial and temporal time) scales
• Evaporation


  • Liquid water changes state into a gas, becoming water vapour
  • It gains energy normally from solar radiation
  • Evaporation increases the amount of water stored in the atmosphere
• Long-term changes in the climate can affect the magnitude of evaporation
• Magnitude of evaporation

  • Varies by location and season
  • Depends on solar radiation, available liquid water, and air temperature and humidity
• Condensation


  • Water vapour changes state to become a liquid
  • It loses energy to the surroundings
  • Happens when air cools to the dew point temperature
• Water droplets from condensation can stay in the atmosphere or flow to other subsystems
• Magnitude of condensation

  Depends on the amount of water vapour in the atmosphere and the temperature
• Cloud formation and precipitation


  • Clouds form when warm air cools, causing water vapour to condense into water droplets
  • Precipitation is the main flow of water from the atmosphere to the ground
• Causes of precipitation

  • Frontal precipitation (warm air rising over cool air)
  • Orographic precipitation (warm air rising over mountains)
  • Convective precipitation (warm air rising from heated ground)
• Cloud formation requires cloud condensation nuclei (e.g. dust) for water droplets to condense on
• Cloud formation and precipitation can vary seasonally and by location
• Cryospheric processes

  • Accumulation and ablation change the amount of water stored as ice
  • The balance of accumulation and ablation varies with temperature
• During periods of global cold, inputs into the cryosphere are greater than outputs
• During periods of warmer global temperatures, the magnitude of the cryosphere store reduces as losses due to melting are larger than the inputs of snow
• The Earth is emerging from a glacial period that reached its maximum 21,000 years ago
• Variations in cryospheric processes happen over different timescales, from thousands of years to annual temperature fluctuations