The Global Hydrological Cycle is of Enormous Importance to Life on Earth

Considered as a closed system - The same amount of water is always maintained (no inputs/outputs) 3 key concepts for understanding how water cycling operates:

• Stores - Reservoirs where water is held (eg. oceans)

Ocean

• Fluxes - Rate of flow between stores

*Fluxes *

• Processes - Physical mechanisms which drive the fluxes of water between the stores .The global hydrological cycle

Example of a closed system driven by solar energy and gravitational potential energy

• A fixed amount of water in the earth-atmosphere system
  • Does not have external inputs or outputs ∴ the total volume of water is constant
  • Water can be held in different states (ice, water vapour etc)
  • In ice age more water held in cryosphere than oceans Cryosphere → Areas of the Earth where water is frozen into snow or ice

Blue water → Water is stored in rivers, streams, lakes and groundwater in liquid form

Green water → Water stored in the soil and vegetation

The major fluxes are driven by processes such as: Precipitation, evaporation, cryospheric exchange and run-off

• Precipitation → The movement of water in any form from the atmosphere to the ground
• Evaporation → Change in state of water from liquid to gas
• Residence times → The average time a water molecule will spend in a reservoir or store
  Oceans: 3, 600 yrs

Icecaps: 15, 000 yrs

Groundwater: Up to 10, 000 (groundwater), 100-200 yrs (shallow groundwater)

Rivers & lakes: 2 weeks to 10 years, 50 yrs large scale

Soil moisture: 2 - 50 weeks

Atmospheric moisture: 10 days

• Fossil water → Ancient deep groundwater from former pluvial (wetter) periods
• Groundwater flow → Slow transfer of percolated water underground through pervious or porous rocks

The Global Water Budget

The annual balance between the fluxes and size of water stores

• Water is generally considered to be renewable however the availability of water varies throughout the year