5.1

• The global hydrological system is a closed system – it has no external inputs or outputs so the volume of water is constant and finite.

  The system has three components:

  • Stores – where water is held
  • Fluxes (flows) – water movement
  • Processes – the physical mechanisms which drive the fluxes.
• The energy motivating the global hydrological cycle comes from the sun and gravity.

  
  

  Solar energy causes evaporation of water from the Earth’s surface. Water is drawn from the soil by plants and evaporated from leaves by the process of evapotranspiration. As humid air rises condensation occurs leading to the formation of clouds. The sun helps melt the snow and ice, releasing water from a store.

  
  

  Gravity causes precipitation, interception or as overland flow, infiltration and helps it flow in the soil as throughflow, percolation into underlying bedrock, this will become groundwater.
• Global water budget is the annual balance of water flows and the size of the water stores. Avg annual precipitation over the whole globe is about 860mm, of which 77% falls on the oceans and 23% on land. Evaporation from the land accounts for 16% of the total precipitation it receives, and 7% of global precipitation returns to the sea as river and groundwater flows.

  Blue water – the visible part of the hydrological cycle – rivers, streams, lakes, seas and oceans.

  Green water – water stored in the soil and vegetation

  Cryosphere – the frozen water in ice, snow and permafrost.
• The most important aspect of the hydrological cycle is the accessibility to fresh water. Only 1% of water budget is accessible. Water stores may have different residence times, but the constant circulation of water makes it a renewable resource.

  Fossil water is water stored in groundwater for millennia. E.g in the Sahara Desert water collected in aquifers at a time when the climate was wetter is being extracted by humans, however in current climatic conditions there is little or no recharge of this water and so effectively this type of groundwater is non-renewable.