5.1 - The global hydrological cycle

A closed system driven by energy

The global hydrological cycle transfers water between stores without creating a new global supply each year.

At the scale of the Earth and its atmosphere, the water cycle is treated as a closed system: transfers across the outer boundary are negligible for ordinary water-budget analysis. Water changes location and state, while the total stock is approximately conserved. The system exchanges energy with its surroundings. It is therefore closed to water, rather than isolated from energy. A catchment inside it can gain or lose water across its own boundary.

Solar energy supplies the latent heat needed for evaporation from oceans, lakes and moist soil. Plants absorb water through roots and release water vapour through leaves by transpiration; evaporation and transpiration together are evapotranspiration. Air movement transports vapour. When moist air cools sufficiently, condensation produces cloud droplets or ice crystals; growth can lead to precipitation. Freezing, melting and sublimation also transfer water between states and stores.

Water raised above sea level has gravitational potential energy. Gravity brings precipitation towards the surface and drives downhill runoff and river flow. It also drives infiltration and percolation where pressure and ground conditions permit. Groundwater follows a hydraulic gradient, which is a difference in water potential; it need not follow the exact surface slope. Gravity does not supply the energy for evaporation. The two energy sources perform different jobs within the same circulation.

Unequal stores and their importance

The largest water store dominates the global stock, but smaller stores often supply the water organisms can use most readily.

The table uses rounded estimates published by the US Geological Survey, based on a global inventory. Percentages refer to all Earth's water, not just freshwater. The categories shown are selected stores and do not form a complete 100% table.

The cryosphere includes frozen water such as glaciers, snow, ground ice and permafrost. The table's ice category excludes ground ice and permafrost; adding the inventory's approximately 0.022% in ground ice/permafrost gives about 1.76% for these major frozen stores. Surface water includes rivers, lakes and wetlands; the lake, river and swamp categories together hold about 0.014% of all water, including saline lakes. Groundwater is also not synonymous with freshwater: the inventory estimates fresh groundwater at about 0.76% of all water. Soil moisture supports roots and exchanges water with the atmosphere, despite its small global stock.

Around 2.5% of global water is fresh, and much of that is frozen or underground. A statement such as “68.7% is in glaciers” uses the freshwater denominator, rather than the all-water denominator used above. Always check the denominator before comparing percentages. A small atmospheric stock matters greatly because it turns over rapidly and controls when and where precipitation occurs. A large ocean stock can support global circulation without being a directly usable drinking-water supply. USGS global inventory.

Balancing annual global flows

A water budget checks that transfers into and out of each store match its change in storage over the chosen period.