2005 - 2006 - Pinsent Masons Water Yearbook 2012

The water cycle and water services
Distribution of water resources
PART 4: APPENDIX 1: THE WATER CYCLE AND WATER SERVICES
The world’s water resources are not a problem. It is their distribution and management in
relation to current and future demand that presents challenges. The ‘Blue Planet’ is aptly
named. Evenly distributed upon a perfectly smooth sphere, water would cover the earth to a
depth of 2.7 km. Freshwater alone would cover the surface to a depth of 70m. However, only
0.16% of the world’s water is contained in freshwater lakes and rivers.
Global breakdown of all water resources (km³)
Salt water 1,348,000,000 97.390%
Freshwater 36,020,000 2.610%
- Frozen 27,820,000 2.010%
- Groundwater 8,062,000 0.583%
- Lakes and rivers 225,000 0.016%
- Atmosphere 13,000 0.001%
Freshwater
Saline or brackish water has at best little utility for life on the land surface. Life upon the land
depends on a minimum access to freshwater in a useable form. As the table below highlights,
barely 10% of freshwater supplies are even potentially readily available for abstraction. The
fragment held in the atmosphere constantly replenishes the river system, in itself a fraction of
surface water supplies.
Global breakdown of freshwater resources
Frozen 77.230%
Groundwater (800-4,000 metres) 12.350%
Groundwater (>800 metres) 9.860%
Freshwater lakes 0.350%
Soil 0.170%
Atmosphere 0.040%
Rivers 0.003%
Plants & animals 0.003%
Water bearing minerals 0.001%
The water cycle
The water cycle refers to the process whereby water is circulated through the biosphere. The
cycle begins with water being precipitated on to the land surface. On reaching the ground, it
either infiltrates the soil or runs off into the river system. Water in the soil is either taken up by
plants where it is returned to the atmosphere through transpiration, or it percolates through the
soil. Once through the soil, it either enters the river system or recharges aquifers (water bearing
rock). From the aquifer, water seeps into the river system and is discharged into the sea through
coastal springs or is stored in the rock. Some water from both river and ground water is taken
up by plants and in turn transpired, but most is discharged into the sea. Evaporation from
seawater, along with a small amount from surface waters, is the main source of atmospheric
water.
The global water balance
Even though more water is precipitated upon the oceans than the land surface in relation to
their total surface area, the actual process involves more water being taken up from the sea
than is returned by precipitation. In total, 500,000 km³ per annum (pa) of water is taken up and
returned through evapotranspiration and precipitation. While 430,000 km³ pa is removed
through evaporation from oceans and 70,000 km³ pa in evapotranspiration from land, 110,000
365 Pinsent Masons Water Yearbook 2005-2006