2005 - 2006 - Pinsent Masons Water Yearbook 2012

PART 4: APPENDIX 1: THE WATER CYCLE AND WATER SERVICES
km³ pa is returned to the land through precipitation against 390,000 km³ pa precipitation into the
sea. This results in a net gain of 40,000 km³ pa on to land. It is this net gain that sustains life
upon the earth’s surface.
Residence times
The longer water is held in a particular place, the less enjoins in the water cycle. While water in
the atmosphere and rivers may account for a small fraction of the global total at any one time,
its relative mobility means that on average 33 times more water is precipitated each year than is
held in the atmosphere at any one time.
Average residence time for water
Oceans 2,500 years
Groundwater 1,400 years
Lakes 17 years
Rivers 16 days
Atmosphere 8 days
Water usage
The intensity of water withdrawal depends to a large extent upon how much water is used for
power station cooling and for irrigation. Groundwater resources are used mainly for domestic
and industrial use, especially in urban areas. These resources are not degraded by domestic
and industrial effluents in the direct way that surface waters are. Instead, aquifers may originate
well away from areas of effluent discharge and thus their integrity remains relatively unimpaired
for quite some time after urban watercourses become unsuitable for use.
Surface water Groundwater
42,650 km³ pa renewable resources