The Water Cycle
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Hydrologic Cycle

The water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. As it is a cycle, there is no beginning or end. Water can change states among liquid, vapor, and ice at various places in the water cycle.

The Water Cycle:

  • Sun heats the oceans' waters and it evaporates into air
  • The hot air rises and is blown over the land by the wind
  • As the air rises it cools and loses its ability to hold the moisture up
  • Clouds form and when the droplets become too heavy they are attracted by gravity to the earth and fall as rain or snow
  • The water eventually drains into rivers and flows back to the oceans.

A hydrogeologist at work on a groundwater protection schemeGroundwater can be a long-term 'reservoir' of the natural water cycle (with residence times from days to millennia), as opposed to short-term water reservoirs like the atmosphere and fresh surface water (which have residence times from minutes to years).

Water tends to permeate to depths of about 5kms below the surface. The water table marks the level of the upper surface of the saturated zone (the area in which water completely fills the rock pore space. Above this level the pore space between the rocks is filled with air and water).

Highly permeable rocks may allow water to move up to 100 metres a day through them. A well must be drilled into an aquifer (through to its saturated zone) to reach an adequate supply of water. Contamination is a real problem. Nitrate, one of the most widely used fertilizers, is harmful in even small quantities in drinking water. Under ideal conditions human sewage can be purified by only about 150 feet of travel through a sandy loam soil. The sewage is purified by filtration and absorption and decomposition by soil organisms. Balancing withdrawal and recharge is essential in water management particularly for cities where there is a significant draw on groundwater. Mexico City has subsided more than 20 feet during the last few decades due to groundwater extraction from pumping wells. The surface drops as groundwater is pumped because the water no longer supports the rock and sediment. Such subsidence can crack building foundations and roads etc. In order to avoid problems of falling water tables and subsidence many cities use artificial recharge to increase the natural rate of groundwater replenishment.

Find out more about groundwater by clicking on the Groundwater Programme pages.

A hydrologist studies water above the ground and a hydrogeologist studies groundwater and helps prevent hazards within the groundwater system and is also involved in finding new sources of groundwater for towns and villages.