The water cycle, also known as the hydrologic cycle or the H2O cycle, describes the continuous movement of water on, above and below the surface of the Earth. Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go, in and out of the atmosphere. The water moves from one reservoir to another, such as from river to ocean, or from the ocean to the atmosphere, by the physical processes of evaporation, condensation, precipitation, infiltration, runoff, and subsurface flow. In so doing, the water goes through different phases: liquid, solid (ice), and gas (vapour).
Over
Exploitation
Water resources, such as lakes and
aquifers, are usually renewable resources which naturally recharge (the term
fossil water is sometimes used to describe aquifers which don't recharge). Over
exploitation occurs if a water resource, such as the Ogallala Aquifer, is mined
or extracted at a rate that exceeds the recharge rate, that is, at a rate that
exceeds the practical sustained yield. Recharge usually comes from area
streams, rivers and lakes. An aquifer which has been over exploited is said to
be over drafted or depleted. Forests enhance the recharge of aquifers in some
locales, although generally forests are a major source of aquifer depletion.
Depleted aquifers can become polluted with contaminants such as nitrates, or
permanently damaged through subsidence or through saline intrusion from the
ocean.
This
turns much of the world's underground water and lakes into finite resources
with peak usage debates similar to oil. These debates usually center around
agriculture and suburban water usage but generation of electricity from nuclear
energy or coal and tar sands mining is also water resource intensive. A
modified Hubert curve applies to any resource that can be harvested faster than
it can be replaced. Though Hubert's original analysis did not apply to
renewable resources, their over exploitation can result in a Hubert-like peak.
This has led to the concept of peak water.
