Overdrafting in the context of "Sustainable yield"

Water resources are natural resources of water that are potentially useful for humans, for example as a source of drinking water supply or irrigation water. These resources can be either freshwater from natural sources, or water produced artificially from other sources, such as from reclaimed water (wastewater) or desalinated water (seawater). 97% of the water on Earth is salt water and only three percent is fresh water; slightly over two-thirds of this is frozen in glaciers and polar ice caps. The remaining unfrozen freshwater is found mainly as groundwater, with only a small fraction present above ground or in the air. Natural sources of fresh water include frozen water, groundwater, surface water, and under river flow. People use water resources for agricultural, household, and industrial activities.

Water resources are under threat from multiple issues. There is water scarcity, water pollution, water conflict and climate change. Fresh water is in principle a renewable resource. However, the world's supply of groundwater is steadily decreasing. Groundwater depletion (or overdrafting) is occurring for example in Asia, South America and North America.

An aquifer is an underground layer of water-bearing material consisting of permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology. Related concepts include aquitard, a bed of low permeability along an aquifer, and aquiclude (or aquifuge), a solid and impermeable region underlying or overlying an aquifer, the pressure of which could lead to the formation of a confined aquifer. Aquifers can be classified as saturated versus unsaturated; aquifers versus aquitards; confined versus unconfined; isotropic versus anisotropic; porous, karst, or fractured; and transboundary aquifer.

Groundwater from aquifers is sustainably harvested by humans through the use of qanats leading to a well. This groundwater is mainly used for agricultral purposes but is used for other reasons such as home use, industrial use and is sometimes used as a source of renewable energy. Groundwater is a major source of fresh water for many regions, although it can present various challenges for the environment, such as overdrafting (extracting groundwater beyond the equilibrium yield of the aquifer), groundwater-related subsidence of land, and the salinization or pollution of the groundwater.

Lake Texcoco (Spanish: Lago de Texcoco; Nahuatl languages: Tetzco(h)co) was a natural saline lake within the Anahuac or Valley of Mexico. Lake Texcoco is best known for an island situated on the western side of the lake where the Mexica built the city of Mēxihco Tenōchtitlan, which would later become the capital of the Aztec Empire. After the Spanish conquest, efforts to control flooding led to most of the lake being drained.

The entire lake basin is now almost completely occupied by Mexico City, the capital of the present-day nation of Mexico. Drainage of the lake has led to serious ecological and human consequences. The local climate and water availability have changed considerably, contributing to water scarcity in the area; subsequent groundwater extraction leads to land subsidence under much of the city. Native species endemic to the lake region, such as the axolotl, have become severely endangered or extinct due to ecosystem change.

Groundwater-related subsidence is the subsidence (or the sinking) of land resulting from unsustainable groundwater extraction. It is a growing problem in the developing world as cities increase in population and water use, without adequate pumping regulation and enforcement. One estimate has 80% of serious U.S. land subsidence problems associated with the excessive extraction of groundwater.

Groundwater can be considered one of the last free resources, as anyone who can afford to drill can usually draw up merely according to their ability to pump (depending on local regulations). However, pumping-induced draw down causes a depression of the groundwater surface around the production well. This can ultimately affect a large region by making it more difficult and expensive to pump the deeper water. Thus, the extraction of groundwater becomes a tragedy of the commons, with resulting economic externalities.