Water Supply Problems: Resource Depletion
As groundwater is pumped from water wells, there usually is a localized drop in the water table around the well called a cone of depression. When a large number of wells have been pumping water for a long time, the regional water table can drop significantly. This is called groundwater mining, which can force the drilling of deeper, more expensive wells that commonly encounter more saline groundwater. Rivers, lakes, and artificial lakes (reservoirs) can also be depleted due to overuse. Some large rivers, such as the Colorado in the U.S. and the Yellow in China, run dry in some years. The case history of the Aral Sea discussed later in this chapter involves the depletion of a lake. Finally, glaciers are being depleted due to accelerated melting associated with global warming over the past century.
Another water resource problem associated with groundwater mining is saltwater intrusion, where the overpumping of freshwater aquifers near ocean coastlines causes saltwater to enter freshwater zones. The drop of the water table around a cone of depression in an unconfined aquifer can change the direction of regional groundwater flow, sending nearby pollution toward the pumping well instead of away from it. Finally, problems of subsidence (gradual sinking of the land surface over a large area) and sinkholes (rapid sinking of the land surface over a small area) can develop due to a drop in the water table.
Water Supply Crisis
The water crisis is a global situation where people in many areas lack access to sufficient, clean, or both. This section describes the global situation involving water shortages, called water stress. Water stress is generally greatest in areas with very low precipitation (major deserts), large population density (e.g., India), or both. Global warming could worsen the water crisis by shifting precipitation patterns away from humid areas and melting mountain glaciers that recharge rivers downstream. Melting glaciers will also contribute to rising sea levels, worsening saltwater intrusion in aquifers near ocean coastlines.
According to a 2006 United Nations Development Programme report, 700 million people (11% of the world’s population) lived with water stress. Most of them live in the Middle East and North Africa. By 2025, the report projects that more than 3 billion people (about 40% of the world’s population) will live in water-stressed areas, with the large increase coming mainly from China and India. The water crisis will also impact food production and our ability to feed the ever-growing population. We can expect future global tension and even conflict associated with water shortages and pollution. Historical and future areas of water conflict include the Middle East (Euphrates and Tigris River conflict among Turkey, Syria, and Iraq; Jordan River conflict among Israel, Lebanon, Jordan, and the Palestinian territories), Africa (Nile River conflict among Egypt, Ethiopia, and Sudan), Centra