Freshwater trends: Will future needs be met?

Despite improvements in the efficiency of water use in many developed countries, the demand for fresh water has continued to climb as the world's population and economic activity have expanded. From 1940 to 1990, withdrawals of fresh water from rivers, lakes, reservoirs, underground aquifers, and other sources increased by more than a factor of four. Increases in irrigation and, to a lesser extent, industrial uses of water have been the largest sources of this growing demand. At the same time, contamination by pollutants has seriously degraded water quality in many rivers, lakes, and groundwater sources, effectively decreasing the supply of fresh water. The result has been increased pressure on freshwater resources in most regions of the world and a lack of adequate supplies in some localities. Water experts and international institutions warn that water shortages could become critical in some regions. In the absence of significant changes in policy and far more effective management of water resources, this could pose serious long-term obstacles to sustainable development in many countries.

The supply of fresh water in a region is limited by the dynamics of the hydrological cycle, in which sea water evaporates and falls over land as precipitation. The renewable supply of water is defined as the surface water runoff from local precipitation, the inflow from other regions, and the groundwater recharge that replenishes aquifers. Because water can, in principle, be reused many times, the availability of water for human use depends as much on how it is used and how water resources are managed as on any absolute limits. With proper treatment, for example, the water returned to rivers by upstream users is also available to downstream users. Nonetheless, the renewable supply is an important constraint to the sustainable use of water within a region. Apart from human use, water is also needed to sustain the natural ecosystems found in wetlands, rivers, and the coastal waters into which they flow.

Pumping water from underground aquifers faster than they can be recharged or diverting so much water from wetlands or rivers that freshwater ecosystems fail are clearly unsustainable practices. To avoid conflict where water resources are shared, upstream and downstream users must agree on how water is to be allocated. Unfortunately, examples of unsustainable water uses can be found in virtually every region--in the depletion of the Ogallala aquifer in the United States and similar overpumping of other aquifers in parts of North Africa, the Middle East, India, and Southeast Asia; in the diversion of river water from the dying Aral Sea in Kazakhstan and Uzbekistan and from the Florida Everglades; in the excessive withdrawals that are causing intrusions of sea water into deltas and coastal aquifers in China, Viet Nam, and the Gulf of California; in the uncontrolled flow of sewage and fertilizer runoff that is hastening eutrophication in some temperate and tropical lakes and many coastal seas; and in the potential for conflict over water in areas such as the Nile River delta, the Middle East, and Southeast Asia.

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