Water circulates through Earth systems via underground aquifers; estuaries and bays; rivers and streams; lakes, ponds and reservoirs; wetlands; ocean and coastal shorelines; and the atmosphere.

A fragile and complex network of living things depends upon a very limited supply of freshwater. Growing human populations and demands on this resource are creating a host of social, economic, environmental and political challenges on scales that range from local to global.

Historically, humans have coped with local scarcity by drawing upon water resources, including non-renewable groundwater, by building dams, digging wells, and otherwise diverting surface water. When natural flows are altered, humans and other species that depend on the natural system suffer.

Rivers, lakes, and wetlands provide essential and typically irreplaceable ecosystem services. These include water purification, habitat support, hydropower, transportation and recreation. A critical link between land and water, wetlands play a key role in filtering pollutants, protecting drinking water and supporting rich biodiversity.

Water pollution from human settlement, industry and agriculture—including the nutrient pollution of eutrophication—threatens the health of humans, other living creatures, and the ecosystems themselves.

Water allocation rather than absolute scarcity lies at the heart of many water problems. Responsible water stewardship, especially across international borders, poses an array of social, environmental, economic and political challenges. Technologies such as desalination, drip irrigation, and water reuse hold promise.

Scientists are encouraging water management plans that replace a sector-by-sector approach with an integrated approach that emphasizes conservation and reuse, and which balances the need for development against the vital importance of clean water and healthy ecosystems.