Water in a changing world

Renewable fresh water comprises a tiny fraction of the global water pool bu t is the foundation for life in terrestrial and freshwater ecosystems. The benefits to humans of renewable fresh water include water for drinking, irr igation, and industrial uses, for production of fish and waterfowl, and for such instream uses as recreation, transportation, and waste disposal. In the coming century, climate change and a growing imbalance among freshwa ter supply, consumption, and population will alter the water cycle dramatic ally. Many regions of the world are already limited by the amount and quali ty of available water. In the next 30 yr alone, accessible runoff is unlike ly to increase more than 10%, but the earth's population is projected to ri se by approximately one-third. Unless the efficiency of water use rises, th is imbalance will reduce freshwater ecosystem services, increase the number of aquatic species facing extinction, and further fragment wetlands, river s, deltas, and estuaries. Based on the scientific evidence currently available, we conclude that: (1) over half of accessible freshwater runoff globally is already appropriated for human use; (2) more than 1 x 10(9) people currently lack access to cle an drinking water and almost 3 x 10(9) people lack basic sanitation service s; (3) because the human population will grow faster than increases in the amount of accessible fresh water, per capita availability of fresh water wi ll decrease in the coming century; (4) climate change will cause a general intensification of the earth's hydrological cycle in the next 100 yr, with generally increased precipitation, evapotranspiration, and occurrence of st orms, and significant changes in biogeochemical processes influencing water quality; (5) at least 90% of total water discharge from U.S. rivers is str ongly affected by channel fragmentation from dams, reservoirs, interbasin d iversions, and irrigation; and (6) globally, 20% of freshwater fish species are threatened or extinct, and freshwater species make up 47% of all anima ls federally endangered in the United States. The growing demands on freshwater resources create an urgent need to link r esearch with improved water management. Better monitoring, assessment, and forecasting of water resources will help to allocate water more efficiently among competing needs, Currently in the United States, at least six federa l departments and 20 agencies share responsibilities for various aspects of the hydrologic cycle. Coordination by a single panel with members drawn fr om each department, or by a central agency, would acknowledge the diverse p ressures on freshwater systems and could lead to the development of a well- coordinated national plan.