Abstract

Renewable fresh water comprises a tiny fraction of the global water pool but is the foundation for life in terrestrial and freshwater ecosystems. The benefits to humans of renewable fresh water include water for drinking, irrigation, 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 freshwater supply, consumption, and population will alter the water cycle dramatically. Many regions of the world are already limited by the amount and quality of available water. In the next 30 years alone, accessible runoff is unlikely to increase more than 10% but the earth's population is projected to rise by approximately a third. Unless the efficiency of water use rises, this imbalance will reduce freshwater ecosystem services, increase the number of aquatic species facing extinction, and further fragment wetlands, rivers, 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 a billion people currently lack access to clean drinking water and almost three billion lack basic sanitation services.
3. Because human population will grow faster than increases in the amount of accessible fresh water, per capita freshwater availability will decrease in the coming century.
4. Climate change will cause a general intensification of the earth's hydrological cycle in the next hundred years, with generally increased precipitation, evapotranspiration, and occurrence of storms, and significant changes in biogeochemical processes influencing water quality.
5. At least 90% of total water discharge from U.S. rivers is strongly affected by channel fragmentation from dams, reservoirs, interbasin diversions, and irrigation.
6. Globally, 20% of freshwater fish species are threatened or extinct, and freshwater species make up 47% of all animals federally endangered in the U.S.

The growing demands on freshwater resources create an urgent need to link research with improved water management. Better monitoring, assessment, and forecasting of water resources will help allocate water more efficiently among competing needs. Currently in the U.S., at least six federal departments and twenty agencies share responsibilities for various aspects of the hydrologic cycle. Coordination by a single panel with members drawn from each department or by a central agency would acknowledge the diverse pressures on freshwater systems and could lead to the development of a well coordinated national plan.

Key words and phrases: aquatic environment, climate change, global hydrological cycle, renewable freshwater supply, water resources, water policy