Water quantity

Dams, diversions, pumps, and other engineering works have profoundly altered the amount and location of water available for both human uses and for sustaining aquatic ecosystems.

• People now withdraw about half of the water readily available for use from rivers.

• Dams have so impeded flows that the length of time that it takes the average drop of water entering a river to reach the sea has tripled.

The changes we have made to forest cover and other ecosystems such as wetlands also have altered water availability and affected the timing and intensity of floods. For example:

• tropical montane forests, which play key roles in regulating water quantity in the tropics, are being lost more rapidly than any other tropical forest type.

• Freshwater wetlands, which store water and moderate flood flows, have been reduced by as much as 50 percent worldwide.

  Agroecosystems Forest ecosystems Freshwater systems