Scarce water supplies in the western US will probably dwindle further as a result of climate change, causing problems for millions in the region, a government report has said.
Climate change could cut water flow in several of the American West's largest river basins by up to 20% this century, the interior department report said.
Those rivers provide water to eight US states, from Texas to California.
The West and South West are among the fastest-growing regions in the US.
The Colorado, the Rio Grande and the San Joaquin are three of the rivers mentioned in the report, which said an 8% to 20% decrease in average annual stream flow is expected.
"Impacts to water are on the leading edge of global climate change," said Mike Connor, commissioner of the Bureau of Reclamation, a US agency that helps provide water to more than 31 million people in 17 Western states.
The report, prepared in response to the Secure Water Act of 2009, outlines increased risks to water resources in the American West for the 21st Century.
Interior Secretary Ken Salazar called water the region's "lifeblood" and said reduced amounts could have a severe impact on the West and South West, which continue to see dramatic increases in population.
Nevada, Arizona and Texas, three of the driest states in the US, are among the fastest growing.
"These changes will directly affect the West's water supplies, which are already stretched in meeting demands for drinking, irrigating crops, generating electricity and filling our lakes and aquifers for activities like fishing, boating and to power our economy," Mr Salazar told Reuters news agency.
The report, released on Monday, will help officials understand the long-term impacts of climate change on Western water supplies and will help develop strategies for sustainable water resource management, Mr Salazar said in a statement.
Other specific projections included in the report are:
- A temperature increase of 5-7 degrees Fahrenheit (2.8-3.9 degrees Celsius)
- A precipitation increase over the north-western and north-central portions of the western US and a decrease over the south-western and south-central areas
- A decrease for almost all of the 1 April snowpack, a standard benchmark measurement used to project river basin run-off
Changes in climate could affect water supplies to a range of users, from farms and cities to hydropower plants, fish, wildlife and recreation, the report said.
"As climate change adds to the challenges we face in managing our water supply, meaningful engagement between the river basin states and the Department of the Interior will continue to be essential," said Anne Castle, Assistant Secretary of the Interior for Water and Science.