Southwestern areas of the United States, reeling from its worst drought in 50 years, may have 10 percent less surface water within a decade due to global warming, a study said Sunday.

While rainfall is forecast to increase over northern California in winter and the Colorado River feeding area, warmer temperatures will outstrip these gains by speeding up evaporation, leaving the soil and rivers drier, a research paper said.

Texas will likely be dealt a double blow with declining rainfall and an increase in evaporation, said the paper based on weather simulations and published in Nature Climate Change.

Overall for the area, "annual mean runoff in 2021-2040 is projected to be 10 percent less than in the second half of the 20th century," co-author Richard Seager of Columbia University told AFP.

This "is a very significant decline given the stress on Colorado River-based water resources" for agriculture and household use, he added.

Runoff is rainfall not absorbed by the soil, running overland or in rivers.

According to the paper, California obtains most of its water from snow on the Sierra Nevada mountain range, while the Colorado River is fed from tributaries created by melting winter snowfall and summer rainfall.

The river provides water to seven US states and Mexico.

Texas, for its part, uses water from rivers and groundwater within its own borders, said the paper.

Average annual runoff for the region overall should drop by about 10 percent, and about 25 percent in spring for the Colorado tributary headwaters.

"Drying intensifies as the century advances," added the paper.

"These projected declines in surface-water availability for the coming two decades are probably of sufficient amplitude to place additional stress on regional water resources."