Study indicates thirsty plants keep deserts' subsurface dry

04/11/05

AUSTIN, Texas–Desert blooms–plants that flourish in arid areas after rains--might reduce water accumulation in soil should the climate shift toward wetter conditions, according to a study conducted by a team led by University of Texas at Austin hydrogeologists.

By the same token, such vegetation keeps water from reaching the water table deep below the surface in such areas.

"Monitoring soil-water response to extreme El Niños in Nevada indicates that vegetation response will dampen the impact of increased precipitation and result in no net downward water movement to aquifers," said Bridget Scanlon, a senior research scientist at the Bureau of Economic Geology at the university.

The paper is to be published this week in the online edition of the Proceedings of the National Academy of Sciences.

Vegetation has, in fact, been drying out the soil in desert basins throughout the southwestern United States since the last glacial period, 10,000 to 15,000 years ago. Satellite data indicate that these vegetation responses to increased precipitation occur in deserts globally.

Because plants can maintain dry conditions, minimizing leaching of wastes into underlying aquifers, important implications exist for radioactive and hazardous waste disposal, the study's results show.

The study provides important insights into links between climate, ecology and hydrology that are critical for water resources and waste disposal.

The hydrogeologists studied eight years (1994-2002) of soil-water storage data in vegetated and nonvegetated lysimeters in the Mojave Desert (Nevada) that are operated by the U.S. Department of Energy.

Lysimeters, similar to the scales that weigh semis on the highway, are buried beneath the desert to precisely measure changes in the amounts of water in the soil.

The eight years included two El Nino weather patterns, which bring wetter and colder than normal weather during winter, each followed by a La Nina pattern, which brings drier and warmer weather during winter.

Even during the El Nino winter of 1997-1998--the largest of the 20th century, with rainfall as high as 2.5 times normal, the vegetation soaked it all up and did it quickly.

"Within two months, vegetation productivity increased tremendously and used up all the excess water," Scanlon said.

When the plants soak up water, they leave the water's chloride behind. By measuring chloride in soil water, therefore, the team also determined that this pattern of soil water movement has been ongoing for millennia.

"So vegetation has been able to maintain very dry conditions in these soils and create upward water movement," Scanlon said.

Study results should apply to deserts globally, as indicated by satellite data, which show large vegetation responses to wet El Nino periods in Australia, South America and Africa.

Source: Eurekalert & others

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