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A New Tally for Water Use by Thirsty Plants
By Ann PerryDecember 19, 2008
Agricultural Research Service (ARS) scientists and their colleagues are helping to refine evapotranspiration rates in dryland riparian corridors by revamping estimates for current and future water use by mesquite and other plants.
Vegetation is an integral part of the riparian ecosystem, and its water use is a key component of the groundwater budget for southern Arizona’s San Pedro River basin. When water evaporates from leaves, stems and other exposed plant surfaces, the process is called transpiration. Water also evaporates from the ground surface and open water. Evapotranspiration, or ET, is the amount of water vapor returned to the atmosphere by both processes.
Russell Scott and David Goodrich, who work at the ARS Southwest Watershed Research Center in Tucson, Ariz., have been measuring ET in riparian zones along the San Pedro River for approximately 10 years. They’ve obtained direct ET measurements using specialized meteorological and plant physiological techniques. They’ve also estimated ET at the landscape scale using remotely sensed data obtained from satellite sensors that measure both vegetation indices and land surface temperatures.
Their results indicate that average approximate ET rates for shrublands and grasslands along old river floodplains are 24 to 28 inches per year. ET rates for mature and dense mesquite woodlands exceed 28 inches annually. Since the region averages only 10 to 12 inches of rainfall every year, these ET rates suggest that the vegetation is drawing on an additional water source—such as groundwater—to make up the difference.
Extrapolating their results across the entire Sierra Vista sub-watershed, these researchers concluded that previous calculations had underestimated groundwater use by the riparian vegetation by 27 to 57 percent. They also found that mesquite accounts for 58 percent of the total riparian vegetation groundwater demand. This level is expected to increase as mesquite expands its range into areas previously dominated by grassland.
Results of these studies will help determine if water supplies are adequate to meet existing and projected demands on groundwater reserves by the growing regional population and by native riparian ecosystems in the San Pedro basin.
ARS is a scientific research agency of the U.S. Department of Agriculture.
