A New Tally for Water Use by Thirsty Plants
By Ann Perry
December 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 Arizonas 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.
Goodrich, who work at the
Southwest Watershed Research Center in Tucson, Ariz., have been measuring
ET in riparian zones along the San Pedro River for approximately 10 years.
Theyve obtained direct ET measurements using specialized meteorological
and plant physiological techniques. Theyve 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 sourcesuch
as groundwaterto 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.