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ARS Home » Pacific West Area » Tucson, Arizona » SWRC » Research » Publications at this Location » Publication #306875

Research Project: Ecohydrological Processes, Scale, Climate Variability, and Watershed Management

Location: Southwest Watershed Research Center

Title: Wide-area ratios of evapotranspiration to precipitation in monsoon dependent semiarid vegetation communities

Author
item GLENN, E. - University Of Arizona
item Scott, Russell - Russ
item NGUYEN, U. - University Of Arizona
item NAGLER, P.L. - Us Geological Survey (USGS)

Submitted to: Journal of Arid Environments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/10/2015
Publication Date: 3/2/2015
Citation: Glenn, E., Scott, R.L., Nguyen, U., Nagler, P. 2015. Wide-area ratios of evapotranspiration to precipitation in monsoon dependent semiarid vegetation communities. Journal of Arid Environments. 117:84-95.

Interpretive Summary: Evapotranspiration (ET) and ratios of ET to precipitation (PPT) are important factors in the water budget of U.S. semiarid rangelands and are in part determined by the dominant plant communities. We determined ET and ET/PPT for shrublands, grasslands and mesquite savannas in southern Arizona and determined the environmental factors controlling ET in each plant community. We then determined ET over wide areas of each plant community using satellite imagery. The goal was to see if landscape changes such as shrub encroachment and replacement of native grasses by invasive grasses impacted rangeland water balance. ET at all sites had a major peak of activity during the July to September monsoon season and a smaller peak of activity in springtime, supported by soil moisture deposited by winter rains. ET responded to the same set of environmental variables in grasslands, shrublands and savannas. Although plant species, fractional cover and PPT differed among sites, plant communities tended to converge in the factors controlling ET. The results did not support claims that suggest vegetation change in this region has increased rangeland ET. So long as plant cover was maintained, ET/PPT was high across plant community types.

Technical Abstract: Aims. E vapotranspiration (ET) and ratios of ET to precipitation (PPT) are important factors in the water budget of semiarid rangelands and are in part determined by the dominant plant communities. We determined ET and ET/PPT for shrublands, grasslands and mesquite savannas in southern Arizona and determined the environmental factors controlling ET in each plant community. We then scaled ET over wide areas of each plant community using satellite imagery. The goal was to see if landscape changes such as shrub encroachment and replacement of native grasses by invasive grasses impacted ET and ET/PPT and therefore watershed hydrology in this biome. Methods. We used Ameriflux eddy covariance moisture flux towers and the Enhanced Vegetation Index (EVI) from the MODIS satellite sensors to scale ET and ET/PPT over wide areas of each plant community type and multiple regression analyses to determine the best set of predictors of ET. We developed an algorithm for ET using potential ET and MODIS EVI that was calibrated at grassland and mesquite savanna tower sites and validated at three other tower sites. Results. ET at all sites had a major peak of activity during the July to September monsoon season and a smaller peak of activity in springtime, supported by soil moisture deposited by winter rains. ET responded to the same set of environmental variables in grasslands, shrublands and mesquite savannas with EVI (driven by PPT) explaining over 70% of monthly and inter-annual variability in ET across sites. Over wide areas, estimated ET/PPT projected from MODIS EVI ranged from 0.75 for a sparsely-vegetated creosote site to 0.93-1.05 for two grasslands, two mesquite savannas and a mixed acacia-creosote shrubland site. Conclusions. Although plant species, fractional cover and PPT differed among sites, plant communities tended to converge in the factors controlling ET. The results did not support hypotheses that encroachment of mesquites into grasslands or that replacement of native grasses with introduced Eragrostis lehmanniana (lehmann lovegrass) have increased rangeland ET. So long as plant cover was maintained, ET/PPT was high across plant community types.