Evapotranspiration (ET) and the ratio 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. The goal of this study was to determine if landscape changes such as tree or shrub encroachment and replacement of native grasses by invasive grasses impacted ET and ET/PPT and therefore watershed hydrology in this biome. USGS scientists determined ET and ET/PPT for shrublands, grasslands, and mesquite savannas in southern Arizona at five moisture flux towers and determined the environmental factors controlling ET in each plant community. Scientists then scaled ET over six areas ranging in size from 4 to 36 km2, representing homogeneous patches of each plant community, using the Enhanced Vegetation Index (EVI) dataset from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor on the Terra satellite. Over these wide areas, estimated ET/PPT projected from MODIS EVI ranged from 0.71 for a sparsely vegetated shrub site to 1.00 for grasslands and mesquite savannas. The results did not support hypotheses that encroachment of mesquite into grasslands or that replacement of native grasses with introduced Eragrostis lehmanniana (lehmann lovegrass) have increased rangeland ET.
Annual MODIS ET versus precipitation for six wide-area sites in southern Arizona rangelands. Plot shows regression line (solid line), 95% confidence intervals (short dashed lines), and the 1:1 line (long dashed line).