COMBINING MODIS ENHANCED VEGETATION INDEX AND GROUND MEASUREMENTS OF EVAPOTRANSPIRATION TO ESTIMATE AGRICULTURAL AND RIPARIAN CONSUMPTIVE WATER USE ON THE LOWER COLORADO RIVER

We used the Enhanced Vegetation Index (EVI) from MODIS to scale evapotranspiration (ET_actual) over agricultural and riparian areas along the Lower Colorado River in the southwestern U.S. Ground measurements of ET_actual by alfalfa, saltcedar, cottonwood and arrowweed were expressed as fraction of potential (reference crop) ET_o (ET_oF) then regressed against EVI scaled between bare soil (0) and full vegetation cover (1.0) (EVI*). EVI* values were calculated based on maximum and minimum EVI values from a large set of riparian values in a previous study. This data shows a satisfactory relationship was found between crop and riparian plant ET_oF and EVI*, with an error or uncertainty of about 20% in the mean estimate (mean ET_actual = 6.2 mm d⁻¹, RMSE = 1.2 mm d⁻¹). The equation for ET_actual was: ET_actual = 1.22 × ET_o-BC × EVI*, where ET_o-BC is the Blaney Criddle formula for ET_o. This single algorithm applies to all the vegetation types in the study. The algorithm was applied to irrigation districts and riparian areas from Lake Mead to the U.S./Mexico border. The results for agricultural crops were similar to results produced by crop coefficients developed for the irrigation districts along the river. However, riparian ET was only half as great as crop coefficient estimates set by expert opinion, equal to about 40% of reference crop evapotranspiration. Based on reported acreages in 2007, agricultural crops (146,473 ha) consumed 2.2 x 10⁹ m³ yr^-1 of water. All riparian shrubs and trees (47,014 ha) consumed 3.8 x 10⁸ m³ yr^-1, of which saltcedar, the dominant riparian shrub (25,044 ha), consumed 1.8 x 10⁸ m³ yr^-1, about 1% of the annual flow of the river. This method could supplement existing protocols for estimating ET by providing an estimate based on the actual state of the canopy as determined by frequent-return satellite data.