An empirical algorithm for estimating agricultural and riparian evapotranspiration using MODIS Enhanced Vegetation Index and ground measurements of ET. I. Description of method

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 US. 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. 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, and offers an alternative to ET_actual estimates that use crop coefficients set by expert opinion, by using an algorithm based on the actual state of the canopy as determined by time-series satellite images.