An Empirical Algorithm for Estimating Agricultural and Riparian Evapotranspiration Using MODIS Enhanced Vegetation Index and Ground Measurements of ET. I. Description of Method

Pamela Nagler,John Osterberg,Edward Glenn,Kiyomi Morino,R Scott Murray

doi:10.3390/rs1041273

Abstract

We used the Enhanced Vegetation Index (EVI) from MODIS to scale evapotranspiration (ETactual) over agricultural and riparian areas along the Lower Colorado River in the southwestern US. Ground measurements of ETactual by alfalfa, saltcedar, cottonwood and arrowweed were expressed as fraction of potential (reference crop) ETo (EToF) 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 EToF and EVI*, with an error or uncertainty of about 20% in the mean estimate (mean ETactual = 6.2 mm d−1, RMSE = 1.2 mm d−1). The equation for ETactual was: ETactual = 1.22 × ETo-BC × EVI*, where ETo-BC is the Blaney Criddle formula for ETo. This single algorithm applies to all the vegetation types in the study, and offers an alternative to ETactual 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.

Highlights

Unlike remote sensing methods based on thermal (NIR) bands on Landsat or other high-resolution satellites, which provide a snap-shot of actual ET (ETactual) at the time of satellite overpass [14,15], vegetation indices (VIs) methods are useful in projecting ETactual over longer time periods, due to the modulated response of VIs to environmental conditions
We developed algorithms based on Equations 1 and 2 relating EG or ETactual to the Enhanced Vegetation Index (EVI) from the Moderate Resolution ImagingSpectrometer (MODIS) sensors on the Terra satellite [33]
This study developed a computation method for estimating ETactual using time-series satellite-derived vegetation index values from MODIS, calibrated with ground measurements of ETo and ETactual

Summary

Introduction

Unlike remote sensing methods based on thermal (NIR) bands on Landsat or other high-resolution satellites, which provide a snap-shot of actual ET (ETactual) at the time of satellite overpass [14,15], VI methods are useful in projecting ETactual over longer time periods (weeks, months and years), due to the modulated response of VIs to environmental conditions. These methods use time-series images from frequent-return sensor systems such as the Moderate Resolution Imaging.

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Methods

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Discussion

Conclusion