Estimating irrigation water use

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Quantifying evapotranspiration and irrigation water use is critical to understanding the water resources in agricultural areas. The Simplified Surface Energy Balance (SSEB) model uses satellite imagery to estimate actual evapotranspiration (ETa) at 1- kilometer resolution. The USGS Oregon Water Science Center investigated downscaling SSEB ETa from 1 kilometer to 250 meters by correlating ETa with the Normalized Difference Vegetation Index (NDVI) from the Moderate Resolution Imaging Spectroradiometer instrument (MODIS). Correlations were studied in three arid to semiarid irrigated landscapes of the western United States during several periods from 2002-2008. The ETa -NDVI correlations were lower than expected, due to an eastward 2-3 km shift in ETa data. The shift is similar to a shift identified in the land surface temperature data from the MODIS satellite. Further study is needed to understand the shift, its effect on SSEB ETa, and the relationship between ETa and NDVI.