Use of the METRIC evapotranspiration model to compute water use by irrigated agriculture in Idaho

In 1995, the U.S. Geological Survey estimated consumptive water use by irrigated agriculture accounted fin, more than 99% of total water use in Idaho. The most recent water-use estimate was done for the year 2000. Unlike previous years, USGS did not report water use by irrigated agriculture. The value of Idaho's irrigated agriculture was $2.58 billion in 1997. The Idaho Department of Water Resources wanted water use statistics for 2000, but knew the process would be expensive and time-consuming. A remote-sensing based method, METRIC, offered an alternative solution. METRIC was developed jointly by the University of Idaho and the Idaho Department of Water Resources under a NASA/Synergy grant. METRIC is an evapotranspiration model that uses the visible, near infrared, and thermal infrared hands of any appropriate satellite to compute a complete enery balance for each image pixel. Landsats 7 and 5 were used for this application, with multiple dates processed for nine nominal scenes for the year 2000 in 28 counties of southern Idaho, the region of the state where irrigated agriculture is concentrated. Land in irrigated agricultural was delineated using National Land Cover Data, other data, and thresholding of evapotranspiration values. METRIC computed 9,313,503 acre feet of evapotranspiration from 3,552,174 irrigated acres, or 2.6 acre feet per acre. The irrigated acreage is biased high due to the generalizing affect of the land use/land cover data set. Nevertheless, the results suggest that past water use data may have under estimated consumptive use. The next phase of the study will compare METRIC-derived evapotranspiration processed from year 2000 MODIS data with the Landsat results.