Remote Sensing of Evapotranspiration
The generation of evapotranspiration maps using Landsat Satellite images is based on METRIC (Mapping EvapoTranspiration at high Resolution and Internalized Calibration). This process was initially funded in the early 2000s by the Idaho Department of Water Resources (IDWR). Since then, evapotranspiration maps for eastern southern Idaho have been generated for 1986, 1996, 2000, 2006, 2008, 2009, 2010, 2011, 2013, 2016, 2017 and 2018. We are currently finalizing "Near Real Time" METRIC processing for 2019. The images/maps are available on line from the IDWR ET GIS website. Additionally, we are working on generation of METRIC evapotranspiration maps for the Treasure Valley area (Boise to Fruitland) for select years to support IDWR and USGS Treasure Valley aquifer modeling project. Treasure Valley METRIC evapotranspiration maps have been completed for 1994, 2000, 2007 and 2015 which have been submitted to IDWR. We are currently working on Treasure Valley maps for 1987, 2010, 1997 and 2004. The process has also been applied at various locations throughout the United States and the world.
In 2009, the Idaho Department of Water Resources — University of Idaho partnership that applies METRIC-based ET in IDWR hydrology and water rights and operations was awarded the prestigious Innovations in American Government Award by the Ash Institute of the Kennedy School of American Governance of Harvard University. The award announcement is at: "Mapping Evapotranspiration from Satellites."
Animations of ET in Idaho are available at: "Evapotranspiration from Landsat" which was produced by the NASA Goddard Space Flight Center.
A two-page Landsat specifications description sponsored by the Western States Water Council that advocates for a four-day Landsat revisit time is available at "Essential Specifications for Landsat -- WSWC April 9 2012.pdf." A four-day revisit time can be accomplished by having four currently-designed Landsats in orbit at the same time, or by launching new Landsat satellites that have twice the path width as the current design. In 2017, the Landsat Science Team formulated future requirements for the Landsat program. Seventeen of twenty-one members polled recommended a one day revisit time (https://landsat.usgs.gov/sites/default/files/documents/landsat_science_team/2017-07_Day1_Crawford_Landsat_10_and_beyond.pdf.) A one day Landsat revisit time would nearly guarantee the ability to consistently produce evapotranspiration images for the entire continental U.S. Most of the continental U.S. requires 16 satellites for a clear image every 16 days 70 percent of the time. . An additional analysis for southern Idaho based on actual Landsat image collections over the past forty years summarizes the impacts of clouds on one versus two satellites for southern Idaho annual ET (pdf).