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 and processing 2020 in near real time. The finalized 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."
An implementation of METRIC on the Google Earth Engine is known as EEFlux. EEFlux has been in development since 2012 by the University of Idaho, University of Nebraska and Desert Research Institute. EEFlux uses an automated calibration scheme to produce ET, ETrF and EToF images for single Landsat scenes. TIF files can be downloaded and processed by users to produce time-integrated monthly ET. The automation of calibration accuracy of the final product by about 5 to 15% as compared to hand calibration by trained and experienced personnel using METRIC processes. Provision is made on the EEFlux web site to make adjustments to final ETrF images. More information is available on the EEFlux web site https://eeflux-level1.appspot.com/
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. 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. This power point shows the impact of the number of satellites and probability of cloud free images to produce cloud-free ET images at least once per 16 days or once per 32 days for the continental U.S. 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).
Evapotranspiration determined with METRIC and satellite imagery was reported in a news story published at weathernationtv.com "Satellite Imagery Sheds Light on Agricultural Water Use" The METRIC process uses Landsat satellite imagery to produce maps of evapotranspiration. EEFlux is a version of METRIC on the Google Earth Engine that provides free public access (https://eeflux-level1.appspot.com/).
A NASA document (pdf) which describes an application of METRIC ET data in Idaho to help with a water rights disagreement. (https://landsat.gsfc.nasa.gov/wp-content/uploads/2014/08/Landsat_Improve_Life_Water.pdf)