MY TURN: Measuring ET with satellite data is an important tool for water management

The Idaho Department of Water Resources (IDWR) was pleased NASA recently announced the availability of a new open-source satellite-based tool (OpenET) that calculates evapotranspiration, which is the amount of water that evaporates from the soil and transpires from plants.

Evapotranspiration, abbreviated as “ET,” is a critical factor in managing and regulating water use, which IDWR does state-wide. In Idaho, crop irrigation is the largest consumptive use of water. Idaho farmers irrigate about 3.8 million acres of farmland. Crop irrigation accounts for more than 95 percent of Idaho’s consumptive water use.

Idaho has long recognized the importance of accurately measuring ET to calculate consumptive water use. In the early 2000s, IDWR and the University of Idaho received a NASA grant to develop processes to map ET from Landsat satellite data.

How does it work? Evaporation consumes energy, causing the evaporating surface to become cooler. This is what makes sweating an effective way for your body to manage its temperature. This evaporative cooling effect is observed in the land surface temperature data gathered by Landsat satellites. The surface temperature data is combined with other measured meteorological data to estimate the land surface energy budget, including the ET component of that budget.

With funding from the NASA grant, scientists from the University of Idaho, including Dr. Richard Allen, a professor of Water Resource Engineering, with support from scientists and GIS experts at IDWR, developed a physics-based model to calculate ET titled “Mapping Evapotranspiration at High Resolution with Internalized Calibration (METRIC).” METRIC uses “remotely-sensed” data from the Landsat satellites to calculate ET for irrigated fields in Idaho.

The development of METRIC and its application by a state agency was unprecedented at the time. Water users in Idaho can be proud that Idaho was the first state in the West to develop and use METRIC ET for water management.

For its efforts, Dr. Allen’s research team, including IDWR, received an “Innovations in American Government Award” in 2009 from Harvard University’s Kennedy School of Government.

In the early 2000’s, IDWR employed METRIC to evaluate a water delivery call filed by an irrigation delivery entity in the Magic Valley. The irrigation entity held senior water rights, which it believed were being injured by groundwater pumping by holders of junior priority water rights. As a result, the irrigation entity, holding senior water rights, asked the Director of IDWR to curtail—or shut off—the diversions by the groundwater pumpers with junior priority water rights.

IDWR investigated the allegation of injury with help from METRIC data. Based on METRIC evidence, which demonstrated no difference in ET and consumptive water use between the alleged “injured” fields and surrounding non-injured fields, IDWR issued an order denying the delivery call. The calling irrigation entity appealed that order to the Idaho Supreme Court. In part on the strength of its METRIC evidence and analysis, IDWR prevailed in the Supreme Court’s final decision.

The initial delivery call is just one example of how Idaho has used METRIC in its management of Idaho’s water resources. Since that initial use, IDWR has also employed METRIC data to develop water budgets, calibrate and validate water models, regulate groundwater withdrawals, resolve water rights disputes, and evaluate endangered species issues.

The new OpenET product, is a tool like IDWR’s METRIC tool that calculates ET. Also, like IDWR’s METRIC tool, OpenET was developed by a collaborative group that included NASA, Dr. Allen, and IDWR staff. Unlike Idaho’s METRIC tool, OpenET is a fully automated web-based application developed for the entire western United States. Dr. Allen was quoted in recent articles about the new OpenET NASA platform, saying, “As someone who has worked on evapotranspiration for more than 40 years, I am thrilled to see multiple, independent models for estimating ET come together on a single, easy-to-navigate platform.”

Water managers in California expect to use OpenET to track water use and look for water savings in the drought-ravaged region. When they do, they will be using this powerful tool to manage water in similar ways to those pioneered by Idaho more than 15 years ago.

We are glad to see the OpenET collaborative group make this technology available to western states. We just hope Idahoans appreciate Idaho’s significant role in bringing this technology to the mainstream.

Gary Spackman is the Director of the Idaho Department of Water Resources.