METRIC Taps Landsat Data to Measure Evapotranspiration

April 24, 2009 By: Cyrena Respini-Irwin

The Idaho Department of Water Resources and the University of Idaho create a computer model that yields accurate assessments, not estimates.

For more than two decades, the Ash Institute for Democratic Governance and Innovation at Harvard Kennedy School has sponsored an Innovations in American Government Awards program to recognize government agencies for creative solutions. According to the institute, "the program serves as a catalyst for continued progress in addressing the nation’s most pressing public concerns."

As increasing populations strain limited freshwater resources, water use is rising to the top of the list of "pressing public concerns," particularly in the western U.S. Among the top 50 contenders in the Innovations competition is an Idaho Department of Water Resources (IDWR) program, “Mapping Evapotranspiration from Satellites,” that uses a computer model and Landsat imagery to help decision makers quantify water usage.

Evapotranspiration (ET) is the return of water to the atmosphere, via evaporation (from surfaces such as soil) and transpiration (from the leaves of plants). Because it can represent the amount of water used by irrigated agriculture, accurate ET measurements are crucial for water management efforts in farming-intensive regions. "More than 90 percent of the water that is used by people [in Idaho] is used by irrigated agriculture," said IDWR Geospatial Technology Manager Tony Morse. "In the arid West . . . if you have no irrigation water, you have no crops."

Accurate ET data is essential to management tasks such as administering water rights, but before IDWR developed the METRIC (Mapping EvapoTranspiration at high Resolution with Internalized Calibration) model, it wasn't possible to measure ET on a field-by-field basis. According to Morse, existing ET equations only gave estimates of potential water use, not actual measurements. "That's why METRIC and Landsat are so important: it is the field — the water right — that is the unit of administration," said Morse. "The director [of IDWR] now uses METRIC as a central tool in his administration of water rights."

At left, water-right polygons outlined in black are superimposed on an aerial photograph. At right, the same polygons are layered over an image of seasonal evapotranspiration. Images courtesy of IDWR.

Pooling Resources

IDWR's work with the METRIC model began in 2000, when the agency received a grant from the NASA/Raytheon Synergy Project to develop a satellite-based method for determining and mapping ET in Idaho. To pursue the project, IDWR partnered with the University of Idaho's Department of Agricultural and Biological Engineering, including Water Resources Professor Richard G. Allen. Basing their work on SEBAL, a model from the Netherlands, the researchers first adapted METRIC to the western United States, then generalized the model so it can be used anywhere in the world. Over the past decade, Morse explained, the two models have evolved together; improvements made to METRIC have benefited SEBAL as well.

Although the data and maps produced with METRIC are primarily intended for water resources management in Idaho, IDWR makes them available to everyone. "We don't limit access to any data we have, unless data are copyrighted or a trade secret in some way," said Morse. IDWR is currently working to make more of its data available via an Internet GIS server.

Applications of ET data are wide-ranging. So far, IDWR has used METRIC data in monitoring aquifer depletion, determining consumptive use for water rights buy-back payments, modeling ground water, and modifying irrigation patterns that reduce stream flow and threaten endangered fish — and that's just a partial list.

Leaning on Landsat

According to Morse, Landsat is the only feasible data source for ET mapping, because it's the only operational satellite that provides both the required thermal sensing capability and an appropriate pixel size for mapping ET from individual fields. In addition, data is acquired constantly — it's not a tasked system — and a quarter-century of archived thermal data allows the comparison of current results with historical trends.

"It's a vital data source," said Morse, lamenting the fact that Landsat has "fallen between the cracks"; managed jointly by NASA and USGS, the Landsat program lacks a dedicated federal agency to champion its ongoing operation. However, IDWR's ET mapping project may increase support for the venerable system. For example, Morse reported that this "fabulous real-world application of Landsat data" has made the Western States Water Council "a vociferous advocate for continued Landsat missions and continued thermal capability for Landsat."

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