The tool is pretty accurate, as it turns out—especially for annual crops and in arid areas where the demand for water can exceed the supply and sustainable water management is crucial.

OpenET was developed by a broad team of federal agency, university and other collaborators, including several USGS Earth Resources Observation and Science (EROS) Center scientists. They contributed an EROS evapotranspiration (ET) model, SSEBop, for the OpenET ensemble of six models that generally represents the average. ET is the amount of water lost to the atmosphere through soil evaporation and plant transpiration and can be estimated by ground-based methods in specific spots or by using satellite data over broad areas, as the OpenET models do. 

Recently released in Nature Water, the study compared OpenET data with data from 152 point-based flux tower stations that provide reliable estimates for ET in a variety of landscapes. The study found “high accuracy” of OpenET use for cropland, according to the article.

Sources/Usage: Public Domain. View Media Details

USGS EROS Research Physical Scientist Gabriel Senay created the SSEBop model, helped develop OpenET and helped design and co-author the comparison study. EROS contractors Mac Friedrichs and Gabriel Parrish were also a part of developing OpenET and co-authors of the study. “The study results strengthen and justify the continued use of remote sensing techniques for estimating evapotranspiration and its application for agricultural water management,” Senay said.

Study co-author Justin Huntington, a research professor at the Desert Research Institute (DRI), reinforced that sentiment. “Evapotranspiration is one of the hardest hydrologic fluxes to measure, and to think we are estimating this flux from space with comparable or better accuracy to ground-based weather stations and meter data for agricultural lands is really remarkable,” he said in a DRI news release. 

“The combined use of the Landsat satellite archive with new Google Earth Engine cloud computing resources has been key, as has our collaboration across different research groups and use of multiple models to better understand model strength and weaknesses and identify areas for improvement,” Huntington added.

Accuracy and Usage Examples of OpenET

For annual crops, the study found that the OpenET average error rate fell within the targeted range set by partners such as farmers and water management agencies. And for crops growing in Mediterranean and desert climates such as California and portions of the Southwest, certain error rates 
were even lower. Accuracy in natural landscapes such as forests and wetlands varied.

OpenET is intended to provide easy access to information at a field scale to help land and water managers monitor water usage and make informed decisions. The ensemble aims for estimates that are at least as accurate, if not more accurate, than an individual ET model. In addition to the ensemble model, the study also compared individual model data to ground-based data.

“One of the main strengths of this study is to reinforce the fact that remote sensing ET is relatively accurate and consistent across models, indicating the maturity of the models, particularly over croplands. Water managers can effectively use either the ensemble or any of the models to quantify field-scale crop water use over large areas and time periods to understand and characterize water use trends and water budgets,” Senay said. The archive of Landsat thermal data, publicly available through EROS, allows for ET comparisons spanning four decades.

OpenET has been used for a variety of purposes since launching in 2021. For example, OpenET helps Sacramento-San Joaquin Delta farmers comply with California law to measure and report accurate water use. In Diamond Valley, Nevada, OpenET helps hay farmers understand the historical and current implications of irrigation as groundwater levels decline. And OpenET helps an Arizona water and power utility understand how forest restoration activities are changing water balances in the Salt and Verde River watersheds.  

An OpenET conference in late February 2024 will bring together OpenET users to highlight and provide feedback on ways OpenET has been used and encourage collaboration. Senay, Friedrichs and Parrish from EROS will attend, with Senay giving a presentation.