Researchers test OpenET’s real-world value

In several states, researchers, agronomists and farmers are exploring how OpenET can support more informed water management decisions. Developed through a collaboration among NASA, the U.S. Geological Survey (USGS), the U.S. Department of Agriculture (USDA) and multiple U.S. universities, OpenET uses satellite imagery to estimate evapotranspiration (ET). That data provides users with field-level insights into crop water use and makes it available nationwide. 

Much of the work to date has focused on California’s Salinas and San Joaquin Valleys, but researchers in Arizona are exploring how the technology might fit within their state’s existing irrigation management tools.  

“My colleague, Diaa Elshikha, an assistant professor and irrigation specialist at the University of Arizona’s Maricopa Agricultural Center, knew the team from California and brought them over to discuss the work that they’re doing,” said Jeffrey C. Silvertooth, PhD, professor and extension specialist in the agronomy/soil science Department of Environmental Science at the University of Arizona. 

Silvertooth said growers have likely heard about OpenET through consultants and companies offering services built around the platform. For him, the value of the discussions was understanding what the tool can do — and its limitations. 

“Some of these new concepts have been floating around for the past several years, and companies have been trying to sell services based on OpenET to farmers and for consultants,” he said. “This program was a good informational link, just to what OpenET is, how it is working, what it can do, and the limitations associated with it.” 

Real-time vs. backward looking data 

Arizona, like a handful of other states, has an established meteorological network with nearly four decades of weather data. Paul Brown, PhD, emeritus professor and assistant dean for the University of Arizona Cooperative Extension, led the development of the Arizona Meteorological Network (AZMET). It now has close to 30 operating stations, mostly in southern Arizona. That real-time capability is one of the biggest distinctions Silvertooth sees between AZMET and OpenET.  

“The parameters that they’re trying to capture with the OpenET system come in through satellite, so you’re always kind of looking backward, and there’s a time lag associated with getting that information,” he said. “AZMET does all that in real time, and you can actually look at it right now today, see what’s happening today, or [access] yesterday’s data.” 

For day-to-day irrigation scheduling, Silvertooth continues to favor weather-based management approaches that rely on Arizona’s established network, which is free to use.  

“For direct management purposes and weather-based water management I really like the Arizona Meteorological Network, because I know it has good data [spanning 40 years],” he said. 

Like OpenET, the AZMET system is free to growers and irrigation districts, and is also open access, meaning it’s free to use. While he prefers that state system, he sees the potential for AZMET and OpenET to complement one another, particularly when it comes to checking model performance against field-based measurements, but he wouldn’t use it as a cure-all for water use estimates on a broad basis.  

OpenET limitations 

In Arizona trials, OpenET tracked alfalfa water use reasonably well but was less accurate for cotton, highlighting the importance of crop coefficients and the assumptions built into evapotranspiration models, according to Silvertooth. 

“The AZMET system is reliable and we can use it to check against OpenET outputs,” he said.   

He added that some important aspects of Western irrigation management are not fully captured in current OpenET. Salinity management, for example, often requires additional irrigation water to leach salts from the root zone. 

“I think it [OpenET] has a useful utilitarian value,” he said. “One of my real concerns is that regulatory agencies will start picking up AI and OpenET and start saying, ‘You don’t get any more water than what OpenET says you need to have, or what was used last year for your crop.’ There are discussions out there like that, particularly out here in the West, and I have real concerns about how OpenET can be used or misused in the hands of regulators.”