The Utah Flux Network: A Dry Tale of Consumptive Use in the West

The Utah Geological Survey (UGS) operates the Utah Flux Network (UFN), a system of four permanent eddy covariance stations with complete energy measurement equipment and redundant sensors. Six more of these stations will be deployed within the next five years. The network is supported by multiple state and federal agencies, including the Utah Division of Water Rights, the Colorado River Authority of Utah, and the U.S. Bureau of Reclamation. Our interests as hydrogeologists have led us to target station deployment in the Great Salt Lake and Colorado River Basins to better understand consumptive water use in these sensitive areas and learn about the impacts of agricultural optimization and other land management practices on evapotranspiration (ET), particularly in a state experiencing a decades-long drought. The stations are set in vastly different ecosystems (playa, wetland, alfalfa), with different environmental challenges, such as periodic inundation, extremely salty “soil,” and shallow groundwater.

The objectives of the UFN are to 1) collect long-term baseline measurements of ET and 2) validate remotely sensed estimates of ET. We are interested particularly in comparing our ground-based measurements of ET with data from OpenET, which may be used by states throughout the West as a unifying data source for ET estimates.

Remotely sensed methods of calculating ET, including OpenET, generally rely on energy closure. As validation points for remote sensing, our network strives for high energy closure, while simultaneously acknowledging that high quality data do not always result in perfect energy closure. We have redundancy in key sensors and have instrumented the stations with a focus on water measurement in the air and soil. Our data management methods follow standard best practices that must be tuned to each station. For example, the data aggregation interval for a station set in the broad playa of the Bonneville Salt Flats is longer than that of a station set in bulrush within Spanish Valley. Friction velocity, soil moisture, and net radiation will also vary widely between sites. In addition, we will be comparing and considering the different measurement scales of station footprints and remote sensing grid cells when validating the remote sensing data.

We are developing a workflow and a series of visual checks for troubleshooting lack of energy closure within our network to more accurately calculate consumptive use and to validate remote sensing methods. We are presenting our protocols to the community with hopes of initiating dialog, receiving feedback from our peers, and offering suggestions to other new networks.