The Utah Flux Network

A Hydrometeorological Network Maintained by the Utah Geological Survey 

by Paul Inkenbrandt & Kathryn Ladig 

Utah Flux Network station locations.

The Utah Geological Survey (UGS) is establishing a network of weather stations throughout the state of Utah, known as the Utah Flux Network (UFN), to measure evapotranspiration. Evapotranspiration is water that is evaporated from the land’s surface and transpired by plants, and it is an important part of Utah’s water budget, as it is one of the main ways water leaves Utah. We will collect long-term baseline measurements of evapotranspiration, compile existing data to make it widely available, and compare our data with remotely sensed (satellite-based) models. 

The main goal of the UFN is to provide ground-truth data for satellite-measurement-based estimates of evapotranspiration, though the network will serve many additional purposes. These data will give water managers tools to deal with drought, allowing them to accurately measure water conserved or used by agriculture. Data from the UFN will also support long-term water conservation and management strategies, such as water banks—for example the pilot Price Water Bank project which converts conserved agricultural water into instream flow near Price, Utah—and measuring consumptive use in the Upper Colorado Basin to help administer the Colorado River Compact. Our data can also be used for hydrologic and climate models because these stations measure important components of the atmosphere, including carbon dioxide (CO₂), water (H₂O), and available energy.

Evapotranspiration and Eddy Covariance

Conceptual movement of wind eddies near a field. Modified from George Burba.

A large portion of evapotranspiration in vegetated areas, including croplands, is due to consumptive use, which is the water that cannot be recovered or reused, including water consumed by plants and water evaporated. Winds blow over the land surface, removing evapotranspired water, allowing more water to be evapotranspired in its place. The wind carries the water vapor and other gasses in turbulent, whirling, circuitous paths known as “eddies.” The UFN uses the eddy covariance technique to measure these rapidly changing eddies.

The eddy covariance technique requires accuracy in data collection and correction to calculate valid evapotranspiration values, which is dependent on available energy. Our measurement goal is energy balance, which occurs when all incoming and outgoing energy components are accounted for and the following equation is met: Rn-G=LE+H, where Rn is net radiation, G is ground heat flux, LE is latent heat flux, and H is sensible heat flux. Latent heat is the energy required for a substance to change state, such as from liquid to vapor during evapotranspiration, and sensible heat is the energy required to change the temperature of a substance. Net radiation is the sum of incoming and outgoing shortwave and longwave radiation. Shortwave radiation is energy emitted by the sun, some of which is reflected by the earth’s surface and some absorbed. The ability of a substance to reflect shortwave radiation, controlled by the substance’s color and texture, is called albedo. Absorbed shortwave radiation warms the earth’s surface, which then emits longwave radiation. Clouds and greenhouse gasses, such as CO₂, absorb the outgoing longwave radiation, warming the lower atmosphere. Balancing these energy fluxes is difficult to achieve and requires great effort to collect quality data, as well as a lengthy data post-processing protocol.

Earth’s energy balance. Solar radiation is predominantly shortwave radiation. Some of this is reflected by clouds and the earth’s surface, and some of this energy is absorbed. Long-wave radiation is emitted by the earth’s surface and captured by greenhouse gasses. From the Intergovernmental Panel on Climate Change.

UFN eddy covariance stations are built from common weather station instruments, including devices that measure the wind speed and direction (anemometers), precipitation buckets, thermometers, and humidity probes. In addition, each station includes equipment to precisely measure the movement of water vapor, CO₂, and energy in and out of the area of the station. Net radiometers and soil heat flux plates help estimate energy stored and released by the soil. Sensible heat flux is calculated using multiple thermometers placed on the station at different heights, as well as some buried in the soil at different depths. All of these instruments are highly accurate, and many of them can measure very rapidly.

The UFN is also investigating the viability and applicability of the surface renewal technique, which is less costly, but not as well documented as the eddy covariance approach. The surface renewal method uses high-speed temperature and wind measurements to calculate sensible heat flux, from which latent heat flux and evapotranspiration can be calculated.

Our Progress

So far, we have constructed four eddy covariance stations in Utah, which comprise the UFN. The stations have been placed in a variety of ecosystems. Two of our stations are on agricultural sites, one is on a wetland, and one is on a salt flat (i.e., playa dominated by halite and gypsum deposits). With the emplacement of these stations, and the potential for more, we are working to develop an effective protocol for managing the stations and their data.