UW hydrologist investigates return flow systemsWritten by Natasha Wheeler
“In Wyoming, we generally make an assumption that 50 percent of the water applied in flood irrigation will actually return to the stream, but we don’t really know how that varies from basin to basin, from field to field or the timing of that process,” notes University of Wyoming (UW) Watershed Hydrologist Ginger Paige.
In the late 1980s, a large study was done on the New Fork River in the Upper Green River Basin, and scientists determined that nearly 70 percent of the water returned to the stream, with a large portion of that return flow occurring late in the season.
“That’s probably the most comprehensive study in Wyoming on return flow,” she comments. “The question is, how does that process vary from basin to basin?”
With graduate students Niels Claes and Bea Gordon, Paige and fellow UW scientists Scott Miller and Andy Parsekian have been investigating water flow for almost two years in the Upper Wind River Basin’s East Fork watershed on the Spence and Moriarity Wildlife Habitat Management Area that is run by the Wyoming Game and Fish Department.
“We’re doing an intensive water balance and field-scale study to try and quantify return flow to Bear Creek within the East Fork watershed. Return flow is an important part of the water balance in Wyoming,” she explains.
Although it is still too early in the study to draw conclusions, the research team is beginning to understand where the potential flow paths are and where they are currently happening.
The New Fork River studied in the 1980s is located in an alluvial basin, which fills up with water and slowly drains over time. The East Fork River being studied now is located in a glacial till and fine loam system.
“In East Fork, we have the other end of the spectrum. If we can figure out how to model processes in both of these systems, we will have two endpoints to try to figure out how these systems behave in terms of return flow under flood irrigation,” Paige says.
Thanks to a grant from the National Science Foundation for the creation of the Wyoming Center for Environmental Hydrology and Geophysics at UW, university researchers have been able to take advantage of high-end equipment for gathering data.
“This project is co-funded by the UW Water Resources Program and the Wyoming Center for Environmental Hydrology and Geophysics,” she notes. “We also received some additional funds from the Walton Foundation through the Haub School of Environment and Natural Resources at UW.”
Tools such as nuclear magnetic resonance (NMR) and electrical resistivity tomography (TMR) allow scientists to image water and water movement under irrigation, which can be coupled with information about stream flow, precipitation and evapotranspiration to create a big picture concept of the return flow process.
“In this case, we are also using both a scintillometer and an eddy covariance flux tower to directly measure transpiration off of the fields,” she adds. “We have some preliminary data, but putting all of the pieces together and really understanding how it changes with time will be one of the factors we have to delve into.”
Data collected from this project will begin to reveal how much water flows through the stream, how much water is consumed by vegetation and how much is lost to groundwater.
“The equipment we are employing altogether in this field is great because we are able to actually look at changes using the NMR and deep-water storage. That’s really hard to do. We always make these assumptions about where the water is going. Now we are getting some very good, detailed measurements on how much water is being consumed by plants by measuring the flux of water vapor off of the field,” she explains.
Nearing the halfway point of the project, the team is finding that the East Fork is a dynamic system with many potential flow paths.
“Although there are not a lot, we do get components of overland flow, but we also see the large heterogeneity in the system. We have springs that appear sort of in the middle of the irrigation system, and we’re looking at what the connections are between the springs and the irrigation,” Paige comments.
One of the biggest challenges of the project turned out to be the beaver population, which wreaks havoc on stilling wells and stream gauges.
“We end up with a lot of beaver, which are very beneficial for riparian areas and healthy watersheds, but when we are trying to measure flow in the stream, they are a pain,” she laughs.
However, she also states, “I think we’ll find information that’s of use to the state but not harmful to irrigators. I think it’s a win-win in terms of the ability to get a better handle on what’s going on with these processes. It’s fun, investigative hydrology.”