Study: Platte trees not as thirsty

LINCOLN --- Trees along the Platte River in central Nebraska are not sucking as much water out of the stream as previously believed, according to a new U.S. Geological Survey study.

Rather than leaving less underground water available for irrigation and other uses, the study indicates that forested areas of the central Platte use roughly the same amount of water as irrigated corn, about half or less of what earlier research indicated.

“Previous beliefs about the way cottonwood trees use water suggested that they will use as much as they can if water is available,” said Dave Rus, a Lincoln-based USGS hydrologist and an author of the study. “But our results showed that was not the case for these forests.”

USGS scientists collected data at two sites along the central Platte that were dominated by cottonwood trees, which generally are believed to be water-thirsty trees when groundwater is available. Results were counter to past research suggesting that cottonwoods deplete the groundwater supply. The study could help managers better understand the role such forests have on water availability.

The study found that the trees' water-thirsty nature may have more to do with their setting than with the tree itself. For example, in arid settings such as the American Southwest, research shows that the trees can be big groundwater users. But in the Platte River valley, where temperatures are lower and rainfall is higher, groundwater supplies underneath the riverside forests actually were replenished during the study.

Such findings also shed light on how the water supply may or may not benefit if the forests were replaced by a different plant community.

“Those benefits are probably much less than originally expected and may actually become more costly if the new plant community is ‘thirstier' than the forests,” Rus said.

USGS scientists collected data at Gothenberg and Odessa, both located in the Big Bend region of the river. In addition to measuring rainfall, soil moisture and groundwater levels during the four-year study, scientists erected 90-foot towers with instruments that measured the amount of water “exhaled” by the vegetation below. This was used to determine the balance of water in the system and to estimate the amount of water the plants were using.

The average forest water use during the study was 22.2 inches per year compared to an estimated 39 inches per year – and up to 72 inches per year – cited in previous studies. Irrigated corn uses about 26 inches per year. Rainfall over the same time period averaged 24.3 inches per year.

Differences in estimates of water use by near-river forests could be a result of methods of study improving through the years, Rus said.

A follow-up study comparing water use between Platte River forests and riparian grassland is ongoing. Rus works at the USGS Nebraska Water Science Center in Lincoln.

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