Trees not as big a culprit along Platte River as previously thought

Results of a recent study point up the danger of making sweeping public policy decisions before all the information is available.

According to a new study by the U.S. Geological Survey, trees along the Platte River in Central Nebraska use less water than we previously believed.

A cottonwood tree, it turns out, will use more water when it needs it, and when it doesn't, not so much.

"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, USGS hydrologist and an author of the study. "But our results showed that was not the case for these forests."

It's true that cottonwoods can use a lot of water in hot, arid locations like the American Southwest, they found.

But where it's cooler and there's a little more rain, groundwater supplies below riverside forests were actually replenished during the study.

USGS scientists collected data at two sites along the central Platte River -- 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 also 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. The full report can be viewed online at http://pubs.usgs.gov/sir/2008/5228/.

What difference could the study make? For one, we're committed to removing woody vegetation along the Republican River with an eye toward meeting Nebraska's obligations under the Republican River Compact. For another, if it's possible the vegetation that replaces the woody vegetation uses more water, a possibility the USGS study suggests, the riparian project is counterproductive.

Although a solution to the Republican River conflict is an urgent need, so is ongoing study of the methods we use to meet that need.