Arkansas River Basin: Project hopes to show that Kansas v. Colorado consumptive use calculations are too low

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From The Pueblo Chieftain (Chris Woodka):

While a research project is attempting to determine better information on how much water crops use, its results are not directly applicable to farming realities in the Arkansas Valley. “There is a stark difference between irrigation on the lysimeter and the surrounding fields,” said Allan Andales, extension specialist for Colorado State University.

The lysimeter project at the Colorado State University Research Center at Rocky Ford uses a scale to weigh a 10­by­10­foot block of soil 8 feet deep to determine how much water is consumed by crops planted on it. So far, alfalfa has been the only crop tested.

The state is funding the project in an attempt to prove that consumptive use of water by Arkansas Valley crops is higher than assumed in the model adopted by the U.S. Supreme Court in the Kansas v. Colorado case over the Arkansas
River Compact. That would mean well owners would have to repay less water.

The model adopted in the case relies on information from field tests in Idaho.

Area farmers are encouraged that the results have shown that nearly all of the water is consumed. This could also affect the model used in 2010 surface irrigation rules by reducing the amount of return flows in the state’s assumptions. In a presentation Wednesday to the Lower Arkansas Valley Water Conservancy District, Andales compared the lysimeter to a “flower pot” that has little interaction with the surrounding fields. Like plants grown in pots, it is likely the alfalfa grown on the lysimeter has become root­bound. Watering on the lysimeter is 99 percent efficient, because there is no way for the water to drain, as it does in the open fields. The efficiency in the fields is roughly 50 percent.

The lysimeter plants are watered at an optimal level, using a hose, while the furrow irrigation in the surrounding fields only captures about 64 percent of water running in the ditch, Andales added.

The lysimeter also fails to account for groundwater tables. Roots of alfalfa plants can use water 10 feet or deeper. Still, alfalfa grown on the lysimeter shows a consumptive use of up to 58 inches of water annually, more than twice what area farmers are able to apply in many years. At that rate, the yield would be about 8 tons per acre, dry weight — productivity that has rarely been seen in the valley. “It’s amazing that alfalfa can use almost 60 inches of water in a year,” Andales said.

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