Consumptive use primer

When changing agricultural water to municipal use the water courts require an analysis of historical consumptive use. Here’s a report on the difficulties of obtaining that information, from Bill Jackson writing for The Greeley Tribune. From the article:

The [New Cache la Poudre Irrigating Co. and Reservoir Co] recently released a draft report on a ditchwide historical use analysis, which Don Magnuson, superintendent, said will probably become necessary for all irrigation companies in the future. The report, he told members of the Greeley Chamber of Commerce Agriculture Committee last week, really focused on consumptive use of water, which he predicted will become more important in future years. “The true value of water is how much you consume and not what is diverted,” Magnuson said. So that becomes even more important when that water is changed from agricultural to another use, and that change drove the need for the analysis.

Consumptive use is that water consumed by plants in a given field. Typically, under a flood irrigation system, about 50 percent of irrigation water is consumed while the other 50 percent either runs off the end of a field, evaporates or escapes past the root zone of the plants and goes into an aquifer. However, each crop consumes different amounts of water. Eventually, that water not consumed by plants returns to streams and rivers both on the surface and through aquifers. But the time that takes varies widely, from a matter of a few hours in the case of surface runoff to months or even years through an aquifer.

Over a 15-year period, the analysis tracked water requirements by crop type throughout the Cache la Poudre system. Those figures show the complicated job of growing and irrigating crops. Over that 15-year period, water requirements, on average, for alfalfa were slightly more than 26 inches through the season, grain corn about 18 inches, dry beans about 10.5 inches, grass more than 26 inches, small vegetables 13 inches, spring grain 17.5 inches and sugar beets 20 inches. The average annual combined diversion and reservoir releases for the study period was about 57,900 acre-feet, or about 1.79 acre-feet per irrigated acre. The average consumptive use was about 26,900 acre-feet or about 0.8 of an acre-foot per acre, so that means a lot of water went downstream for someone else to use. An acre-foot is about the equivalent of covering a football field with 12 inches of water. Throw into the mix the different types of soils, weather conditions and increased demands from growth, and things just get more complicated.

More South Platte River Basin coverage here.

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