From The Pueblo Chieftain (Chris Woodka):
The lysimeter physically weighs the amount of water being used on a crop, rather than estimating use through equations. At the same time, a weather station at the site calibrates temperature, moisture, wind speed and other environmental factors to take the guesswork out of where the water comes from and where it goes. Results from 2008-10 show that with 10 to 12 inches of rainfall and 40 to 44 inches of irrigation water, nearly all of the water was used by the alfalfa crop. Very little of the water drained…
Still, the research has far-reaching implications about how consumptive use is treated in the courtroom, said Dale Straw, a Division of Water Resources researcher. The Penman-Montieth model replaced the Blanney-Criddle model as the way water use is estimated after the U.S. Supreme Court ruled in favor of Kansas’ interpretation in the Kansas v. Colorado lawsuit over the Arkansas River Compact. That decision means that well users in Colorado repay depletions to the aquifer at the highest possible rate. Within the state, Water Court decisions have tended to underestimate the use of water because of pressure from objectors during change cases, Straw said.
The state hopes to improve the Penman-Montieth model by introducing data specific to the Arkansas Valley. Currently, the model is based on data collected in Idaho. The Penman-Montieth takes more weather factors into consideration, and the wind in the Arkansas Valley appears to be the biggest variable not taken into account, Straw said.
More Arkansas River basin coverage here.