The state is being asked to help fund a study that looks at farmers’ contentions that estimates for return flows to the Arkansas River are inflated. A standard of 10 percent for tailwater — water that sheets off fields during irrigation before it can soak in — is used in mathematical models adopted during the 24-year Kansas v. Colorado U.S. Supreme Court case under the Arkansas River Compact. Those models also affect consumptive use rules that apply to surface water improvements such as sprinklers or drip irrigation.
The Arkansas Basin Roundtable last week forwarded a $175,000 grant request to the Colorado Water Conservation Board to determine if that number is too high.
“Farmers on the Fort Lyon did not believe 10 percent was really happening,” said Leah Martinsson, a lawyer working with the Lower Arkansas Valley Water Conservancy District, which is applying for the grant.
Straight line diagram of the Lower Arkansas Valley ditches via Headwaters
The ditch is more than 100 miles long and irrigates 94,000 acres and usually water short. That increases the likelihood that the estimate of tailwater runoff is too high, since much of the water never makes it back to the river, she explained. The higher the tailwater number, the greater the obligation from farmers to deliver water to the Arkansas River. So, reducing the figure in the group augmentation plans filed with the state would mean a reduction in the amount of replacement water.
While the concern of Fort Lyon farmers is the model used in the consumptive use rules, it also could affect the hydrologic-institution model that guides Colorado’s obligation from wells.
“If we are prepared with good technical data, we will go in and try to change the H-I model,” said Bill Tyner, assistant division engineer with the Division of Water Resources.
It would not be the first attempt to change the model. The state also is funding an ongoing lysimeter study at Rocky Ford to determine if evapotransporation rates in the Arkansas Valley are higher than assumed in the model.
Another study is looking at whether ponds that feed sprinklers leak more than the model assumes.
