Tracy Kosloff, P.E., Water Resources Engineer, Colorado Division of Water Resources
According to the 1948 Arkansas River Compact, irrigation return flows from Colorado that were historically used in Kansas cannot now be consumed by improved irrigation practices in Colorado. The State Engineer’s Office found that approximately 6,100 acres on ditches along the Arkansas River were converted from flood irrigation to sprinklers between 1998 and 2008. Beginning in 2006, the State of Kansas notified Colorado that they were concerned about increased crop consumption due to the irrigation improvements.
A variety of factors affect whether a change in an irrigation method will have an impact on historical seepage and return flows. In most cases, the sufficiency of water supply to a field is the most important factor. For historically water-short locations, crop demand is not met for every acre. However, when irrigation improvements are implemented, more water may be delivered to the crop root zone at the time when it is needed, potentially increasing crop evapotranspiration. Such improvements can result in more consumptive use and a reduction in seepage and return flows. Although drying out the corners of fields can offset some of the increased consumptive use due to sprinkler irrigation, it may not offset the entire effect.
The Compact Rules Governing Improvements to Surface Water Irrigation Systems in the Arkansas River in Colorado (“Rules”) were developed to ensure historical return flows are maintained. The Rules were developed after extensive public review and with the guidance of an Advisory Committee with wide representation. The Rules were filed in Division 2 Water Court in September of 2009. Since then, the State Engineer’s Office has worked with the parties who filed statements of opposition to refine the Rules prior to the water court trial, which is scheduled for November 2010. The Rules are anticipated to take effect in January 2011.
The Advisory Committee developed the idea of the Irrigation System Analysis Model (ISAM), a peer reviewed computer program that can be used by applicants to help avoid the expense of individual engineering reports. The ISAM is used to compare water consumption and return flows before and after the improvements. The ISAM requires the following minimal data inputs: how many acres the system irrigates, any change in acreage due to the improvement (such as drying up corners when converting to sprinkler), the surface water rights used (or number of ditch shares), and whether there is supplemental irrigation from a well.
The following types of improvements must comply with the Rules if they are implemented on or after the effective date of the Rules (anticipated to be January 2011):
• Methods to reduce seepage from canals;
• Adding ponds for short term storage; and
• Certain sprinkler upgrades.
Any person who wants to continue using a sprinkler or drip irrigation system that was installed on or after October 1, 1999, must obtain written approval from the Division Engineer.
If the ISAM or an engineering report shows that irrigation improvements have an effect on seepage and return flows, water users will need to supply replacement water. Preliminary estimates are that up to 400 acre-feet per year of replacement water would be required for the area upstream of John Martin Reservoir. Downstream of John Martin Reservoir, up to 1,400 acre-feet per year would require replacement.
There are three ways to get approval for improvements from the Division Engineer:
• Rule 8 Applications – individual irrigators only, cannot involve water sources for compliance other than the subject water rights;
• Rule 10 Compact Compliance Plan – can be for a group of farmers and can involve the use of water rights other than the subject water rights to maintain historical seepage or return flows; and
• Rule 11 General Permits – for those improvements in areas of the Arkansas River Basin unlikely to affect compact compliance.
The CWCB provided funds for the Lower Arkansas Valley Water Conservancy District (“Lower Ark”) to act as a liaison between water users and the Division Engineer. The Lower Ark hired Leonard Rice Engineers, Inc. to develop a Rule 10 Compact Compliance Plan. The Lower Ark will collect data from farmers and work with the Division Engineer to calculate replacement requirements. The Lower Ark intends to charge a sign-up fee and then a per acre-foot fee, about $25 per acre-foot, for those opting to join their Rule 10 Compact Compliance Plan.
Additional information on the proposed rules is available on the Division of Water Resources website by clicking here: link to proposed rules.