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I I I I I I I I I I I I I I I I I I I INTRODUCTION This report presents the results of a feasibility study that was completed for the Lower Arkansas Water Management Association (LAWMA) under a loan from the Colorado Water Conservation Board. There were two objectives for the study. The first was to formulate a detailed augmentation program to replace depletions from LAWMA well pumping both to Colorado surface rights and to "usable flow" at the Colorado-Kansas Stateline. The second was to determine the financial feasibility of the augmentation program. The accomplishment of these objectives required estimating the well pumping by the LAWMA members during water years 1986-94, estimating the depletions from this well pumping, and assessing various sources of replacement water that are or will be available to LAWMA in the future. The need for this feasibility study has been precipitated by the recent rulings of the Special Master and the U.S. Supreme Court in Kansas v. Colorado. These rulings indicate that post-compact well pumping in the Arkansas River main stem area has depleted usable Stateline flow. Curtailment of this post-compact well pumping appears imminent unless programs are developed to replace the depletions to the Arkansas River from such pumping. At stake here is a significant part of the agricultural economy of southeastern Colorado. According to recent studies by the Colorado Division of Water Resources, about 150,000 acres were irrigated in Water District 67 in 1985. Water for irrigating this land is derived from several ditches and reservoirs and from more than 500 large capacity irrigation wells. According to the December 1990 Water Budget'...] developed by Boyle Engineering Corporation, the total irrigation supply in Water District 67 averaged approximately 395,000 acre-feet annually during 1976-85, of which more than 100,000 acre-feet were derived from wells. Colorado agricultural statistics indicate that the income from this irrigated land exceeds $40 million annually. Thus, irrigated agriculture in Water District 67 is a significant part of lJ This water budget provided the basis for a significant part of Colorado's technical case in the liability phase of the trial in Kansas v. Colorado. It was actually a series of water budgets that represented the hydrologic characteristics of the water supply and water uses in the main stem area, including separate eccountings for the irrigation under each major ditch system, the alluvial and various surficial or bench aquifers, and surface and ground water contributions to the Arkansas River. The Water Budget Documentation explains the technical approach and assumptions in the water budget and summarizes significant data involved therein and the results derived therefrom. This is a very detailed analysis of the major ditch systems and was relied upon heavily in this feasibility study. 3