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June available flows often exceed 100,000 and can range over 600,000 af, greatly exceeding the range of <br />likely diversion capacities (200 — 400 cfs, or approximately 12,000 — 24,000 af/month) for the canal. <br />Inflow to the reservoir was computed in the yield analysis spreadsheet as the minimum of legally <br />available water in the river, and the user-specified diversion capacity. <br />The original scope of work for the study included use of more senior rights at the diversion site, which <br />would result in alternative inflow time series representing potentially greater flows available to the <br />project. This objective was abandoned during the course of the study for the following reasons: <br />1. Preliminary results demonstrated that there were relatively few months when increasing <br />available water would result in greater inflow to Grand Valley Lake. "Reasonably" sized <br />diversion and conveyance facilities constrained diversions more often than legal availability of <br />water. In other words, a junior right yields an adequate amount of water to support the range of <br />reservoir capacities considered herein. <br />2. The diversion site lies between the Aspinall Unit and the Gunnison River at Whitewater gage, at <br />which target flows were developed by the U.S. Fish & Wildlife Service (USFWS) under the <br />Colorado River Recovery Program. In December 2008, the U.S. Bureau of Reclamation (USBR) <br />published a Programmatic Biologic Assessment describing changed operations of the Aspinall <br />Unit to help meet the targets, within their discretionary operations. USFWS is currently <br />reviewing the document. Transferring a senior right upstream, into the reach which is the subject <br />of the biological assessment seems suspect. The transfer would presumably occur only at great <br />cost and with an unpredictable outcome. <br />B. Demands <br />Without knowing how the reservoir would operate based on monthly needs of pre-defined water users, <br />AECOM used typical demand patterns based on information in the Clay Report, requirements of the <br />Colorado River Recovery Program for the 15-Mile Reach, reported annual diversions by Orchard Mesa <br />Irrigation District for irrigation (not pumping or power use), and readily available demand information <br />for Ute Water Conservancy District and the City of Grand Junction. The demand time series, driving <br />reservoir releases, has a significant impact on reservoir yield. Not only the size, but the monthly pattern <br />of use impacts firm yield (the amount of water that can be delivered from the reservoir each and every <br />year of a study period that contributes to the purposes of the project). The process of estimating firm <br />yield with the spreadsheet reservoir operations model is an iterative process: the annual demand is <br />increased until the active pool in the reservoir is emptied and demand is unmet for one month of the <br />study period. (Use of this publicly available data for these two nearby water suppliers is not intended to <br />imply any interest by them in participating in the Grand Valley Lake project. Their data was used solely <br />38 <br />AECOM <br />2/22/10 <br />