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underestimates crop consumptive use or the irrigated area is much greater, then more of these ditches may <br />be water-short. That aside, the conclusions reached in this memorandum are based upon the data provided <br />and the assumptions made. <br />3.0 CONCLUSIONS AND RECOMMENDATIONS <br />The investigation found that 2 of th e 3 USGS gages used to determine the USBR cutoff dates were affected <br />by storage and releases from upstream reservoirs. Ho wever, based upon just 2 years of data at one gage <br />(Gunnison River at Gunnison), it app eared that the reservoir operations did not appreciably impact the <br />cutoff date. At the other gage (Uncompahgre at Col ona), the reservoir that impacts the gage is newly <br />constructed and has yet to be fully utilized, so no conclusion could be reached based upon the available <br />data. However, it is suspected that reservoir ope ration can significantly impact the stream hydrograph, and <br />any further use of the USBR cutoff procedure should take these operations into account. <br />The investigation found that in two of the HCUs th at the USBR's report represented as not being water- <br />short, there are at least six ditches in which th e water supply limits the amount of consumptive use that <br />could have occurred. Some of those ditches were lo cated on tributary streams, and the available physical <br />supply probably caused the ditches to be water-short. In other cases, it appears that the ditch water rights <br />were not in priority. The dates when the ditch b ecame water-short could not be accurately estimated using <br />the USBR procedure. In most cases the dates are at least 1 month off. In ot her cases the USBR procedure <br />would indicate that the water supply is limite d when this analysis shows it was not. <br />For a basin-wide study, the USBR's c oncept of determining when a ditc h (or ditches) becomes water-short <br />by relation to a "representative" str eam gaging station is an acceptable a pproach, particularly since water is <br />diverted under the doctrine of prior appropriation. Ther efore, as the streamflow decreases, the more junior <br />water rights can be legally precluded from diverting if th e demands of senior rights are not satisfied. In an <br />individual HCU, a single cutoff date may not be a ppropriate for determining when individual ditches <br />become water-short. However, the focus of the USBR analysis for the Colorado River System Consumptive <br />Uses and Losses Report, 1981-1985 is a determination of the "annual" basin-wide consumptive use. If the <br />singular cutoff date is determined by a basin-wide approach that would weight the overestimation of <br />consumptive use for a reasonably large number of water-short ditches versus the underestimation of <br />consumptive use for a similarly large number of th e remaining water-short ditches, such that the <br />overestimation is offset by the underestimation, on an a nnual basis, then the concept of a singular cutoff <br />date is acceptable. With the data provide d, that determination could not be made. <br />For evaluation of the remaining river basins in the CRD SS, more attention should be given to the selection <br />of the ditch structures that are to be analyzed to evaluate the USBR cutoff dates and to the procedures used <br />for determining the amount of acreage that does not r eceive a full water supply. Ditch structures should be <br />selected from HCUs that the USBR report identifies as alfalfa-short and pasture- short acreages. Additional <br />communication with the Division Engineers and water commissioners in those HCUs could provide <br />information that would allow the CRD SS team to focus its data collecti on and analysis on structures that are <br />water-short. <br />7 <br />A 275 01.09.95 1.14-24 Walter <br />