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SPDSS Task 58 Memo <br />August 5, 2004 <br />Page 16 of 22 <br />TR15 - Weekl~p Consumptive Use and Precipitation in the Lower South Platte River Basin <br />This publication contains weekly consumptive use data, expressed in inches, for the major <br />irrigated crops grown in the lower South Platte River Basin. These data are based upon the <br />historical climatic record of 1947 - 1975 at U. S. Weather Bureau stations at Fort Morgan, <br />Sterling, and Julesburg. Precipitation by weeks is tabulated from the sum of daily amounts. No <br />adjustment in the precipitation data was made to estimate the amount which is "effective" in <br />meeting consumptive use requirements. Consumptive use was calculated using the modified <br />Blaney-Criddle Method given in USDA-SCS Technical Release No. 21, "Irrigation Water <br />Requirements." As these results are based on a typical calculation using the modified Blaney- <br />Criddle method, this document is thought to provide little additional value to existing computer <br />models (see previous section) for comparison and validation of consumptive use estimates <br />generated for SPDSS. <br />Basin Project Studies <br />A number of State (Colorado Water Conservation Board or Colorado Water Resources and <br />Power Development Authority) supported studies have been performed in which consumptive <br />use estimates were an integral element. These studies include the Cache la Poudre Basin Study, <br />the Clear Creek Project Study, the St. Vrain Basin Study, Hardin Project Study, Narrows Project <br />Study, etc. Based on an initial review of the documentation for these studies, these studies <br />typically used the modified Blaney-Criddle method (without elevation adjustment or local <br />calibration). These studies are thought to provide little additional value to that available from <br />existing computer models for comparison and validation of SPDSS consumptive use estimates. <br />Small Project Water Right En~ineerin~ <br />There have been hundreds of water court applications in the SPDSS study area for changes of <br />water rights, augmentation plans and substitute water supply plans for which consumptive use <br />determinations have been made. These exist at varying levels of detail, for varied study periods <br />and are generally site specific to an individual ditch or water right system. These analyses do not <br />provide regional estimates of consumptive use that would be of value for comparison or <br />verification of SPDSS consumptive use estimates. <br />SUMMARY/CONCLUSIONS <br />Investigations have been performed of how calculations of PCU are typically made in Colorado <br />and how those calculations may change in the future. These investigations were also intended to <br />collect information that could be used to confirm and verify consumptive use estimates prepared <br />for the SPDSS. Investigations were principally performed through 1) interviews with parties <br />who prepare consumptive use estimates (including State representatives), 2) literature searches, <br />and 3) literature reviews. These investigations have yielded the following primary conclusions. <br />Methods -Crop consumptive use calculations made in Colorado are predominantly <br />based on the modified Blaney-Criddle method as described in SCS's Technical <br />