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these flows is to combine daily data from the diversion record of the Maybell Canal with the <br />concurrent record of the Maybell gage. The latter method was selected for this analysis. <br />Existing human water use in the Yampa Basin depletes about 120,000 acre -feet (AF) annually <br />out of an average annual yield of 1.13 million acre -feet (MAF). Future water development for <br />human consumption in the Yampa River Basin is expected to deplete up to 50,000 AF more <br />(170,000 AF total) from the river by the year 2045' °2 . By modeling the estimated human water <br />demand against an historical set of hydrologic and atmospheric conditions, CRDSS can predict <br />the magnitude and monthly distribution of discharge in AF under similar conditions in the future. <br />The Recovery Program funded a study by Modde et al.3 (1999) to determine stream flow needs <br />of endangered fishes in the Yampa River. The study focused on riffle habitat due to its <br />importance to production of invertebrate biomass and its sensitivity to changes in flow. Based <br />on a series of physical habitat variables (e.g., depth, velocity, wetted width, etc.), the authors <br />found that the rate of loss of riffle habitat was greatest at flows below 93 cfs. Historically, at <br />times river flows have fallen below 93 cfs with no apparent harm to the Colorado pikeminnow. <br />Moreover, native fishes in the Colorado River Basin have evolved under highly variable flow <br />conditions. Therefore, the authors concluded that these fish can tolerate and may benefit from <br />"transgressions" below this 93 cfs flow target at their historic frequency, magnitude and duration. <br />This hydrologic analysis was designed to compare CRDSS calculated historic with CRDSS <br />modeled future (ca. 2025 and 2045) supply and demand conditions within the context of this 93 <br />cfs flow target. <br />CRDSS modeling assumptions for the Yampa River <br />1. Modeled four different levels of demand <br />a. Natural (no demand) <br />b. Calculated historic <br />c. Near -term future (ca. 2025) <br />d. Long -term future (ca. 2045) <br />2. Functions as if the river were under water rights administration; water is delivered in <br />priority to satisfy individual demands served by water rights as needed and available.' <br />3. Explicitly includes all water diversions and return flows. <br />4. Functions on a database with a 17 -year period of record which serves as a hydrologic <br />template for modeling an overlay of alternative water supply and demand scenarios. <br />'BBC Research & Consulting. 1998. Yampa Valley Water Demand Study. Final Report. Denver, CO. <br />2 Hydrosphere Resource Consultants. 1995. Yampa River Basin Recommended Alternative Detailed Feasibility ' <br />Study. Final Report. Boulder, CO. <br />' Moddej., W.J. Miller, and R. Anderson (eds.). 1999. Determination of habitat availability, habitat use, and flow ' <br />needs of endangered fishes in the Yampa River between August and October. Final Report submitted to the <br />Recovery Implementation Program, U.S. Fish and Wildlife Service, Denver, Colorado. 134 pp. <br />'Calculated historic = natural flow minus historic consumption estimate; 2025 and 2045 = natural flow minus 2025 ' <br />or 2045 predicted consumption. <br />'Based on historic hydrologic and atmospheric data, using Blaney - Criddle to estimate irrigation demand, and <br />assuming that Stagecoach Reservoir initially was full. , <br />Appendix B 2 <br />