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<br />MWSl Project <br />Phase 11 Effluent Management Summary Report - DRAFT <br /> <br />study, exchange potential is assumed not to exist during periods of "free river" <br />conditions. In these conditions junior water rights can freely divert v.ith having to <br />exchange. ) <br /> <br />August 22, 1995 <br /> <br />Exchange potcntial is difficult to quantifY for planning purposes. It varies both <br />seasonally and from year to year as a function of stream flows and call conditions. <br />Exchangc potential occurs primarily in the irrigation season of April through October <br />when downstrcam irrigation rights call substantial amounts of water past upstream junior <br />municipal water rights. Exchange potential tends to be highest in years when there is a <br />relatively large volume of runoff throughout the summer coupled with relatively high <br />downstream irrigation demands due to hot dry summer conditions and/or the effects of a <br />previous year's drought which has reduced the carryover contents of downstream <br />reservoirs and depleted the soil moisture profile. <br /> <br />In the future exchange potential will tend to diminish. As the metro area grows, <br />downstream calls will tend to tend to become less senior and possibly shorter in duration <br />as the level of wastewater discharge and overall urban runoff increases, providing a <br />growing supply to a fixed set of downstream demands. To the degree that the priority <br />and duration of downstream calls is diminished, upstream water rights will be able to <br />increase their combined diversions, thereby reducing the level 6r continuous flow <br />between upstream municipal points of diversion and water rights located downstream of <br />the metro Denver area. <br /> <br />As a part of Phase II investigations of exchange potential two previous studies <br />were examined, one conducted by W.W. Wheeler & Associates and one by Cheryl Signs <br />Engineering. The Wheeler study used a daily model to estimate the exchange potential <br />on the South Platte River between the Burlington ditch headgate and Strontia Springs <br />over the historical 1947-1974 hydrologic period of record. This study reported an <br />average exchange potcntial of 44,000 acre feet per year. The results of this study are <br />summarized in Figure 9. <br /> <br />These results are senSlllve to a number of factors including instream flow <br />requirements, assumed levels of wastewater discharges and urban runoff, and changes in <br />water rights. As an example, the Wheeler study assumed that cxchanges would be <br />limited by a 15 cfs instream flow constraint through the exchange reach. If a minimum <br />stream flow requirement of 60 cfs is assumed, exchange potential would drop to 32,000 <br />acre feet per year. Using the Wheeler data, the general effect of potential instream flow <br />requirements for the urban South Platte on exchange potential is illustrated in Figure 10. <br />Because of this sensitivity and the importance of exchange potential to effluent <br />management opportunities, further study of exchange potential is recommended for Phase <br />III of the MWSI project. <br /> <br />Exchange potential would be slightly greater at Chatfield Reservoir compared to <br />Strontia Springs due to intervening gains from Plum Creek and Deer Creek. <br /> <br />19 <br />