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USBR Natural Flow Estimates <br />USBR natural flow estimates were available for: <br />• 09379500 San Juan R near Bluff, Utah <br />• 09251000 Yampa R at Maybell, CO <br />• 09260000 Little Snake R at Lily, CO <br />Graphical representation of USBR natural flows are included in the respective time series <br />graphs in Exhibit 1. USBR natural flow estimates were only available for the period 1975-1990. <br />The natural flows show good agreement with StateMod generated baseflows for the period <br />1975-1985. After 1985, the natural flows deviate from the San Juan and Yampa model <br />baseflows. The USBR natural flow estimates after 1985 are all evenly divisible by 1000, unlike <br />the estimates for the period 1975-1985. If the USBR baseflow calculations are carried out with <br />all terms at less precision, after 1985, that may account for some of the difference between <br />CRDSS baseflows and USBR baseflows. <br />Exhibit 3 contains comparisons, for the period 1975-1990, of USBR natural flow estimates to <br />StateMod generated baseflows in Subtask 11.7 and Phase IIIa. <br />Conclusions and Recommendations <br />The data estimation technique tested in Task 11.7 is acceptable for estimating historical <br />diversions, as evidenced by the strong agreement between Phase IIIa and Task 11.7 baseflow <br />hydrographs. Its weakest aspect is its limited ability to reflect expansion of decreed use over <br />the decades. Limiting the diversion by water right administration number is a reasonable way to <br />include effects of basin development through time. However, demandts with option "- <br />fwadminnum 0.0" removes the water right constraint on any structure that has a free or <br />extremely junior right. While this option made sense for verification under a recent period of <br />record, it may not be the best approach when generating baseflows for the early part of the <br />century. Executing this subtask exposed this issue, but we were not able to evaluate its <br />importance as we were working with 1975-1991. We do know that 20 to 33 percent of the <br />structures have free water rights, depending on which basin is being considered. Leaving them <br />unconstrained throughout the entire study period may cause us to significantly overestimate <br />diversions during the early years of the period. (Demandts options and functionality were <br />modified to address this issue prior to extending baseflows and simulating over the Pre-Existing <br />POR .) <br />A second concern is that currently, demandts constrains all diversion estimates to the current <br />water right, regardless of whether the diversion amount is estimated, read from a replacement <br />file, or retrieved from the database. This logic was acceptable in Task 11.7, as all diversions <br />were estimated, but should not be used in Task 11.9. In Task 11.9, actual diversion records <br />should be used whenever they are available. (Demandts options and functionality were modified <br />to address this issue prior to extending baseflows and simulating over the Pre-Existing POR .) <br />Boyle recommends the following changes to demandts: <br />1) The -rights option should be altered to constrain estimated data only, and not recorded <br />diversions. <br />Appendix E E-86 <br />