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Reservoir Operating Studies FINAL OPERATIONS STUDIES FOR THE ENLARGED ELKHEAD RESERVOIR The final reservoir operations studies made using the Yampa River basin model considered three basic scenarios: 1) current potential conditions, 2) year 2040 conditions without the reservoir enlargement, and 3) year 2040 conditions -wiW the reservoir enlargement. Several mainstem flow enhancement options were also examined under the latter scenario. These various scenarios were defined so as to characterize the full range of likely water management futures in the basin and to permit the with/without comparisons necessary for environmental and biological assessments of the enlargement project. The key assumptions and results of these groups of model scenarios are presented below. Detailed tables of scenario results can be found in Appendices E, F, and G. Current Potential Conditions Scenario Assumptions This scenario represents the hydrologic conditions that would ensue if current potential demands were active over the entire 53-year study period and if existing reservoirs were operated according to contracts and agreements presently in place. It is = a "no action" scenario since water demands will change in the future even without development of additional water storage or other changes in basin water management. Rather, it represents a "baseline" that facilitates the characterization of future hydrologic conditions as differences from actual present conditions. In terms of water demands it reflects a smaller amount of depletion than does the "current potential conditions with consulted depletions" scenario used to develop the preliminary flow recommendations at Maybell. Water demands in this scenario are met using existing direct flow water rights and, if necessary, storage deliveries from the reservoirs. All reservoirs are represented at their present capacities. There is no mainstem instream flow right in place nor are there any reservoir operations to enhance mainstem flows at any locations. Scenario Results Water Deliveries Water shortages to demands occur in 34 of the 53 years of the study period in this scenario. However, all of these shortages are to agricultural demands, many of them on small tributaries, that have no storage supplies. The average annual total shortage is 917 of and the maximum annual total shortage is 11,773 of occurring in 1934. Reservoir Levels There is very little use of reservoir storage in this scenario because the demands with storage contracts can be met from direct flow supplies in all but the driest years. Figure 2-1 shows the end-of-month contents of the three modeled existing reservoirs. As can be seen, there is little draft for water supply purposes except in the dry periods of the 1930's, `50's, and `70's. The regular annual cycles that are evident are the result of evaporation and, in the 2-12