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DESCRIPTION OF THE MODEL 5 Grand Dry Fork Valle PID Government y MCID Highline Irrigation GVWUA Canal District Canal Grand Sulphur Gulch Junction Colorado River Reservoir Palisade Cameo 15-mile or Study reach (Palisade to Grand Junction) OMID Plateau Creek EXPLANATION GVWUA Grand Valley Water Users Association MCID Mesa County Irrigation District OMID Orchard Mesa Irrigation District PID Palisade Irrigation District Roan Creek De Beque Upper Colorado River Basin Figure 2. Primary study reach hydrologic components considered in the stochastic mixing model. from Sulphur Gulch Reservoir, inflow from Plateau Creek, and return flow from the Orchard Mesa powerplant. The amount and quality of streamflow upstream from De Beque is a result of dissolved solids transported by runoff, ground-water inflow (baseflow), and reservoir releases. Daily streamflow losses over the study reach include diversions, reservoir pumping, and evaporation. The Government Highline and Grand Valley Canals divert Colorado River water downstream from the De Beque streamflow gaging station, but upstream from the Palisade streamflow gaging station, for use by the Grand Valley Irrigation Company, Grand Valley Water Users Association, Orchard Mesa Irrigation District, Palisade Irrigation District, and Mesa County Irrigation District. The quality of water that will be released to the Colorado River from the Sulphur Gulch Reservoir would be the result of mixing seasonally pumped Colorado River water with runoff to the reservoir and concen- tration of salts by reservoir evaporation. In conceptualizing the stochastic mixing model, the fol- lowing assumptions were made. No change in reservoir operations or transmountain diversions occur upstream from the city of De Beque. It is assumed that the operations associated with reservoirs and transmountain diversions in the headwaters of the Colorado River will continue with the same (or similar) pump and release schedules. The assumption of similar operations is valid so long as the mean streamflow deter- mined by the stochastic model does not change in time (stationarity). 2. streamflow record at Colorado River near Cameo adequately describes flow at the proposed Sulphur Gulch Reservoir pump and release point. This assumption is realistic because there are no anthropogenic or perennial tributary sources of water between the proposed pump and release point and the streamflow-gaging station located about 1 mi downstream. Variability in streamflow and salinity measurements adequately reflects periods of extreme wet and dry climatic conditions. The period of record used (1974- 2001)reflects wet to dry conditions; however, future climate conditions may be different and result in extreme wet or dry (drought) conditions. As long as climatic stationarity is in effect, the minimum and maximum values of the probability distribution functions describing daily flow may be adjusted to incorporate extreme values; for example, the probability distribution function describing minimum annual (calendar year) streamflow (657,210 ft3/s) at the gage near Cameo could be changed to the new minimum annual streamflow (604,026 ft3/s) experienced during the 2002 drought. Because of the comparatively minor difference in minimum annual flows (about 8 percent) and similar probability of occurrence (of less than about 1-percent chance) the simulations were not repeated with the new distribution. 4. Instantaneous routing between pump and discharge points. Instantaneous routing assumes that there is no time delay between water sources or sinks in the study