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18 Probable Effects of the Proposed Sulphur Gulch Reservoir on Colorado River Quantity and Quality <br />near Grand Junction, Colorado <br />10,OOG <br />Q <br />CL <br />~_ <br />J <br />J <br />~~ <br />Zw <br />0 J 1,000 <br />J ~ <br />O W <br />!~ ~ <br />0 <br />W <br />J <br />O <br />C~ <br />~_ <br />f~ <br />100 <br /> <br /> <br />® <br />~ ,~i <br />L~ <br /> <br /> <br /> <br /> <br /> • ^ <br />0.01 0.10 1 10 100 <br />STREAMFLOW, IN CUBIC FEET <br />PER SECOND <br />EXPLANATION <br />Measurements-Dry Fork <br />Measurements-Sulphur Gulch <br />~ Realization 1 <br />Realization 2 <br />Fitted <br />Figure 14. Stochastic simulation ofdissolved-solids concentration in runoff at Dry Fork at <br />Upper Station (09095300) near De Beque with measurements at mouth of Sulphur Gulch <br />(3916071081535001. <br />(fig. 14). The fact that these two Sulphur Gulch measurements <br />appear at the margin of variability indicates that the use of this <br />equation is nominally conservative. On the other hand, addi- <br />tionalmeasurements are needed to decide whether the equation <br />should be shifted. The stochastic equations for streamflow, <br />evaporation, and salinity together with deterministic equations <br />for reservoir surface area and streamflow exceedance are incor- <br />poratedinto the mixing model. The mixing model itself is com- <br />posed of linked hydrology and water-quality submodels that are <br />described in the following sections. <br />Hydrology Model <br />The objectives of the hydrology model are twofold: <br />(1) compute the availability of pumpable water to the proposed <br />reservoir, and (2) provide flows at key hydrologic points in the <br />study reach that can be used by the water-quality model. <br />User-defined input that is required by this model includes pump <br />rate, beginning and ending pumping period days, Grand Valley <br />Irrigation senior and junior water rights, and maximum allow- <br />able return flow at Orchard Mesa Irrigation District check <br />structure. <br />To compute the amount of pumpable water requires six <br />primary steps. First, the daily flow in the Colorado River and <br />downstream demand (diversions) are generated on the basis of <br />random sampling of corresponding probability distributions. <br />The available daily flow then is computed by taking the differ- <br />encebetween these random variables. Second, the cumulative <br />distribution function for daily flow is computed to determine <br />the streamflow exceedance. Third, quartiles associated with <br />these daily flow exceedances are determined. Fourth, a check is <br />conducted to see if flow exceeds diversions (total demand). If <br />flow exceeds the total downstream demand (diversions), the <br />demand is subtracted from the Colorado River flow. If this dif- <br />ference is greater than zero, then that quantity is assigned as the <br />minimum divertible flow; otherwise, the minimum divertible <br />flow is set to auser-defined value. Fifth, a check for peak-flow <br />cut criteria is conducted. During the peak-flow check, if flow in <br />the Colorado River is between 12,900 and 26,600 ft3/s then a <br />zero value is assigned; otherwise the minimum divertible flow <br />value is used in the subsequent operation. In the subsequent <br />operation, the actual divertible flow is assigned as the lesser <br />value of the available flow or the peak-flow cut criteria. Sixth, <br />the pumpable flow is set to the minimum value between the <br />divertible flow and maximum pump rate. A flow chart describ- <br />ing the hydrologic model operations is provided (fig. l5), and a <br />description of the Excel cell-based equations is included as <br />Appendix 1. <br />Water-Quality Model <br />The objective of the water-quality model is to compute the <br />dissolved-solids concentrations (salinity) at points where water <br />enters or exits the study reach. User-defined input that is <br />required by this model includes initial reservoir volume and <br />concentration, total reservoir volume, seasonal release amount, <br />seasonal release period, and peak-flow release amount. The <br />background concentration at selected locations depends on <br />