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o 0 314 2 Page 41 IV. SCENARIOS - DESCRIPTIONS AND RESULTS A. Interstate Water Use Changes 1. Effects of runoff increases given current institutions and demands The MODSIM network optimization model was specified for the Colorado River system, as described previously, and then a series of computer runs was performed in which the parameters of inflow, demand, and water-use priorities were varied systematically. The first of those runs represented the baseline case, i.e., a situation characterized by no vegetation manipulation, climatic conditions equal to a repeat of the 1906-1977 period of record, water demands and storage facilities representative of 1980, and current interstate and intrastate water allocation institutions remaining unmodified. Then, a second run was performed in which the only change from the baseline assumptions was that of vegetation manipulation on the Arapahoe National ~orest, and consequent increased flows into the Colorado River. ~ As shown in the first two columns of Table IV-l, the streamflow augmentation from vegetation manipulation produced almost no change in water consumption anywhere in the Colorado River system under the assumptions of the baseline scenario. In other words, the increased flows went largely unused, in the sense of the uses which were modeled (instream uses such as hydroelectric power generation, salinity control, fishery habitat maintenance, and water-based recreation would have been affected). This result is due to the fact that there Were no physically satisfiable yet unsatisfied demands in the Upper Basin, and the flows were stored in Lake Powell and not released for Lower Basin use except in years of high run-off, at which times Lower Basin demands were already fully met. In such years, if storage space existed in Lake Mead, increased flows would be stored therein, subject to downstream demands, but usually would be ~