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2071 tf/:z5' ~r~ 1T1t" August 24, 1997 San Juan River Basin Modeling Effort Natural Flow Analysis The Natural Flow analysis is essentially complete. Through reviews and critiques, tbe analysis has been refined. Revised irrigated acreage and cropping patterns have completed based on recommendations by tbe states of Colorado and New Mexico. Cascade Reservoir, IE, Electra Lake, in tbe Animas River drainage, has been added as an explicitly modeled reservoir. A narrative description or template of the Natural Flow analysis is complete. Tables 1 is the average monthly summary over tbe 24 year period, 1970-93 wy of Natural Flows. Table 2 shows the Gains between selected Natural Flow stations. Figures 1 and 2 are line graphs of these values. Comparison of Natural Flows with Colorado's CRDSS base flows have been accomplished. Of the 25 Natural Flows core stations, 15 were compared witb CRDSS base flows by plotting montbly differences from Oct 1974 thru Sep 1991. (See Line Graphs) Oftbese, 13 stations were chosen for detailed analysis of the components Natural Flow. These components consist of the gaged flow, major reservoir change in content and evaporation, irrigation consumptive use, minor reservoir evaporation, transbasin diversions and subbasin diversions, livestock depletion and stockpond evaporation, and M&I depletions, These comparisons were plots of average monthly bar charts grouped by depletion type. Of these, 6 stations were deemed to represent the same physical areas so that comparison of depletions would be reasonable. These stations are: San Juan River at Caracas BOR average annual (1975-91) Natural Flow is only 7900af (1.3%) less than Colorado's base flows. A look at the bar chart and table shows tbat tbe main differences are reservoir evaporation, CD rates, and transbasin and subbasin diversions. Irrigated acres are almost identical but BOR's CD is only 2/3 of Colorado's CD, BOR is shorting these lands based on the Type I study, which relates irrigation shortages to streamflow. San Juan River at Achuleta BOR average annual (1075-91) Natural Flow is about 36,000af (2.7%) less than Colorado's.base flows. August's and September's average difference is greater than 10%. Almost the entire annual difference is from tbe reservoir evaporation category. Animas River at Durango BOR average annual (1075-91) Natural Flow is about 1,488af (0.24%) less tban Colorado's base flows. The biggest differences are in reservoir evaporation and accounting for Mineral Resources. Both irrigated acres and irrigation cu look pretty good. I believe the biggest difference in Cascade Reservoir results from about two years of missing EOM data, In this case BOR estimated a normal filling and spilling pattern and Colorado used a straight interpolation of known eom values. Animas River at Cedar Hill BOR average annual (1075-91) Natural Flow is about 5,100af (0,68%) less than Colorado's base flows. The monthly variations are higher than at the Durango