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<br />Results of extensive analyses of climatological precipitation records from the Shasta and <br />Trinity Watersheds are presented. Because SLW records are not available in these areas, <br />the effort concentrated on determining how surface precipitation may serve as a proxy for <br />hours in which seeding might be effective. Final estimates of pQtential increases possible <br />through seeding are presented for a range of low, average, and high precipitation years. <br />These estimates incorporate SCPP results, limited icing and precipitation data collected <br />specifically for this study, and seeding suspension criteria adjustments based on historical <br />inflows to Shasta and Clair Engle reservoirs. <br /> <br />A general program design for an operational seeding program is presented. Both the Shasta <br />and Trinity Watersheds were considered as potential target areas. However, augmentation <br />of inflow to Shasta Lake is probably not a cost-effective alternative. Seeding to increase firm <br />yield to Clair Engle Lake, however, does appear feasible. The Trinity Alps provide an <br />excellent high elevation target for snowpack augmentation, and the catchment area is <br />sufficiently small to be treated effectively with modest equipment and personnel resources. <br /> <br />The design proposed would attempt to seed all storms, or portions of storms, expected to <br />result in enhanced snowfall in the target area. Seeding would be conducted using two <br />aircraft simultaneously dispensing AgI (silver iodide). A randomization scheme incorporating <br />blocking would be employed; 60 pct of the experimental units would be seeded and the <br />remaining population would be left for controls. Experimental units would consist of 4-h <br />blocks. This design would allow an eventual statistical evaluation, based primarily on <br />surface precipitation measurements, to be made. However, an adequate prediction of how <br />many experimental units would be necessary to achieve statistical significance is not possible <br />at this time. The" statistical investigations would be supplemented by a physical <br />observations program intended to confirm that seeding agent delivery was adequate and that <br />postulated microphysical effects were occurring. These measurements would not be <br />conducted for each experimental unit. <br /> <br />Based on the data presented from past seeding experiments and results of the climatological <br />studies, precipitation increases potentially achievable through seeding are presented for the <br />Trinity Watershed. Mter correction for the 3:2 (seed:no seed) seeding randomization scheme <br />presented in the design, these increases range from 64,000 to 113,000 acre-ft for low and high <br />precipitation years respectively. <br /> <br />The total benefit of additional water was estimated at $100 per acre-ft, based solely on <br />current revenues from power generation, M&I (municipal and industrial), and agricultural <br />usage. This estimate is probably conservative because additional benefits resulting from <br />other components, such as improvements in water quality, additional water for fisheries <br />management, and recreation, are not included. <br /> <br />Assuming a program duration of 10 yr, the estimated cost for all components, including <br />environmental compliance and monitoring, is about $1,080,000 annually. The resulting <br />benefit-cost ratios range from 5.9:1 to 10.4:1 for low and high precipitation years respectively. <br /> <br />Uncertainties exist in both the estimates of additional water production possible and the costs <br />to conduct the program. However, the benefit-cost ratios are sufficiently high so that the <br />program would be justifiable even if water production has been overestimated by a factor of <br />2 and the costs underestimated by an equivalent amount. Errors of this magnitude are not <br />felt to be present. <br /> <br />x <br />