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<br />~ <br /> <br />. <br /> <br />contour was located. This problem may be solvable either by relocation of propane dispensers, and/or the <br />use of AgI generators positioned at somewhat lower elevations. It may be that AgI generator placement <br />well upwind may enable seeding ofthese difficult-to-target areas. These possibilities should be explored <br />during the design phase. Generally, it is believed that high elevation seeding sites can be found along <br />most of the length of each barrier without violating wilderness area boundaries. <br /> <br />To obtain estimates of aerial coverage by the seeding devices, Holroyd had to use several assumptions <br />including that seeding effects began 10 minutes after seeding initiation and end at 40 minutes, and <br />dispersal occurs within a 15-degree angle sector centered on the wind direction. The times and angle <br />settings were estimated from seeding trials in the Grand Mesa and the Wasatch Plateau of central Utah <br />(appendix A). Holroyd used digital terrain at 0.5-kilometer resolution to determine seeded area pixel <br />numbers in 500-feet elevation bands down to 9000 feet elevation. With areal coverage known for each <br />seeding device, an estimated treatment effect can be applied to natural precipitation estimates for selected <br />elevation bands, then values summed over all seeded areas to obtain a total volume SWE. <br /> <br />Table 4.3 presents the seeded area coverages in pixel totals for elevation bands that are identified at mid- <br />elevations (9250, 9750 feet, etc.). The table gives estimates of additional water from cloud treatment <br />using the pixel areas within seeded plumes, for 240 and 360 degree winds, assuming for all elevation <br />bands a 26-inch average SWE for 240 degree winds and 2 inch SWE for the 360 degree winds. The <br />SWE values were obtained by area-weight averaging the 1961-1990, 1 April, SWE for five Park Range <br />and seven Medicine Bow snowpack sites. The overall outcome of the area-weight averaging is 28.1 <br />inches of SWE. The use of two inches of SWE for the 360 degree wind cases is an estimate based on <br />study of wind rose information from the Storm Peak Laboratory. <br /> <br />. Additional water volumes from cloud seeding were estimated for average, dry and wet years based on 50 <br />and 150 percent of average SWE, for the areal coverage estimated by Holroyd. Also, water estimates <br />were developed for the higher areal coverages of 40 and 60 percent of total area (above 9000 feet <br />elevation and contributing to the North Platte River). Holroyd's calculations led to 28 percent areal <br />coverage (elevation band area weighted) by seeding plumes. <br /> <br />Estimates of additional water for the average year were 59,727, 85,326, and 127,989 acre-feet, for the <br />28-, 40-, and 60-percent areal coverages, respectively. The comparable values for the dry year were <br />29,863,42,663, and 63,994 acre-feet, and 89,590, 127,989, and 191,983 acre-feet for the wet year. The <br />values for the wet year may be an overestimate in some winters if seeding suspension criteria suspends <br />operations. <br /> <br />Table 4.3 does not take into account the varying opportunity across year type to conduct cloud seeding; <br />namely, the possibly differing cloud numbers and level of favorable seeding circumstances in available <br />clouds. The table values assume uniform opportunity across normal, dry and wet years. The design <br />phase of the project will look into this issue. It may be that dry years offer more inefficient natural clouds <br />than normal and wet years, thus presenting more treatment opportunities (or fewer opportunities). <br /> <br />26 <br />