The Feasibility of Operational Cloud Seeding in the North Platte River Basin Headwaters to increase Mountain Snowfall

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' 4.5. Estimate of Operational Seeding Benefits ' Estimates of additional water and associated operational costs were developed assuming that cloud treatment would be conducted primarily over the western and eastern barriers surrounding the North Park of north- central Colorado. Clearly, additional water estimates depend on the areal coverage by the seeding device network selected. The sample seeding device placement selected for developing water ' estimates was developed by Holroyd (unpublished internal Reclamation memo, 2000) and is shown in figure 4.1. Holroyd used digital terrain information to position 55 seeding devices for efficient seeding coverage for a wind direction of 240 degrees (used in figure 4.1) and separately for winds of 360 degrees. The predominant wind direction during winter storms possesses a westerly wind component for a large proportion of most storm durations (wind roses developed and available from Randolph Borys, director of the Storm Peak Laboratory). ' Seeding device numbers and placement may be impacted by the presence of large wilderness areas on both, the west and east barriers of the Headwaters Region. The Holroyd seeding device placement in figure 4.1 does not include some high - elevation areas because of terrain limitations where the 9000 -foot contour was located. This problem may be solvable either by relocation of propane dispensers, and /or the use of AgI generators positioned at somewhat lower elevations. It may be that AgI generator placement ' well upwind may enable seeding of these difficult -to- target areas. These possibilities should be explored during the design phase. Generally, it is believed that high elevation seeding sites can be found along most of the length of each barrier without violating wilderness area boundaries. To obtain estimates of aerial coverage by the seeding devices, Holroyd had to use several assumptions including that seeding effects began 10 minutes after seeding initiation and end at 40 minutes, and ' dispersal occurs within a 15- degree angle sector centered on the wind direction. The times and angle settings were estimated from seeding trials in the Grand Mesa and the Wasatch Plateau of central Utah (appendix A). Holroyd used digital terrain at 0.5- kilometer resolution to determine seeded area pixel numbers in 500 -feet elevation bands down to 9000 feet elevation. With areal coverage known for each seeding device, an estimated treatment effect can be applied to natural precipitation estimates for selected elevation bands, then values summed over all seeded areas to obtain a total volume snow water equivalent ' (SWE). Table 4.3 presents the seeded area coverages in pixel totals for elevation bands that are identified at mid- elevations (9250, 9750 feet, etc.). The table gives estimates of additional water from cloud treatment using the pixel areas within seeded plumes, for 240 and 360 degree winds, assuming for all elevation bands a 26 -inch average SWE for 240 degree winds and 2 inch SWE for the 360 degree winds. The ' SWE values were obtained by area - weight averaging the 1961 -1990, 1 April, SWE for five Park Range and seven Medicine Bow snowpack measurement sites. The overall outcome of the area - weight averaging is 28.1 inches of SWE. The use of two inches of SWE for the 360 degree wind cases is an estimate based on study of wind rose information from the Storm Peak Laboratory. Additional water volumes from cloud seeding were estimated for average, dry and wet years based on 50 and 150 percent of average SWE, for the areal coverage estimated by Holroyd. Also, water estimates were developed for the higher areal coverages of 40 and 60 percent of total area (above 9000 feet elevation and contributing to the North Platte River). Holroyd's calculations led to 28 percent areal coverage (elevation band area weighted) by seeding plumes. Estimates of additional water for the average year were (rounded) 60,000, 85,000, and 128,000 acre- feet, for the 28 -, 40 -, and 60- percent areal coverages, respectively. The comparable values for the dry 25