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t <br /> some areas before that date. Therefore data exist for a planning document for a winter cloud seeding re- <br /> two winters only, a short climatological record com- search project called the Colorado River Snowpack <br /> pared to the more traditional Snow Telemetry Enhancement Test(CREST; Super et al. 1993). Fur- <br /> (SNOTEL) and snow course datasets. Unlike these ther criteria from this document were that the candi <br /> datasets, however, SNODAS provides spatially con- date region usually has at least 20 km west-east ex- <br /> tinuous data at 1 km resolution. The model has been tent of 9000 feet elevation area and that it is largely <br /> validated (Cline et al. 2004) in the Colorado Moun- or wholly outside designated wilderness areas. These <br /> tains against the Corps of Engineers widely-validated criteria were used to select proposed areas for cloud <br /> SNTHERM model. seeding experimentation, whereas this document is <br /> concerned with all potential target areas for opera- <br /> 2.1 Determination of Target Areas tional seeding. it would be impractical to conduct <br /> experimentation within a wilderness area, since key <br /> A major variable in estimating water yield from instrumentation would not be allowed. Some opera- <br /> seeding will be selection of the seeding target areas. tional projects target wilderness areas, although their <br /> We split this task into two sections: existing areas seeding generators are located outside the boundaries <br /> already being seeded by operational programs (in of those areas. So for the present study, several addi- <br /> Utah and Colorado only), and potential new areas. tional areas,including those with wilderness designa- <br /> Geographic Information Systems (GIS) maps were tions and barriers no wider than 5 km, have been <br /> obtained for the former. Selection criteria for existing added.Finally,the potential new areas were included <br /> areas vary, but are in general elevation-based. In only if they are outside existing target areas. <br /> Utah, this criterion is 7000 feet or higher,whereas in <br /> Colorado it is above 8000-9000 feet. These criteria The CREST locations were given spatial extents <br /> were informally adopted and reflect the higher eleva- through use of GIS software, since the planning <br /> lions in Colorado. In any event, the existing areas document gave only general text descriptions of loca <br /> were used as provided, with no modification except tions(the authors had to manually search 1:1,000,000 <br /> to exclude parts outside the Colorado River drainage aeronautical charts). Based on the slightly different <br /> area.The existing target areas are listed in Table 2. criteria (stated above) and more sensitive mapping <br /> with GIS software, we identified five additional po- <br /> Table 2.Existing Target Areas tential target areas (Table 3). The Bureau of Recla- <br /> mation report (Water and Power Resources Service <br /> Colorado Utah 1980) identified the entire Mogollon Rim in Arizona <br /> 1.Upper Arkansast 11.Fishlake Mtns.t as a potential target,whereas we exclude all of it(ex- <br /> 2.Gunnison North 12.Boulder Mtn.t cept for the San Francisco Mountains) because it is <br /> 13.Uinta Mtns. South largely below 9000 feet elevation. The snowfall at <br /> 3.Gunnison South such low elevations,particularly if they are at south- <br /> 4.Vail 14.Dixie Natl.Forest- erly latitudes as in Arizona,would frequently occur at <br /> 5.Beaver Creek relatively warm subfreezing temperatures, during <br /> 6.Grand Mesa North which silver iodide would be ineffective. Seeding by <br /> 7.Grand Mesa South liquid propane gas expansion, which can create ice <br /> crystals at warmer temperatures (thresholds -1°C vs <br /> 8.San Miguel Mtns. -5°C for silver iodide), might be a viable alternative <br /> 9.Western San Juans for such locales. <br /> 10.Eastern San Juans <br /> Portion of area outside Colorado River Basin An essential point is that,despite the CREST cri- <br /> teria application, there is still substantial subjectivity <br /> For potential areas, a strict 9000 foot base in selecting any seeding target area.All new potential <br /> threshold was used as a criterion in the Colorado ba- areas are listed in Table 3.Both existing and potential <br /> sin regions of Arizona,New Mexico Colorado,Utah, areas are shown by the map in Fig. 1. <br /> and Wyoming.This criterion follows that specified in <br />