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Determine effective ice nuclei concentrations in the area as 8. function of temperature Determine the ratio of ice crystal concentra- tion to ice nuclei concentrations in the area as a function of cloud top temperature. Define the orographic component of the vertical motion field. Present technology is now available to derive reasonable estimates of the se quantities. b. General conclusions The agreement of the three independent samples with current cloud physics theory, and the , internal consistency among the samples increase confidence that snowfall can be increased on the mountain barrier when seeding is conducted under the right conditions. Almost as apparent is the fact that snowfall can be decreased on the mountain E:ummit when seeding ii:nproper conditions. The modest snowfall increases reported by most cloud seeding projects (generally 15'70 or less) appears to be composed of both increases and decreases that nearly cancel one another over a period of time. Clearly, a potential for m~difi~;tion does not always~xist'and cloud treatment will have to be tailored for existing conditions. C. Weather Modification Geography The Colorado River Basin above,Lee's Ferry contains about 109,500 square miles. The average annual runoff for this Basin is equivalent to 2. 3 inches of precipitation (Crow, 1967). Some 130/0 (aJxut 14,200 square miles) of the area basin produceE! a runoff equivalent to 10 or more inches of prec ipitation. Some 9500 square miles produces from 1 to 10 :lnches, and the remainder of the Basin, some 85,800 square miles, produces runoff equivalent to about 1 inch or less of precipitation. This very large difference in yield of various portions of the Basin results from 1. The substantially greater amounts of precipi- tation in the mountainous areas resulting from orographic influences. 2. The greatly increased evapotranspiration losses at the lower elevations due to higher temperatures. Clearly, the initial stages of a weather modification operational adaptation program in the Colorado River Basin should be concentrated in the 13'70 of the Basin that produces over 10 inches of precipitation and' about 77'70 of the annual runoff. 1. Site Selection for Pilot Project The 13'70 of the Colorado River Basin that produces most of the runoff lies almost exclusively above 9,000 feet elevation. The 9,000 to 10,000 it contours, consequently, can be used to define the sub-areas of the basin that should receive primary consideration for initial weather modification efforts of a pilot project. Commercial programs of weather modification operations in mountainous areas. have in " general been conducted for individual watersheds. Moderate or large scale weather modification opera- tions for larger sub-areas within the Colorado Hiver I drainage, however, will affect a number of such watersheds. Consequently, subdivisions of the Colorado River Basin for purposes of larger scale weather modification efforts should be defined for mountain ma s s i fs that should be the primary targets rather than individual drainage areas. Respective mountain ma s s i fs form the headwaters for a number of individual watersheds. The Colorado River Basin has, consequently, been divided into seven areas, ' (Figure 8 ) that form the primary targets for weather modification efforts to increase streamflow in the Basin. The seven sub-areas are: 1. San Juan Mountains 2. The central massif between the mainstem of the Colorado and the Gunnison Rivers 3. The Upper Basin of the Colorado River Basin above Kremmling 4. The White Mountains at the headwaters of the White River 5. The Park Range and headwaters of the Yampa River 6. The Uinta Mountains 7. The Wind River Range at the headwaters of the Green River The critical areas of snow collection on several of the mountain massifs in these sub-areas can be noted from Figures 9 - 13. These aerial photographs of the respective mountain massifs were taken May 22-28, 1968. Figures 9 and 10 show the San Juan massif ill the distance. Figure 9 shows the western portion of this range while Figure 10 shows the central portion. Figure 11 is a closer picture of the central portion of this range. By the May 22 date when tre photograph was taken the lower elevation snowfall had melted and the remaining snowfall essentially defines the area above 9,000 to 10,000 ft msl that produces most streamflow. This can be seen from Figure 12 which shows the streamflow measured on Vallecito Creek which is fed primarily by snowmelt during late May, June, and early July in most years. This figure shows daily streamflow of 200 cfs or more had been recorded on only three days prior to the taking of the above aerial photo- graphs of the area on May 22. Following May 22, an additional 54 days had streamflow greater than 200 cfs, with the peak day reaching 1190 cfs on June 2, The snowmelt period in 1968 had a' very noticeable dip in the middle of the season due to a cold spell with minimum temperatures being 250 , 270 , and 280 , respectively, on the three days when the streamflow dropped below 500 cfs. Figure 13 shows the snowcover on the central Colorado River Massif; sub-area #2, from directly overhead. This photograph was also taken on 22 May 1968. Figure 14 shows the delineation of snowfall areas over another sub-ba::;in area, sub- area #5, the Park Range. This photograph was made on May 28, 1968. Figures 15 - 20 show a closer view of the wintertirre snowfall over these barriers and the base sites of three of the facilities for experimental program of weather modification. Figure 15 shows the CSU mountain laboratory (12,000 ft msl) atop Chalk Mountain in the central portion of the Colorado Rockies near Climax, sub- area #2. This is a fall season photograph. Figure 16 shows the same lab oratory under a winter- time regime. Figures 17 and 18, respectively, 27