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e e Agreement Number 2-AB-30-0011l0 Development of a Program Plan for a Precipitation Management Demonstration Activity June 15 through July 17, 1992 Phone conversations were held with representatives of each of the Colorado River Basin States between June 22 to July 14. They revealed general satisfaction with the experimental site selection processes as described in the document handed out during the June 8, 1992, Denver meeting. Accordingly, detailed experimental plans and budgets will be developed for both the Grand Mesa of Colorado and the Wasatch Plateau of Utah. A question of obvious interest in developing a winter orographic cloud seeding program is, "how much additional runoff can be expected for a given increase in snowpack?" The relationship between snowpack water equivalent and runoff is known to be complex, depending upon antecedent soil moisture, characteristics of individual watersheds, air temperatures and rains during the snowmelt period, and other factors. However, existing snowcourse and streamflow data can provide a first approximation of the runoff. that might be expected from snowfall enhancement. \ Snowpack-runoff relationships were investigated for several watersheds in the Upper Colorado River Basin. Fourteen high elevation watersheds were identified that had at least 15 years of streamflow measurements, essentially unmodified by upstream diversions or reservoirs. Total April through July runoff was compared with nearby April 1 snowcourse. observations for the available periods of record (16-30 years), using linear regression analysis. The least squares line of best fit to each watershed's data was used to estimate the incremental runoff for a 10 percent increase in the ~ snowpack. Runoff increases of about 8 percent were indicated for 3 watersheds on the south-facing slopes of the Uintas in Utah. Values were 6 and 8 percent, respectively, for 2 southwest-facing watersheds in the Wind River Range of Wyoming. Seven Colorado watersheds had runoff increases between 10-14 percent, and two watersheds on the Wasatch Plateau of Utah had increases of 19 and 21 percent, respectively. It appears that a 10 percent snowpack increase during an average winter will result in at least a 10 percent increase in spring and summer runoff at most locations. Some watersheds had less runoff, possibly due to increased evapotranspiration on their south-facing slopes. It is expected that the peculiarities of individual years may cause them to v8:ry significantly from these runoff estimates based on "average conditions." A study was conducted into the frequency of snowfall events on the Grand Mesa and Wasatch Plateau, using Soil Conservation Service SNOTEL data available the past decade. It appears that the frequency of daily snowfalls, and the April 1 snowpack water equivalent, are similar for the two potential experimental areas. During an average winter (November through March), 35-40 percent of all days have measurable snowfall on both mountain barriers, resulting in April 1 snow water equivalents near 20 inches. A literature review was started to compare SLW (supercooled liquid water) information between the Grand Mesa and Wasatch Plateau. The availability of SLW is essential for cloud seeding potential. Microwave radiometer SL W observations were made on or near both locations, but for limited periods. Original data from the Grand Mesa, collected by Reclamation in 1983-1985, are being examined. It is hoped that SL W flux estimates can be made to compare with similar estimates published in final reports for the Tushur Mountains of southern Utah.