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6. DESIGN OF PHYSICAL CLOUD SEEDING EXPERIMENTS FOR THE WASATCH PLATEAU . 6.1 Introduction Demonstration of a validated weather modification technology to enhance water supplies in the Sevier River Basin should be pursued in a two phase program. The first should be a "physical phase" followed by the second "statistical/physical phase", discussed in section 7. The purpose of the physical phase is to increase physical understanding of the important processes in winter clouds, and how they are affected by seeding. Seeding would be confmed to short-tenn (few hour) experiments aimed at producing measurable effects on clouds and snowfall over a limited but well-instrumented target area. Comprehensive physical experiments monitor the most important links in the chain of physical events following seeding. Such links include documentation that the seeding agent was reliably released, that the seeding agent was transported and dispersed into the SL W zone, that ice particles were created by the seeding material, and that artificially created ice crystals grew with time and settled to the surface as snow. Comprehensive physical experiments have been made increasingly practical by recently developed instrumentation. Reclamation scientists have participated in such experiments starting with the HIPLEX summer program in the late 1970's and, during the 1980's, in winter projects in California, Colorado, and Montana. While some of these experiments strongly indicated increased precipitation, others failed to do so, at least at the intended point target. In the latter cases, it often was possible to detennine plausible reasons for the apparent failure because of the comprehensive nature of the observations. That led to improved understanding and better experimental design. In addition to the comprehensive physical experiments, a series of physical experiments with more limited objectives would investigate specific processes. One example of a more limited physical experiment is investigation of the T &D of seeding agents. This can be accomplished with an instrumented aircraft and limited supporting observations (e.g., rawinsonde data). Some T&D experiments can be conducted when full stonn conditions do not exist, while the cloud base is still above the terrain pennitting very low-level sampling. Another example of a limited physical experiment is study of the concentration of ice particles created by release of a seeding agent under particular temperature/moisture/dispersion conditions. Again, such observations can be made primarily by an instrumented aircraft with no attempt to follow the seeding signature to the surrace. The purpose of both the comprehensive, and the more limited physical experiments is to significantly advance understanding of the physics involved in winter orographic storms. This is required before a statistical/physical randomized program can be properly designed. The purposes of the statistical/physical phase are to show that snowfall can be increased over a target area of significant size, and to quantify the multi winter magnitude of the increase. These purposes cannot be met by comprehensive physical experiments which are limited in time and space by their intensive observational requirements. The statistical experiment would rigorously test precisely stated hypotheses, using predetennined seeding and observational criteria, and confinnatory analysis. In addition, any statistical results must be physically plausible to be considered convincing. At the conclusion of the two phases a validated technology would exist for seeding winter clouds over the Sevier River Basin in an effective and efficient manner. The impacts of a scientifically demonstrated ability to enhance snowfall would be far reaching as the technology could be readily transferrable to other regions of Utah and the Rocky Mountains in general. The benefits and costs of conducting a properly designed operationaI program would be well known. Water interests would then be in a position to make 47