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oon Je The CREST will consist of physical and statistical investigations concurrently conducted at two sites in the Basin_ Recommended sites are the Grand Mesa of west-central Colorado and the Wasatch Plateau of central Utah. Final site selection will be made only after appropriate National Environmental Policy Act and public involvement processes are completed. These two mountainous areas typify many mountains in the Upper Colorado Basin, and have characteristics that make them especially suitable for experimentation. Their lack of isolated peaks allows low-level aircraft sampling during storms, which is essential for monitoring cloud responses to seeding. Both areas have highways and trails that provide access to ground instrumentation sites needed for direct observations (e.g_, snowfall measurements) and remote sensing (e.g., radar monitoring). In addition, these areas have been operationally seeded in recent winters with support from local water user groups. Recent scientific investigations have been conducted in each area which documented the frequent existence of apparently seedable clouds. The CREST will be conducted in two phases with associated transferability studies. Phase I will concentrate on collecting physical evidence to demonstrate that seeding can increase snowfall during portions of many winter storms. Phase II will concentrate on collecting the statistical and physical evidence necessary to quantify the seasonal snowpack increases and resulting streamflow. Transferability studies will assist, after completion of the CREST, in transferring the results from the two experimental areas throughout the Basin. Phase I will consist of a series of "direct detection" experiments that use new instrumentation to monitor key links in the chain of physical events from the release of seeding material to precipitation reaching the ground_ Each direct detection experiment will be brief(4-6 h) and will target a small well-instrumented area. A combination of direct and remote sensing approaches will observe the transport and dispersion of the seeding agents and resulting ice crystals, and will follow resulting snowfall to the surface. Supporting measurements will document the seeding methods and atmospheric conditions for which seeding is effective and those for which it is ineffective. Starting in Phase I and continuing throughout the program, a sophisticated, physically based numerical model will be used in conjunction with the observations to investigate mountain airflow and cloud and precipitation processes. The combination of comprehensive physical observations from the seeding experiments, and state-of-the-art numerical modeling, will provide rapid progress in understanding seeding responses for a wide range of storm conditions. Phase II, a statistical seeding experiment for each experimental area, will be carried out during the final four winters. Its design will be based upon results obtained from the direct detection seeding experiments and numerical modeling. The purpose of the statistical experiments will be to document the magnitude of seasonal snowfall increases from seeding a large target area_ A major physical monitoring effort will continue throughout Phase II to ensure that the statistical experiments are properly designed and conducted. Special observations and numerical modeling simulations will be accomplished for six other mountain regions in the Basin during the CREST. These investigations will concentrate on the availability of SLW-the "raw material" needed for seeding to be effective, and on proper targeting of seeding material to convert excess supercooled liquid water into snowfall. The results Dfthis work will expedite transfer of cloud seeding technology throughout the Basin. xiv