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<br />EXECUTIVE SUMMARY <br /> <br />The Denver Water Board (DWB) contracted with a commercial company to seed <br />orographic (mountain-induced) clouds over a large mountain region in central Colorado <br />during the 2002/03 winter. It was hoped that seeding with a network of ground-based <br />sHver iodide (AgI) generators would enhance the mountain snowpack. Subsequent spring <br />and summer stream flows could then be expected to increase over what would have been <br />the case in the absence of seeding. Cloud seeding was attempted because prolonged severe <br />drought resulted in many water storage reservoirs being well below capacity. <br /> <br />A brief overview of winter orographic cloud seeding status is presented along with <br />discussion of the limitations of statistical evaluation of non-randomized operational <br />projects, which attempt to seed all suitable storms. Supercooled liquid water cloud, the <br />necessary "rdw material" for cloud seeding to be effective, has been shown to be abundant <br />over Colorado and other western states during the winter. The major problem with <br />ground-based AgI seeding of winter orographic clouds has been and remains the transport <br />and dispersion of adequate concentrations of Agl into the supercooled portions of the <br />clouds at temperatures where the AgI nuclei are active as ice-forming agents. Evidence is <br />reviewed from some long-term operational projects, which demonstrates that adequate Agl <br />targeting was infrequent in these programs. Some plume tracking investigations have <br />shown that routine good targeting can be achieved primarily when the release points for <br />the seeding agents are located well up the windward slopes of mountain barriers. <br /> <br />This report discusses results of a "first look" physical evaluation done under <br />contract to the DWB. The approach used trace chemical analyses of seasonal snowpack <br />samples. End of seeding season snow pits were dug at ten target and three presumed <br />upwind control locations. The entire seasonal snowpack was carefully sampled with <br />special clean tools and each snow sample was analyzed for silver concentration by an <br />independent laboratory with long experience in this methodology. Finding silver levels <br />above natural background concentrations, near 5 parts per trillion, does not prove that <br />seeding produced more snowfall. Scavenging by natural snow and other mechanisms can <br />bring silver from the silver iodide (AgI) seeding agent-plumes to the surface. However, the <br />lack of silver enhancement in the snowpack meaDS that the Agl plumes were Dot routinely <br />transported over the sampling location. Such a finding indicates failure to routinely seed <br />the intended cloud regions. Although trace silver analyses of the seasonal snowpack cannot <br />by itself demonstrate cloud seeding success, it can show whether AgI seeding plumes were <br />usually transported over intended "target area" mountain barriers. <br /> <br />Results of silver-in-snow analyses over the intended DWB target area showed that <br />only one of ten target stations had silver concentrations in the expected range for effedive <br />seeding. Two other target stations had Agl-produced silver enhancements by about a <br />factor of 2-3 above natural background. Such increases are generally considered too low <br />for meaningful snowfall augmentation. One of the three control locations also had <br />enhanced silver, suggesting mistargeting unless the sample was contaminated in some way. <br /> <br />2 <br /> <br />