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<br />windward slope, substantially reduces targeting unccrtainties40. In mountain ranges with <br />extensive wilderness areas, siting of thosc devices can be problematic because they are not <br />allo\l.'cd in such areas. <br /> <br />Knowledge of cloud seeding will continue to advance through basic and applied research, and <br />through seeding or related hydrometeorological cxperiments. California can benefit from <br />ongoing research projects. particularly those being conductcd in the state. An excellent example <br />is the Hvdrometeorological Testbed (flMT) program, expected to be conducted in the American <br />River Basin from 2006-201141. This basin overlaps or is near to existing weather modification <br />programs in the Upper American, Tahoe-Tmckee, Carson-Walker, and Upper Mokelumne <br />Basins. The IIMT program will dcploy transportable and mobile scanning precipitation radars, <br />wind profiling radars, precipitation profiling radars, and GPS sensors for measuring precipitable <br />water vapor. Additional instmments will include precipitation gauges. raindrop disdromcters, <br />surface meteorological stations, soil moisture/temperature probes. radiosondcs and stream levcl <br />loggers. The IIMT will provide a wealth of data important to weather modification in California, <br />and it is advisable that futurc cfforts to optimize operational seeding programs establish a <br />collaborativc and synergistic data exchange with the IIMT program and others like it. The <br />IIMT, however, \....ill not involve cloud seeding and so cannot provide answers to all the <br />remaining questions facing weather modification in California and elsewhcrc. <br /> <br />b) Are there any adverse impacts? <br /> <br />Questions about potential unintended impacts from cloud seeding have been raised and <br />addressed throughout the history of weather moditication. Common concerns are: I) dowm....ind <br />elTects, i.e., enhancing precipitation in one area at the expense of those downwind ("Robbing <br />Peter to pay Paul" or "cloud rustling"), 2) long-tenn environmental or health efrccts of seeding <br />matcrials. and 3) consequences of additional snow. As for I), cvidence docs not show that <br />secding clouds with silver iodide causes a decrease in downwind precipitation; in fact, <br />sometimes there may be an incrcase as far as 100 miles do\\'nwind of the target area:. The <br />amount of atmospheric moisture passing over a mountain barner that is converted to <br />precipitation is usually 10% or less of the total water budget. If this natural precipitation is <br />increased 10% by cloud seeding, only I % of the original atmospheric moisture supply is <br />depleted by seeding. Moreover, winter cloud seeding is done on the upwind side of mountain <br />ranges. These clouds usually dissipate on the dO\\!l1\vind or lee side of the range, a natural elTcct <br />called the "rain shadow." This is \\'hy areas downwind of mountains. like Eastern Colorado and <br />Nevada. arc much drier than upwind areas. So the atmospheric moisture supply on the <br />downwind side of mountain ranges will not likely precipitate anyway (unless aided by weather <br />modilication). <br /> <br />Regarding concern 2), Reclamation extensively studied environmental and health impacts42:4J;4-:. <br />The toxicity of silver and silver compounds (from silver iodide) was shown to be of low order. <br />According to Reclamation. the small amounts of silver used in cloud seeding arc 100 times less <br /> <br />16 <br />