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<br />in the Colorado mountains failed to confirm the inverse relationship between aerosol <br />concentration and precipitation rate reported by Borys et al. (2003). <br /> <br />A recent study compared historic (1950-2002) daily precipitation and wind data in the <br />"polluted" Denver metro area and "pristine" sites along the Colorado Front Range north <br />of Denver, to identifY the possibility of precipitation suppression by air pollution. <br />Decreasing trends of upslope precipitation for elevated sites relative to upwind polluted <br />sites were found, without a statistically significant trend for a more pristine area. As this <br />trend has occurred during a period of industrialization and urbanization, it suggests that <br />anthropogenic air pollution has led to the suppression of orographic precipitation west of <br />the Front Range over the last half century (Jirak, Cotton, and Woodley, 2005). A similar <br />long-term study showed precipitation losses over topographical barriers downwind of <br />major coastal urban areas in California and in Israel that amount to 15% to 25% of the <br />annual precipitation (Givati and Rosenfeld 2004). <br /> <br />Legal Issues <br /> <br />To address potential flooding liability, all ongoing projects have suspension criteria <br />designed to stop seeding any time there is a flood threat. Also, the type of storms that <br />produce large floods are naturally efficient in processing moisture into rain so there is <br />little benefit to be gained from seeding. <br /> <br />Colorado and Utah have statutes addressing the issue of water augmented by weather <br />modification. Under Colorado law, a permit can be obtained to appropriate the right to <br />use surface water made available by seeding (Colorado Legislative Council, 1971). Utah <br />would allocate its use to the most senior water appropriator whose allocation was not <br />already filled by water naturally in the stream (Dewsnup and Jensen, 1977). California <br />law says that water gained from cloud seeding is treated the same as natural supply with <br />regard to water rights (DWR, 2004). <br /> <br />Conclusions on Effectiveness <br /> <br />With regard to uncertainties, the NRC (2003) concludes that future work should <br />emphasize the understanding of processes rather than on modification. Further, they <br />assert that the initiation of large scale operational WxMod programs would be premature <br />because many fundamental problems need to be answered frrst. The NRC indicates that <br />expanding existing programs, which are essentially uncontrolled experiments, will not <br />answer critical questions and that long-term (possibly decades) of patient investigations is <br />the optimum route to success. Their recommendation is a coordinated national program <br />to conduct sustained research. <br /> <br />The NRC further states that although there bave been some randomized experiments, <br />most could not provide evidence sufficient to reject the null hypothesis of no seeding <br />effect. A common conclusion is that there were indications of seeding effects based on <br />physical measurements, but the data were insufficient to reach statistical conclusions. <br />The NRC report concludes that there is ".. .no conclusive scientific proof of the efficacy <br /> <br />-21- <br />