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<br />A. Executive Summary <br /> <br />Operational weather modification (cloud seeding) has been conducted in California since the <br />early 19505 - one of the longest records of seeding in the world. Cloud seeding has been <br />done in many arcas, but the most continuous programs ha....e been in winter over the Sierra <br />Nevada. This seeding has been intended to augment snowfall and snowpack. The additional <br />snowpack melts and runs olT. providing more water for various uses such as hydroelectric <br />po\\lcr, agriculture, municipal and industrial needs, recreation, and endangered species <br />habitat. A recent study (Appendix A) estimates thai cloud seeding increased water supplies <br />in seeded Sierra watersheds by morc than 5% until about 1990. when seeding etTectiveness <br />began to declinc. The reasons for this decline are unclear, but may bc associated with <br />atmospheric warming and/or changes in seeding methods. <br /> <br />Seeding of mountain clouds in winter is the most scientifically credible form of intentional <br />large-scale weather modification. This strong conclusion is supported by several professional <br />scientific organizations, including the American Meteorological Society and the World <br />Meteorological Organization. The conclusion is based upon statistical evidence that such <br />seeding, if properly designed and conducted, can augment seasonal precipitation by about <br />10%. This percentage may serve as a goal for California's seeding programs. This paper <br />describes the following characteristics of winter '\leather modification: <br /> <br />. Its relevancc and need in California <br />. Its history and current state <br />. Its environmental and health impacts <br />. Its downwind effects on precipitation <br />. Its costs and potential benefits, relative to other water augmentation technologies <br /> <br />Past research has demonstrated that cloud seeding poses minimal environmental and health <br />risks. and that there is almost no evidence for decreases in precipitation downwind of seeding <br />target areas. The principal conclusions of this paper arc that: <br /> <br />. Population growth in California will cause ,...ater demands to regularly outstrip water <br />supplies in the near future, especially during inevitable droughts. Therc is evidence <br />that thc current period of atmospheric warming anJJor air pollution may be <br />decreasing natural snowfall and therefore fresh water supplies. Unless steps are <br />taken. these situations will exacerbate conflicts and instigate litigation over those <br />supplies. Cloud seeding is much less expensin Ihall olher \,;aler augmenlation <br />lechnologies and has large benefiHo-cosl ralios. Therefore seeding is an <br />allraclive oplion 10 help allf~\.iale water supply problems. <br />. Applied research is needed 10 optimize Ihe effecli..-eness of operalional seedin~ <br />programs in Ihe slate. This applied research would address the reasons for the <br />apparent reccnt decline in cloud seeding elliciency and. once those reasons arc <br />identified, would investigate ways to optimize seeding practices, The approach will <br />survey the latest scientific advances in cloud physics. remote sensing, atmospheric <br />science. seeding technologies and evaluation strategies. and then recommend the best <br />courses of action to maximize the contribution of operational cloud seeding programs <br />