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<br />conservatively estimated that all the California seeding projects generate an additional 4% or <br />300,000-400,000 acre-feet of water annually22. <br /> <br />The long period of record for California seeding projects has allowed a statistical analysis of <br />their effectiveness (Appendix A of Silverman). This analysis was based on streamflow data <br />from the Lake Almanor, Upper American. Mokelumne River, Kings River. Kaweah River, Kern <br />River. and Eastern Sierra operational cloud seeding programs. Silverman used the regression <br />ratio statistic:!), '....hich he adjusted for biases that can occur when operational data are compared <br />to historical records in an a IXJSterior; evaluation of non-randomized seeding programs. The <br />results of this analysis support the claim of seeding-indue cd increases in streaml10w of more than <br />5% in the Sierras. at least before 1990. <br /> <br />3. Declines in Seeding Effectiveness and Possible Causes <br /> <br />The aforementioned analysis (Appendix A) also found a disturbing decline in seeding <br />effectiveness since about 1990. The decline was more dramatic in watersheds of the southern <br />Sierra Nevada than those of the northern Sierra. The overall sccding elfects through 2003 in the <br />northern watersheds decreased. but were still positive and statistically significant. In some of the <br />southern watersheds, however, the positi....e seeding clfects found aner 1990 were diluted. and <br />there was no longer any statistical evidence of a seeding signal. The analysis further showed a <br />'....arming atmosphere since 1950. and speculated that this wanning might have contributed to the <br />declines in seeding effectiveness. In ....iew of the signi lieanec of these findings. further statistical <br />studies should be conducted using program-specific controls and additional targets in order to <br />eonfinn or revise. as necessary, the statistical results of this preliminary analysis. There is <br />considerable evidence that atmospheric warming will continllclO. Silver iodide is only clfective <br />at temperatures of about -SC'C or colder so atmospheric warming could be decreasing the <br />frequency of suitable clouds and, therefore, opportunitics for ground-based seeding. <br /> <br />Another possible cause of the apparent decline of seeding effectiveness is related to <br />anthro/x)xenic effects on clouds. A long-teml study:!-l showed precipitation losses over <br />topographic barriers downwind of major coastal urban areas in California amounting to /5rf)-- <br />25% of the annual precipitation. These losses occurred during the 20lh Century in increasingly <br />polluted areas. \.I,'hereas no such trends were observed in similar nearby pristine areas. The <br />authors later investigated2~ an '.orographic enhancement factor" in Israel (ratio of precipitation in <br />inland hilly areas from 500~1000 m in elevation. to that at upwind coasts and plains) from 1950- <br />2002. segregating seeded and non-seeded days. They found that. as in California. increasing air <br />pollution decreased orographic precipitation: the decreases were of such magnitude as to cancel <br />increases from cloud seeding. Physical evidcnce of cloud and aerosol changes induced by air <br />pollution downwind of urban areas has been documented by satellite~6;27 and aircraOZg <br />mcasurcments. See Figure 3 for an example of the satellite analyses in California. <br /> <br />9 <br />