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• April 2009 Silverman 13 Vail Targets 1.GBH z OPT 3.GBO 4. MID 5.GUP s.GM- 7.PNY 8.TMW 50 1 43: S 20 10 s .1a - 1985 1900 1996 2000 2005 Water Year Fig. 3. Time evolution of the seeding effect(%change in streamflow) estimate decreases with increasing sample size. through physical and hydrological studies such See Table 2 for an indication of the 90 percent as silver iodide tracer experiments. confidence limits for the final year(2005)of each target's evaluation. (2) Of the 8 primary seeding targets in the Vail Basin,statistically significant increases in stream- It can be seen from Fig. 3 that there are no sig- ow due 6 to f eedin n 5 g hem Pt+6. % to nificant and/or abrupt changes in trend tut might be indicative of a significant change seeding ef- GPT, GBH and GUP); not quite statistically sig- festiveness and, in turn, a significant change in nificant seeding-induced increases in streamflow some aspect of the meteorology and/or seeding was found in 2 of them (MID and GBL); and no procedure that affected the seeding effective- seeding effect was found in one of them (TMW). nees. The maximum seeding effect of+28.8%occurred at Bighom Creek (GBH) and decreased rapidly 6. SUMMARY with increasing distance for seeding targets both northwest and southeast of GBH. An independent statistical evaluation of the Vail operational cloud seeding program over its pe- (3) The time evolution of the seeding effect on rind of operations from 1977 to 2005 was con- the Vail primary seeding targets suggests that dulled using ratio statistics and,in particular,the the seeding-induced changes in streamfiow were bias-adjusted regression ratio.The effect of seed- steady and consistent over time. Mg on eight(8)primary seeder targets in the Vail Basin was evaluated using the control that gives 7. REMARKS the most precise evaluation results possible with the available data.The following is a summary of It is emphasized that this study is an a posteriori the main findings of this evaluation study: evaluation of a non-randomized seeding opera- tion. In addition, this evaluation is an exploratory (1)The evaluation results suggests, as one pas- study that involves consideration of a multiplicity able explanation.that the dispersion of the silver of analyses, some of which are suggested by the iodide seeding agent tends to be narrowly fo- results of previous analyses. With such a large cused rather than uniformly distributed across all number of tests, a few am Nicely to yield signifi- the primary seeding targets. This and other pos- cant results purely by chance. In view of these • sable explanations need to be Investigated further considerations, the results of the evaluations in -Scientific Papers-