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An Independant Statistical Evaluation of the VAIL Operational Cloud Seeding Program

Bernard A. Silverman

Abstract


An independent target-control statistical evaluation of the Vail operational cloud seeding program over its period of operations from 1977 to 2005 was conducted using ratio statistics and, in particular, the bias-adjusted regression ratio. The water year (October-September) streamflow expressed in Acre-Feet (AF) served as the response variable in the evaluations. The effect of seeding on eight (8), closely-spaced sub-basins in the Vail watershed was evaluated using the controls that give the most precise evaluation results possible with the available data. Evidence for statistically significant seeding effects ranging from +6.3% to +28.8% was found for 5 of the 8 seeding targets. The maximum seeding effect is centered on Bighorn Creek (GBH) and decreases for targets both northwest and southeast of GBH. An analysis of the time evolution of the seeding effect suggests that the percent change in streamflow at each of the target sub-basins was about the same from water year to water year.

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