A Statistical Evaluation of the Kern River Operational Cloud Seeding Program

A target-control statistical evaluation of the KernRiver Basin operational cloud seeding program was conducted using ratio statistics. The cumulative effect of seeding from water year 1977 through water year 2006 was calculated in terms of confidence intervals because they provide information on the strength of the seeding effect whereas null hypothesis significance tests infer only whether there is any seeding effect at all. The effect of seeding on several targets in the Kern River Basin was evaluated using the controls that give the most precise evaluation results possible with the available data. Evidence for positive, statistically significant and cost effective seeding effects was found at all 3 target sites in the Kern River Basin that were evaluated with estimated increases instreamflow due to seeding ranging from +8.4% to +12.2%, depending on the target location. Physical studies that help explain the statistical results and that could lead to more cost-effective seeding operations are indicated.Pooling of the estimates of the seeding effects for the Kern River, Kings River and San Joaquin River Basin operational cloud seeding programs indicated that the common effect of seeding on the three River Basins is +6.4% with 90% confidence thatthe true effect of seeding is somewhere between +3.9% and +9.0%. The probability that the seeding effect is greater than 0% and 1% (threshold of cost-effectiveness) are both 100%. Thus, there is strong statistical evidence in support of the hypothesis that cloud seeding in the watersheds of the Southern Sierra Nevada Mountains is a cost-effective technology for increasing streamflow by significant and societally important quantities.