Target/Control Analyses for Santa Barbara County's Operational Winter Cloud Seeding Program
Keywords:cloud seeding operations, weather modification operations
An operational winter cloud seeding program has been conducted most winter seasons in the Santa Barbara, California area since 1981. This program has been sponsored by the Santa Barbara County Water Agency (SBCWA). There have typically been two target areas; the Upper Santa Ynez drainage above Cachuma Dam located in the eastern part of Santa Barbara County and the Twitchell Reservoir drainage (usually referred to as the Huasna-Alamo target area) located in the northern portion of Santa Barbara County and the southern portion of San Luis Obispo County. This operational program was implemented following the completion of the Santa Barbara II research program which provided indications of positive seeding effects some of which were statistically significant.
The operational program has typically used both airborne and ground based seeding modes to target “convection bands” as they approach the two target areas. In the fall of 2013, North American Weather Consultants (NAWC) was asked by the Santa Barbara County Water Agency if there might be a method applied to the ongoing, long-term operational cloud seeding program that could indicate the possible effectiveness of the program. Up to this point there had not been any attempts made to evaluate this program. NAWC offered to attempt an historical target/control evaluation of the program. After conducting a search for potential long-term target and control precipitation measurement sites that included quality control checks of these sites, three acceptable control sites and four acceptable target sites (two in each of the intended target areas) were identified. Linear and multiple-linear regression equations were developed for each of the target areas based upon these sites using historical periods without any cloud seeding in either the control or target areas. Good correlations were obtained between the control and target sites with r2 values ranging from 0.84 to 0.91.
When these regression equations were used to predict the amount of precipitation for the December-March period for the two target areas then compared to the actual amounts of precipitation the average results for all the seeded seasons were:
· Upper Santa Ynez Target Area: Estimated increases of 19% to 21% from the linear and multiple-linear equations.
· Huasna-Alamo Target Area: Estimated increases of 9% from both the linear and multiple-linear equations.