A Brief History of Evaluations Performed on the Operational Kings River Winter Orographic Cloud Seeding Program

Abstract

Abstract: An operational winter orographic cloud seeding program has been conducted in the Kings River Drainage located in the southern Sierra Nevada Mountains of California most winter seasons since water year 1955. Both ground based and airborne seeding modes have been used to disperse silver iodide into naturally occurring winter storms. Several evaluations have been performed to estimate the effectiveness of this seeding program. These evaluations have considered different seeded time periods dependent upon when the evaluation work was published. Since the program has not been randomized, these evaluations have all been based upon the historical target/control technique. Most of these evaluations have used annual streamflow as the response variable. One recent analysis was based upon April 1st snow water content. Control sites were selected from the Yosemite National Park region, an unseeded area, and southern Sierra Nevada sites located south of the Kings River Drainage. Linear regression equations were developed relating the control and target areas annual streamflow values during historical periods without any seeding. High correlations were obtained with r2 values ranging from 0.90 to 0.98. Both linear and multi-linear equations were developed for April 1st snow water contents. High correlations were obtained with r2 values ranging from 0.91 to 0.93. When these equations were used to predict the amount of natural streamflow or snow water contents during seeded years and then compared to the observed values during the seeded years, the average estimated increases in annual streamflow ranged from +3.3 to +6.1%. Similarly, the estimated increases in April 1st snow water content ranged from +4.9% (linear regression) to +5.7% (multi-linear regression).