Technical Reports Document Library

North American Weather Consultants completed its second weather modification program for the Edwards Underground Water District. The program ran from April 1, 1986 to September 30, 1986. The program was based in Uvalde, Texas where the weather radar, aircraft, and program director resided.

Seeding operations were performed on days and used a total of 19,368 grams of silver iodide. A total of 85.4 aircraft hours and 981 weather radar hours were logged during the program.

The project experienced dry conditions during July and the first half of August. Operational potential was further reduced by seeding suspensions due to excessive soil moisture content, as June was a wet month.

No evaluation of seeding effectiveness has been attempted in this operations report. A target-control type evaluation is possible, although the precipitation data for such an evaluation is delayed several months until it becomes available from the National Oceanic and Atmospheric Administration. Even if the data were available, the determination of a seeding effort for a one or two year project is difficult due to the large natural variability in precipitation coupled with the expectation of a relatively small seeding signature (10 -20 percent). Normally, several years are required to document such a seeding effect with any degree of statistical significance.

NAWC nonetheless believes, however, that this seeding program was effective. This belief comes from experience on similar projects in other areas such as one long term program that has been conducted by the Colorado River Municipal Water bistrict near Big Spring, Texas. Even though there were a number of seeding suspensions on the Edwards program during the summer of 1986, considerably more seeding was accomplished on the project than the first year of operations in 1985….