Results of Operational Seeding over the Watershed of San Angelo, Texas

Authors

  • William L Woodley Woodley Weather Consultants Boulder, Colorado
  • Mark E Solak Atmospherics, Inc. Fresno, California

Abstract

An assessment of five years of warm-season cloud seeding over the watershed of San Angelo, Texas is presented. The goals of the seedings were the replenishment of surface reservoirs, channel dams and surface aquifers and increased precipitation over the residential areas to reduce demand for municipal water. It was recognized that increased rainfall also would benefit the farming and ranching communities. During the program a total of 125 kgm of AgI were expended during the course of 2,315 separate seeding events at cloud top or at cloud base. Most of the seedings took place within 30 n.mi. of San Angelo as intended, primarily to the west and southwest of the City. Assessment of the effect of seeding made use of target-control regressions that had been derived from historical rainfall records. Historical monthly precipitation data were accumulated for long-term rainfall stations within the target and outside to the west and to the south. The period of record was 1960 through 1984 inclusive. Six control stations and nine target stations were used in the analysis. The analysis, suggests a positive effect of seeding (i.e. more rainfall) in each of the five years. The probability of this happening by chance may be a low as 3%. An overall effect of seeding of about +17% for the target for all years of operation is indicated. Sensitivity testing supports the interpretation that seeding was responsible for a sizeable portion of this apparent increase in rainfall.

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Scientific Papers