A Reanalysis of the Wolf Creek Pass Cloud Seeding Experiment

Abstract

A six-season, randomized-by-season cloud seeding experiment consisting of three seeded seasons and three non-seeded seasons was conducted by Colorado State University (CSU) during the middle and late 1960's in the Wolf Creek Pass region of the San Juan Mountains of southwest Colorado. The results of the seeding have been reported in a series of papers as having produced statistically significant increases in precipitation at Wolf Creek Summit when the 500 mb temperature was ≥?23°C. Furthermore, it has been reported that increases in precipitation produced statistically significant increases in the runoffs from three target watersheds when compared to the runoffs from three control watersheds. In this paper the results of the Wolf Creek Pass Experiment (WCPE) are reexamined. It is shown that the three non-seeded seasons occurred during meteorological conditions which brought ?warm aloft? (500 mb temperatures ≥ ?23°C) storm days with unusually light precipitation over a wide region of Colorado, northern New Mexico, southern Utah and northern Arizona. This bias produced high values of seed/no-seed precipitation ratios at Wolf Creek Summit which led to the misperception of large increases in precipitation due to cloud seeding. It is also shown that nearly all central and southwest Colorado watersheds with similar exposures to the target watersheds for the WCPE had high runoffs during the three seeded seasons compared to the three control watersheds chosen. Hence, the increases in runoff reported from the three target watersheds were part of a large-scale pattern due to natural causes rather than to cloud seeding.