Development of hydrologically-based cloud seeding suspension criteria in the Western United States

Cloud seeding operations to enhance winter precipitation and augment water supply sources have been regular annual programs in Utah, Colorado, Wyoming, Nevada, Idaho, and California in the Western United States. The winter cloud seeding program generally runs from November to April and is continued until the snow water equivalent (SWE) reaches a specified monthly average SWE level. Cloud seeding operations are suspended if snowfall exceeds a given threshold. For example, cloud seeding operation in Utah is suspended when SWE reaches 200%, 180%, 160%, and 150% of the average SWE values on January 1, February 1, March 1, and April 1, respectively. Local hydrological factors such as reservoir levels, soil moisture content, and possible risks of flash flood are often evaluated while suspending the cloud seeding operations. The current SWE-based criteria have been used since the 1980s, however there is no supporting document or scientific study that describes how these indices were derived. Additionally, the same indices have been adopted for the entire state without considering the hydrological and metrological variability among the watersheds. This study presents an objective method for deriving SWE-based indices for cloud seeding suspension criteria. The revised suspension criterion were calculated based on statistical analyses that establish a relationship between the SWE and seasonal natural streamflow for different watersheds in Utah. The updated indices were derived by inverting the regression equation to determine the SWE value associated with the 95th percentile of the historical seasonal cumulative streamflow, and these values have been used in Utah since 2019. The revised suspension criterion are 17-30% higher in magnitude than those previously used, and the difference is even larger if the results are compared for particular river basins. The revised suspension criterion capture the spatial and temporal variability of hydro-meteorological conditions within and between the watersheds. The developed method is easy to replicate in other states and watersheds.

Automated summary

Cloud seeding operations in Western United States have been regular annual programs to enhance winter precipitation and water supply. The study proposes an objective method to derive SWE-based indices for cloud seeding suspension criteria, which have been successfully implemented in Utah. The revised suspension criterion are 17-30% higher in magnitude than previous ones, capturing the spatial and temporal variability of hydro-meteorological conditions within and between watersheds.