A Proposal for a Change in Winter Snowpack Suspension Criteria Used in the Conduct of Cloud Seeding Programs in Utah

Cloud seeding has long been recognized as a method of precipitation augmentation. In any given year, natural precipitation may range from exceptionally low to record highs. Clearly augmentation at low and average accumulations produces substantial water supply benefits and is the primary purpose for cloud seeding – to increase water supply. However, there needs to be clear criteria by which, in high accumulation years, cloud seeding is to be suspended due to the potential for snowmelt flooding. Years that have high snowpacks also have the highest potential for longer term, sustained high streamflow and flooding, years with lower snowpacks can and do produce flood events but normally require more extraordinary climatological phenomena to produce flooding and thus lower overall potential. As there are many and sundry causes for flooding and to be clear, the flood potential to be mitigated by snowpack cloud seeding suspension is specifically springtime snowmelt flooding/runoff. Other suspension criteria are used in the conduct of the Utah cloud seeding programs (e.g. no seeding during storms with high freezing levels that could produce winter flood events). Various other flooding mechanisms such as rain on frozen soil, urban flooding from impervious surfaces, flooding resulting from channel blockages, etc. would not be impacted by or mitigated by snowpack suspension criteria.

To address the snowmelt flooding issue, streamflow points were selected that were either unimpaired or minimized upstream management. The USDA Natural Resources Conservation Service SNOTEL stations were generally selected based on high elevation and geographic location relevant to the watershed being investigated. An attempt to correlate Snow Water Equivalent (SWE) index values to observed historical flood events was largely unsuccessful. The 95th percentile or approximately the 20-year recurrence interval was used for the index suspension criteria because it assures a robust water supply for the water year and yet is low enough to reduce flood potential as many municipalities utilize the 100-year recurrence for storm water design. (KEYWORDS: cloud seeding, SNOTEL, water supply, Utah, suspension criteria)