A Stable Isotopic Profile of Atmospheric River Precipitation Impacting the Sacramento Valley

Atmospheric river storm systems (ARs) are an integral part of California's hydrologic cycle. These systems provide up to 60% of the state's total precipitation annually, but extreme AR events can pose a significant risk to residents of low-lying flood prone areas such as the Sacramento Valley. Greater understanding of atmospheric river dynamics is vital to improving both water resource management and hazard mitigation planning in California.

While much is known about global-scale AR characteristics, few studies have focused on smaller-scale, more localized behaviors. Fewer still have investigated the use of isotopic tracers as a means of identifying and tracking atmospheric river precipitation. This study compared δ18O values of AR and non-AR precipitation collected along two transects of Northern California's Sacramento Valley to establish a local isotopic lapse rate for atmospheric river precipitation.

Precipitation was collected at 26 sites along two roughly 140-mile transects stretching from the coast to the Northern Sierras for the entirety of the 2018-2019 storm season, with the assistance of CoCoRaHS citizen science volunteers. Storms were categorized as either AR or non-AR events using a combination of satellite imagery, integrated water vapor (IWV)/wind speed and integrated vapor transport (IVT) threshold criteria. Samples were analysed using an LGR Liquid Water Isotope Analyzer, and results plotted against site-specific spatial data to calculate an isotopic lapse rate relative to elevation and distance from the ocean.

Preliminary analysis of a subset of samples revealed a small but statistically significant difference in mean δ18O values between the AR and non-AR groups. Elevation was shown to exert a slightly greater influence on δ18O values than distance. Final results of this study will be used to create a local isotopic profile for potential use in regional hydrologic and paleoreconstruction applications.