A Stable Isotopic Comparison of Atmospheric River and non-Atmospheric River Precipitation Impacting Northern California

Atmospheric river storm systems (ARs) are an integral component of California’s hydrologic cycle, providing an average of 40–60% of the state’s total annual precipitation in a mere handful of events per season. The remainder of California’s precipitation is provided by more common high-latitude (non-AR) storms that frequently form around the Gulf of Alaska. Both storm types can produce heavy rainfall, but the most extreme rainfall and flooding events occurring within the state are historically associated with AR activity. Modeled projections of the possible effects of climate change on AR behavior suggest greater interannual and intraseasonal variability and potentially enhanced intensity. This is of particular concern in northern California, where the steep topography and seasonal snowpack of the Sierra Nevada Range puts the low-lying Sacramento-San Joaquin Valley at risk for floods and extreme weather hazards that strong AR storms can trigger. Although much is known about global and regional-scale AR characteristics, less attention has focused on localized AR interactions with varied topography. This study represents the first steps towards employing a novel method, the analysis of oxygen (¹⁶O/¹⁸O) and hydrogen (H/deuterium) stable water isotopes, for tracking AR precipitation through the hydrologic cycle and complex terrain of northern California by determining an average isotopic range and local lapse rates for AR and non-AR storms impacting the area.

We compared the isotopic composition of AR and non-AR precipitation collected at 26 sites along two transects of Northern California with help from CoCoRaHS Network citizen science volunteers from October 2018 through April 2019. Storms sampled throughout the study period were categorized as AR or non-AR storms based on integrated water vapor and vapor flux criteria, satellite imagery, and back-trajectory analysis using the HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory) model. Comparisons of the two storm types found AR δ¹⁸O and δD values to be significantly more enriched in heavier oxygen and hydrogen isotopes than non-AR precipitation at eleven of the fourteen Transect B sites. The majority of AR precipitation δ¹⁸O values for the transect overall fell between -10‰ and -1‰, and non-AR values between -18‰ and -3‰. Additionally, isotopic lapse rates determined for the AR storms revealed that elevation explained more of the variability in precipitation isotope ratios than distance from the coast, with a 2‰/km decrease in δ¹⁸O moving up in elevation and corresponding ~0.03‰ decrease in δ¹⁸O per km moving inland away from the coast. By identifying a distinct isotopic range and lapse rates for AR precipitation, this information can benefit local-scale hydrologic monitoring, water resource management, and paleoclimate reconstruction efforts.