A Tale of Two Storms: Inter-Storm Variability of Stable Water Isotopes in a Solute Transport Model

Stable isotopic methods in hydroclimate monitoring are powerful for improving water resources management, but applications are limited, especially in semi-arid regions where such management is needed most. Here, we show that we can address shortcomings related to the lack of a seasonal signal using stable water isotopic signatures measured in precipitation over the East San Francisco Bay area, California, during two contrasting events sampled at more than 20 locations in the winter of 2023. The observed range in δ¹⁸O in the rain samples is similar for both storms. However, the distributions do not overlap—the mean air temperature and δ¹⁸O during Winter Storm Olive (February 2023) were 2°C and − 12‰, respectively, while a warm atmospheric river event (March 2023) had a mean temperature of 9°C and δ¹⁸O of −6‰, close to the long-term average δ¹⁸O measured in local precipitation. The Winter Storm showed expected trends in δ¹⁸O related to geography (i.e., lower with greater distance inland and elevation), while the atmospheric river δ¹⁸O pattern was more spatially uniform. We use hydrometric data from a gaged watershed in the study area and isotopic signatures of rain sampled during the two storm events and apply a solute transport model (StorAge selection) with a travel-time approach to examine predicted watershed responses and potential water tracing applications. In this virtual experiment, we find that event size exerts a strong control on the relative amounts of runoff versus pre-event water in the stream, while uncertainty in stream hydrograph separation is related to the degree of contrast between precipitation/runoff and pre-event water. Key to flood prediction, adaptation, and mitigation, especially in coastal urban areas, is knowledge of the contributing water sources and timing of stream flow. The strong contrast in stable isotopes between these two events, close in time and over the same area, illustrates the potential to use stable isotope signatures to track the transport and mixing of events through natural and engineered watersheds that are threatened by climate whiplash events.