ABSTRACT:

Atmospheric rivers (ARs) are key drivers of regional water supply and flood risk in subtropical and mid-latitude regions, generating interwoven beneficial and hazardous impacts. The original AR scale, developed for early-warning communication, ranks ARs from 1 (“primarily beneficial”) to 5 (“primarily hazardous”) based on atmospheric vapor transport. However, the AR scale does not account for physical processes on the land surface that can strongly influence flood response. Analyzing over 70,000 AR events across 142 catchments in California and central Chile, here we show that runoff efficiency, primarily controlled by antecedent soil moisture, is the dominant source of peak streamflow variability not explained by the AR scale. Based on this insight, we present a modified AR scale for flood impacts that incorporates antecedent moisture conditions. This modification doubles the scale’s correspondence with peak streamflow and increases the number of floods classified as hazardous by over 25%, raising AR flood detection rates to 87% in California and 72% in central Chile. These findings demonstrate that incorporating critical land surface conditions into hazard classification can enhance early-warning tools for communicating not just hazard presence, but likely impact.

ABSTRACT:

This repository contains the data and code used to analyze the impact of antecedent soil moisture conditions on flooding caused by atmospheric rivers for our paper:

Webb, M. J., Albano, C. M., Harpold, A. A., Wagner, D. M., & Wilson, A. M. (2025). Wet Antecedent Soil Moisture Increases Atmospheric River Streamflow Magnitudes Non-Linearly. Journal of Hydrometeorology. https://doi.org/10.1175/JHM-D-24-0078.1

"In this study, we analyze how antecedent soil moisture (ASM) conditions contribute to variability in streamflow during atmospheric river (AR) events and how that changes across climatic regimes and physiography in 122 U.S. West Coast watersheds. We identify a robust non-linear relationship between streamflow and ASM during ARs in 89% of watersheds. The inflection point in this relationship represents a watershed-specific critical ASM threshold, above which event maximum streamflow is, on average, two to four and a half times larger. Wet ASM conditions amplify the hydrologic impacts of more frequent but weaker, lower moisture transport AR events, while dry ASM conditions attenuate the hydrologic impacts that stronger, higher moisture transport AR events could otherwise cause. Our research shows that watersheds prone to ASM-amplified streamflows have higher evaporation ratios, lower cold-season precipitation, lower snow-to-rain ratios, and shallower, clay-rich soils. Higher evaporation and lower precipitation lead to greater ASM variability during the cold season, increasing streamflow during wet periods and buffering streamflow during dry periods. Lower snow fraction and shallower soils limit the antecedent water storage capacity of a watershed, contributing to greater sensitivity of streamflow peaks to ASM variability. Incorporating ASM thresholds into hydrologic models in these regions prone to AR-amplified streamflow could improve forecasts and decrease uncertainty."