ABSTRACT:

Climate change is projected to increase the frequency of extreme atmospheric river sequences and the likelihood of long-duration flooding in susceptible regions. At the same time, socially vulnerable communities often face disproportionate exposure to natural hazards, including those intensified by climate change. In this study, we simulate flooding in the high-elevation Truckee River watershed near Reno, Nevada, under a hypothetical future 30-day sequence of atmospheric rivers from the ARkStorm 2.0 scenario. We examine how flood hazard and socioeconomic exposure evolve over the course of the event and compare these daily conditions to those represented by more conventional static design floods. We find that flood duration changes the relationship between flood hazard and socioeconomic exposure. After the flood peak, streamflow and flooded area decline to levels associated with 5- and 30-year design floods, while socioeconomically vulnerable exposure remains comparable to that of a 125- to 175-year design flood. The remaining flooding during the flood recession becomes increasingly concentrated in densely populated urban areas. Socially vulnerable residents are disproportionately exposed to flooding throughout the event, and this disparity increases substantially among residents exposed for longer periods. Socially vulnerable residents were 5.6 times as likely to experience at least 10 days of flooding as non-socially vulnerable residents. Our results show that static design floods do not capture the accumulation of exposure or how disproportionate exposure evolves during a long-duration flood. Accounting for flood duration and social vulnerability could provide a more complete assessment of exposure from long-duration floods.

ABSTRACT:

Atmospheric rivers (ARs) are key drivers of regional water supply and flood hazard 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 landfalls 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 simple modification to the AR scale for flood hazards that incorporates antecedent moisture conditions. This modification close to doubles the scale’s correspondence with peak streamflow and increases the number of flood-generating ARs classified as hazardous by over 30%, raising AR flood detection rates to 81% in California and 64% in central Chile. These findings demonstrate that incorporating critical land surface conditions into hazard classification can enhance early-warning tools for communicating hazard likelihood.

See Webb, M. J. et al. Antecedent moisture enhances early warning of atmospheric river flood hazards. Nat Commun https://doi.org/10.1038/s41467-026-69286-3 (2026) doi:10.1038/s41467-026-69286-3.

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."