ABSTRACT:

This repository contains the data and code used to analyze the impact of antecedent soil moisture conditions on flooding caused by atmospheric rivers. Here, we analyze how antecedent soil moisture (ASM) conditions contribute to variability in flood peaks during AR events and how that changes across climatic and physiographic regimes in 122 U.S. West Coast watersheds. We identify a robust non-linear relationship between peak streamflow and ASM during ARs. The inflection point in this relationship represents a watershed-specific critical ASM threshold, above which peak streamflow is, on average, over three times larger. Wet ASM conditions amplify the impacts of more frequent but weaker, lower moisture transport storms, while dry ASM conditions attenuate the flooding that stronger, higher moisture transport events could otherwise cause. Our research shows that watersheds prone to ASM-influenced flooding have higher evaporative indices, 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 flood season, increasing flooding during wet periods and buffering flooding during dry periods. Lower snow fraction and shallower soils limit the antecedent water storage capacity of a watershed, contributing to greater sensitivity of flood peaks to ASM variability. Incorporating antecedent moisture condition thresholds into flood models in these regions prone to AR-influenced flooding during ARs could improve forecasts and decrease uncertainty.