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Observed Impacts of Duration and Seasonality of Atmospheric-River Landfalls on Soil Moisture and Runoff in Coastal Northern California

Responds to the need for better forecasts of extreme precipitation and floods
Investigates unique hourly observations collected over six years near California’s Russian River
Fills key gaps limiting the prediction of ARs and, especially, their impacts by quantifying the duration of AR conditions and the role of duration in modulating hydrometeorological impacts
Shows precursor soil moisture conditions and their relationship to streamflow
Shows (on the basis of 91 well-observed events during 2004–10) that the passage of ARs over a coastal site lasted 20 h on average and that 12% of the AR events exceeded 30 h
Finds that differences in storm-total water vapor transport directed up the mountain slope contribute 74% of the variance in storm-total rainfall across the events and 61% of the variance in storm-total runoff volume
Finds that ARs with double the composite mean duration produced nearly 6 times greater peak streamflow and more than 7 times the storm-total runoff volume
Finds that when precursor soil moisture was less than 20%, even heavy rainfall did not lead to significant streamflow
States that predicting which AR events are likely to produce extreme impacts on precipitation and runoff requires accurate prediction of AR duration at landfall and observations of precursor soil moisture conditions

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