Flood runoff in relation to water vapor transport by atmospheric rivers over the western United States, 1949–2015

Atmospheric rivers (ARs) have a significant role in generating floods across the western United States. We analyze daily streamflow for water years 1949 to 2015 from 5,477 gages in relation to water vapor transport by ARs using a 6 h chronology resolved to 2.5° latitude and longitude. The probability that an AR will generate 50 mm/d of runoff in a river on the Pacific Coast increases from 12% when daily mean water vapor transport, DVT, is greater than 300 kg m⁻¹ s⁻¹ to 54% when DVT > 600 kg m⁻¹ s⁻¹. Extreme runoff, represented by the 99th quantile of daily values, doubles from 80 mm/d at DVT = 300 kg m⁻¹ s⁻¹ to 160 mm/d at DVT = 500 kg m⁻¹ s⁻¹. Forecasts and predictions of water vapor transport by atmospheric rivers can support flood risk assessment and estimates of future flood frequencies and magnitude in the western United States.