Floods due to Atmospheric Rivers along the U.S. West Coast: The Role of Antecedent Soil Moisture in a Warming Climate

Abstract

Precipitation extremes are projected to become more frequent along the U.S. West Coast due to increased atmospheric river (AR) activity, but the frequency of less intense precipitation events may decrease. Antecedent soil moisture (ASM) conditions can have a large impact on flood responses, especially if prestorm precipitation decreases. Taken together with increased antecedent evaporative demand due to warming, this would result in reduced soil moisture at the onset of extreme precipitation events. We examine the impact of ASM on AR-related floods in a warming climate in three basins that form a transect along the U.S. Pacific Coast: the Chehalis River basin in Washington, the Russian River basin in Northern California, and the Santa Margarita River basin in Southern California. We ran the Distributed Hydrology Soil Vegetation Model (DHSVM) over the three river basins using forcings downscaled from 10 global climate models (GCMs). We examined the dynamic role of ASM by comparing the changes in the largest 50, 100, and 150 extreme events in two periods, 1951–2000 and 2050–99. In the Chehalis basin, the projected fraction of AR-related extreme discharge events slightly decreases. In the Russian basin, this fraction increases, however, and more substantially so in the Santa Margarita basin. This is due to increases in AR-related extreme precipitation events, as well as the fact that the relationship of extreme precipitation to extreme discharge is strengthened by projected increases in year-to-year volatility of annual precipitation in California, which increases the likelihood of concurrent occurrence of large storms and wet ASM conditions.

Uncovering Flooding Mechanisms Across the Contiguous United States Through Interpretive Deep Learning on Representative Catchments

“…Taking the station located in the northwesternmost point in Figure 8a as an example, the recent rainfall-induced and historical rainfall-induced floods account for 56.6% and 43.4% of all the identified flow peaks, respectively. This finding is consistent with Cao et al (2020), who recognized that floods in the basin were associated with large storms and soil moisture excess resulting from sustained rainfall.…”

Assessing compound flooding potential with multivariate statistical models in a complex estuarine system under data constraints

“…The threshold should be high enough so that selected events can be considered extremes (to not violate the asymptotic justification of the extreme value model) while also obtaining enough events to derive robust distribution parameters to reduce variance. Previous studies used thresholds resulting in approximately 3-5 events per year (e.g., Cao et al, 2020;Serafin & Ruggiero, 2014). POT exceedances must be declustered to ensure that the selected events abide by the IID rule (Davison & Smith, 1990).…”

A soil moisture monitoring network to assess controls on runoff generation during atmospheric river events

“…These fluctuations are site‐specific, which reflects the value of dense soil moisture networks that capture a range of soil moisture responses during periods of frequent AR activity. During such periods, precipitation is a stronger controller of soil moisture behaviour than evapotranspiration is (Cao et al, 2020), where early soil moisture recession characteristics shortly following the events set the antecedent conditions of the subsequent events.…”