Watershed memory amplified the Oroville rain-on-snow flood of February 2017

Abstract

Mountain snowpacks are transitioning to experience less snowfall and more rainfall as the climate warms, creating more persistent low- to no-snow conditions. This precipitation shift also invites more high-impact rain-on-snow (ROS) events, which have historically yielded many of the largest and most damaging floods in the Western United States. One such sequence of events preceded the evacuation of 188,000 residents below the already-damaged Oroville Dam spillway in February 2017 in California's Sierra Nevada. Prior studies have suggested that snowmelt during ROS dramatically amplified reservoir inflows. However, we present evidence that snowmelt may have played a smaller role than previously documented (augmenting terrestrial water inputs by 21%). A series of hydrologic model experiments and subdaily snow, soil, streamflow, and hydrometeorological measurements demonstrate that direct, “passive” routing of rainfall through snow, and increasingly efficient runoff driven by gradually wetter soils can alternatively explain the extreme runoff totals. Our analysis reveals a crucial link between frequent winter storms and a basin's hydrologic response—emphasizing the role of soil moisture “memory” of within-season storms in priming impactful flood responses. Given the breadth in plausible ROS flood mechanisms, this case study underscores a need for more detailed measurements of soil moisture along with in-storm changes to snowpack structure, extent, energy balance, and precipitation phase to address ROS knowledge gaps associated with current observational limits. Sharpening our conceptual understanding of basin-scale ROS better equips water managers moving forward to appropriately classify threat levels, which are projected to increase throughout the mid-21st century.

Subseasonal Prediction of Impactful California Winter Weather in a Hybrid Dynamical‐Statistical Framework

“…Predictability of extreme precipitation in a probabilistic sense could also be explored. This is especially important as both extreme winter precipitation and early winter wildfires may become more common (Cayan et al, 2022;Gershunov et al, 2019), raising the possibility of compound extreme events such as short-duration high-intensity rainfall, which can cause devastating post-fire debris flows (Oakley et al, 2017(Oakley et al, , 2018a and landslides (Oakley et al, 2018b;Rengers et al, 2020), rain-on-snow flooding (Haleakala et al, 2023), as well as other precipitation patterns driving mass movements such as avalanches (Hatchett et al, 2017). Improving lead time to prepare for these types of events and likelihood of occurrence is crucial to prevent loss of life and mitigate damage to property (Oakley et al, 2023).…”

Anticipating How Rain-on-Snow Events Will Change through the 21st Century: Lessons from the 1997 New Year’s Flood Event

“…This is repeated from the top snowpack layer down to the bottom snowpack layer. Notably, ELM (and many other land surface models) do not represent some of the internal snowpack physics that occur during RoS or melt events that recent observational and modeling studies increasingly highlight the importance of (Brandt et al, 2022;Heggli et al, 2022;Haleakala et al, 2022;Katz et al, 2023). For example, preferential flow paths (or "flow fingers") within the snowpack rather than a uniform wetting front can influence the snowmelt dynamics (McGurk and Marsh, 1995;Brandt et al, 2022).…”

“…Compound extreme events can amplify flood hazards, especially to vulnerable communities, infrastructure, and ecosystems due to a combination of hydrometeorological drivers that overlap in space and/or time (Zscheischler et al, 2018). Rain-on-snow (RoS) events are one such type of compound extreme events (McCabe et al, 2007) that enhances flood risk, especially if the snowpack is actively contributing meltwater (Brandt et al, 2022) or if antecedent soils are saturated (Haleakala et al, 2022). Such situations often occur in maritime mountains such as the Sierra Nevada in the WUS (Guan et al, 2016).…”

Increasing risk of dam failures in the United States due to compound risk of rainfall clusters as climate changes

“…Since 2000, there have been reports of 630 dam failures in the contiguous United States (CONUS) attributed to hydrologic factors 5 . In February 2017, after a series of storms capped by an atmospheric river landfall, the Oroville Dam in California, the tallest dam in the US, faced a spillway failure and came dangerously close to overtopping [6][7] . The conventional dam design criteria require the spillway to be designed to handle the "inflow design flood", which is usually identified as the "Probably Maximum flood" (PMF) using the "Probable Maximum Precipitation" (PMP) 8 .…”