Flooding caused by rain falling on snowpack could more than double by the end of this century in some areas of the western U.S. and Canada due to climate change, according to new research from 抖阴旅行射 Boulder and the National Center for Atmospheric Research (NCAR).
The greatest flood risk increases are projected for the Sierra Nevada, the Colorado River headwaters and the Canadian Rocky Mountains鈥攑laces where residents are no strangers to flood concerns. Conversely, lower elevations in coastal regions of California, Oregon, Washington and maritime British Columbia could see decreases in rain-on-snow flood risk.
The findings were published today in the journal听Nature Climate Change.
Rain-on-snow events vary widely in timing and scale but can cause costly and damaging flooding as rapid snowmelt triggered by heavy and prolonged rainfall converge in a cascade that can overwhelm downstream rivers and reservoirs. In 2017, California鈥檚 Oroville Dam nearly failed catastrophically due to such an event, leading to the evacuation of 188,000 people and $1 billion in infrastructure damages.
鈥淩ain-on-snow events can be intense and dangerous in mountainous areas, but they are still relatively poorly understood,鈥� said Keith Musselman, lead author of the study and a research associate at 抖阴旅行射 Boulder鈥檚听. 鈥淲e can infer a little bit from streamflow, but we want to get better measurements and model more of the variables involved.鈥�
To study the past, present and potential future of rain-on-snow events, the researchers turned to a state-of-the-art weather modeling dataset developed at NCAR. Known as CONUS 1, the dataset contains weather simulations across the continental U.S. in the current climate and a warmer future based on projected climate trends. The enormous data trove鈥攚hich took NCAR鈥檚 Yellowstone supercomputer more than a year to compile鈥攐ffers unprecedented detail and resolution.
鈥淭his high-res dataset allows us to resolve mountains in granular fashion and examine the factors that combine to melt the snowpack when a warm storm comes in and hits cold mountains like the Sierra,鈥� Musselman said.
The authors found that in a warmer climate, less frequent snow-cover at lower elevations would decrease the risk for rain-on-snow flood events in areas like the U.S. Pacific Northwest. By contrast, at higher elevations where winter snow will still accumulate despite climate warming (such as in the High Sierra and much of the Rocky Mountains), rain-on-snow events could become more frequent due to increased rainfall that might once have fallen as snow. The events will also become more intense.
The rain and melt produced during rain-on-snow events is projected to increase for a majority of western North American river basins as rain rather than snow affects more mountain watersheds, increasing the corresponding flood risk by as much as 200 percent in localized areas and potentially straining existing flood control infrastructure.
鈥淲e were surprised at how big some of the projected changes were,鈥� Musselman said. 鈥淲e didn鈥檛 expect to see huge percentage increases in places that already have rain-on-snow flooding.鈥�
The findings represent an important first step toward better understanding rain-on-snow flood risk in the context of anthropogenic climate change, which could significantly shift the timing and extent of future flood regimes.听
The researchers hope that continued investment in snowpack monitoring networks and efforts such as NASA鈥檚 Airborne Snow Observatory will provide additional ground information, allowing hydrologists and climate scientists to verify their models against observations and better inform flood risk assessment now and in the future.
The study was co-authored by NCAR researchers Flavio Lehner, Kyoko Ikeda, Martyn Clark, Andreas Prein, Changhai Liu, Mike Barlage and Roy Rasmussen.听NCAR is sponsored by the National Science Foundation.
