In mid-February 2021, Bloomington, Indiana was hit by a winter snowstorm that dropped several inches of white, fluffy snow around town. Schools and businesses closed. However, as other people were wondering “How will I get my car out of the driveway?”, I caught myself wondering “Will it rain soon?” What? Why?
Rain-on-snow melt events have been receiving a lot of attention in scientific articles I have been reading. Aptly named, these events occur when a warm rain falls on an existing snowpack. Along with other sources of heat, this causes the snow to melt. The meltwater and rainwater from this event can then cause extreme floods. Counterintuitively, rain-on-snow melt can also lead to a decrease in water availability during the summer, since the meltwater is not stored in the landscape or groundwater. Rain-on-snow melt events occur in temperate, high-latitude, and mountainous regions around the globe.
One reason these phenomena have been getting attention is their significance. Historically, seven of every ten extreme floods in the United States have had some contribution from rain-on-snow melt1. Rain-on-snow melt also contributed to Canada’s most expensive natural disaster: a 2013 flood in the province of Alberta that caused over $6 billion in recovery and damages to infrastructure and property2. In the Great Lakes Basin, which includes northern Indiana, rain-on-snow melt is included in 25% of the most severe snowmelt events3. However, rain-on-snow melt can also have its benefits. For instance, it has a cooling effect on stream temperatures4, which could be beneficial to aquatic life like trout that often prefer cold waters.
Another reason that rain-on-snow melt events are getting attention is their vulnerability to climate change. Following rising air temperatures and changing precipitation patterns, the frequency of these events in the Great Lakes Basin has decreased by over one-third in the past half-century5, echoing a general decline throughout North America6. However, there are other parts of the globe, such as the Balkan regions of Europe and Alaska, where they are increasing in frequency7,8. These shifting patterns could impact winter flooding, summer water supplies, and stream temperatures in ways that are not yet fully understood. Thus, it is important that assessments of water resources and climate change incorporate the changing nature of rain-on-snow melt events.
At Indiana University Bloomington, the Ficklin Hydroclimatology Lab is investigating the impacts of rain-on-snow melt and its changing occurrence in a warming climate. We are using computer simulations, along with data of climate and the flow of water through the landscape, to better understand how these events affect winter floods and summer water availability. How will rain-on-snow patterns in the Great Lakes Basin be affected by future climates? Can we improve the performance of computer simulations of climate change impacts to rivers and streams when we include them?
To answer these questions, we use a modified computer program for the management of freshwater resources known as the Soil and Water Assessment Tool. This program uses data about local climate, landcover, soils, and elevations to simulate stream characteristics, including streamflow, water quality, and water temperature. We are finding that simulations including rain-on-snow melt perform better than those that do not include it. This is particularly noticeable when investigating how winter floods, summer low streamflows (a.k.a. “hydrological droughts”), and snowpacks are simulated.
We are also finding that rain-on-snow melt can have substantial effects on streams in the Great Lakes Basin. Preliminary results for this analysis were presented at the American Geophysical Union Fall Meeting (a large scientific conference) in December 2020. We showed that rain-on-snow melt could increase the intensity of winter floods by 39%. This could be bad for keeping your feet dry! The results also showed that summer low streamflows could be 45% lower after large rain-on-snow melt events occur. Because water quickly leaves a river system during rapid snowmelt, there is less water stored as groundwater, which feeds streams during the summer. So…about those water supplies? In addition to the academic impacts of this research, these findings suggest that rain-on-snow melt can have crucial implications for ecosystems, communities, and economies depending on freshwater resources in a changing climate.
When it snows, keep an eye out for the next big rainstorm and see the exciting way it affects the creeks.
1 Li D, Lettenmaier DP, Margulis SA, Andreadis K (2019) The Role of Rain-on-Snow in Flooding Over the Conterminous United States. Water Resour Res 55:8492–8513. https://doi.org/10.1029/2019WR024950
2 Pomeroy JW, Stewart RE, Whitfield PH (2016) The 2013 flood event in the Bow and Oldman River basins: causes, assessment and damages. Can Water Resour J 41:105–117. https://doi.org/10.1080/07011784.2015.1089190
3 Suriano ZJ (2020) Synoptic and meteorological conditions during extreme snow cover ablation events in the Great Lakes Basin. Hydrol Process 34:1949–1965. https://doi.org/10.1002/hyp.13705
4 Leach JA, Moore RD (2014) Winter stream temperature in the rain-on-snow zone of the Pacific Northwest: Influences of hillslope runoff and transient snow cover. Hydrol Earth Syst Sci 18:819–838. https://doi.org/10.5194/hess-18-819-2014
5 Suriano ZJ, Leathers DJ (2018) Great lakes basin snow-cover ablation and synoptic-scale circulation. J Appl Meteorol Climatol 57:1497–1510. https://doi.org/10.1175/JAMC-D-17-0297.1
6 Il Jeong D, Sushama L (2018) Rain-on-snow events over North America based on two Canadian regional climate models. Clim Dyn 50:303–316. https://doi.org/10.1007/s00382-017-3609-x
7 Pan CG, Kirchner PB, Kimball JS, et al (2018) Rain-on-snow events in Alaska, their frequency and distribution from satellite observations. Environ Res Lett 13:1–15. https://doi.org/10.1088/1748-9326/aac9d3
8 Sezen C, Šraj M, Medved A, Bezak N (2020) Investigation of rain-on-snow floods under climate change. Appl Sci 10:1–12. https://doi.org/10.3390/app10041242