That snow is critical to water management in the western US is old news. We have long relied on snowpack in the West as a way to naturally store water for human use. In fact, most reservoirs and dams were designed and are operated around this snow: its accumulation in the winter, and its melting in the spring, when the bulk of mountain snow turns to water, finding its way into streams and rivers as snow “runoff.”
Yet despite the societal importance of snow, one team of researchers, after combing through the scientific literature, could find no detailed, systematic study accounting for how much runoff comes from snow across the entire western US. What’s more, this team was also unable to find an analysis of how this ratio of snow-derived runoff might change in the future as our climate continues to warm.
The team of researchers, led by Dongyue Li, recently tackled these problems in a paper published in Geophysical Research Letters.
Because of spotty coverage of snow measurement sites, Li and colleagues instead used a hydrological model that reliably simulates snowpack in the western US. The researchers also took advantage of simulations of snow accumulation, melt, and runoff to streams and rivers done as part of CIRC’s Integrated Scenarios project. From these simulations, Li and colleagues were able to estimate the ratio of snow-derived runoff to total runoff from rain as well as snow. The numbers they came up with are telling.
Snow-derived runoff accounts for 53% of total runoff across the western US, according to Li and colleagues’ findings. However, in mountainous areas, runoff from snow jumps higher still, accounting for 70% of total runoff on average. In the Cascades of Washington and Oregon, the snow runoff ratio goes even higher, accounting for 78% of total runoff. What this means for water management shows up in a related set of numbers.
The largest 21 reservoirs in the western US, making up over half of all reservoir storage for the region, receive 67% of their water from melting snowpack, according to Li and colleagues. And while that’s impressive, let’s put this in further context by considering another percentage: 37%. That is the percentage of precipitation that falls as snow over the entire western US. How can the 37% snowfall result in snow-derived runoff that makes up over half of total runoff? It turns out that snowfall is more efficient at generating runoff than rainfall.
This fact both explains why the percentages of snow runoff to total runoff listed above might seem disproportionately high while also pointing to just how important snow really is to the western US. It also gives us an idea of what might happen to runoff in a warmer future as more precipitation falls as rain and less as snow. The short answer is that a warming climate means that the western US will receive less runoff even if total precipitation stays the same because rain is not as efficient at generating runoff as snow. The longer answer comes as Li and colleagues’ final analysis.
After calculating current snow runoff to total runoff ratios, the researchers took the further step of examining hydrological simulations under two future climate scenarios that assume two different trajectories of greenhouse gas concentrations. The team used the Representative Concentration Pathways (RCP) 4.5 and 8.5, where RCP 4.5 represents a middle-of-the-road warming scenario and RCP 8.5 represents an extreme warming scenario. (If you need a brief description of the RCPs, check out the CIRC website.)
By the end of this century, according to the researchers’ analysis, the percentage of snow-derived runoff to total runoff across the western US is expected to decline from today’s 53% to 40% under RCP 4.5 by the end of this century. Under the high warming scenario RCP 8.5, the decline is even more dramatic, going from 53% to just 30% by the end of this century. In the Cascades, the changes are more dramatic still. The ratio of snow runoff is expected to go from today’s 78% to just 44% by 2100 under RCP 8.5.
Although Li and colleagues don’t take the extra step of calculating what these changes mean for water management in the West (for example, reservoir inflows), they do conclude their study with a strong warning: the declines in snowpack under a warmer climate are a direct “first-order threat” to our region’s water supply.
Study: Geophysical Research Letters.
Citation: Li, Dongyue, Melissa L. Wrzesien, Michael Durand, Jennifer Adam, and Dennis P. Lettenmaier. “How Much Runoff Originates as Snow in the Western United States, and How Will That Change in the Future?.” Geophysical Research Letters (2017). https://doi.org/10.1002/2017GL073551
David Rupp is a researcher for OCCRI at Oregon State University. He works on CIRC’s Climate Science and Climate Tools efforts. Interested in climate variability and change, and, in particular, in how these two factors impact the hydrological cycle and water resources, David’s work assessing how well Global Climate Models perform in the Northwest has become the foundation of much of CIRC’s Climate Tools and Community Adaptation efforts, including the Integrated Scenarios and Willamette Water 2100. More reviews written by David Rupp.