Monday February 3, 2025
Water Desk via Maven’s Notebook —
Snow is a cornerstone of the American West’s water supply, but just how important is it to the region’s streams, rivers and reservoirs?
In the popular press and academic papers, the sizable share of runoff that originates as snowmelt is often cited as a reason why the West’s snowpack is so crucial to both cities and farms, not to mention the region’s wildlife and very way of life.
But when a team of researchers set out to study the question, they found a wide range of estimates cited in 27 scientific papers. They concluded that “a detailed study of the contribution of snow to the runoff over the western U.S. has not been conducted.”
To clarify the connection between the snowpack and streamflow—and project how climate change is altering the relationship—the scientists used computer simulations and hydrological modeling in a 2017 paper in Geophysical Research Letters to estimate snow’s significance for runoff across the West. Here’s what they found:
- 53% of total runoff in the West originated as snowmelt, even though only 37% of the precipitation fell as snow.
- In mountainous parts of the region, snowmelt was responsible for 70% of runoff. Specifically, it was 74% for the Rockies, 73% for the Sierra Nevada and 78% for the Cascades (see graphic below).
- A quarter of the West’s land area, primarily in the high country, produced 90% of total runoff on average.