Snow simulations predict future changes in rain-on-snow events across the upper Gallatin River watershed, a Greater Yellowstone Ecosystem headwater system

The upper Gallatin River watershed, an alpine headwater system in the Greater Yellowstone Ecosystem, in Wyoming and Montana. As global and regional air temperatures rise, mountain headwaters across the Greater Yellowstone Ecosystem (GYE) are projected to receive more precipitation falling as rain. While the hydrologic effects of this snow-to-rain transition depends on a variety of factors, it can lead to an increased occurrence of rain-on-snow (RoS) events. To investigate these changes, we used high-resolution (30 m) SnowModel simulations of the upper Gallatin River watershed. Simulations were run for 2001–2013 using two scenarios: (1) historical meteorology as control and (2) pseudo-global-warming (PGW) where control air temperature and precipitation conditions were perturbed to represent mean end-of-century conditions under a high-emissions scenario. SnowModel outputs show that changes in PGW precipitation and snow accumulation varied with elevation. Warmer air temperatures at low elevations (< 2500 m) led to less snow accumulation and less precipitation falling as snow. Colder baseline air temperatures for elevations above 2500 meters (m) resulted in minor reductions in winter snowfall fraction. For PGW simulations, spring (April-June) months experienced higher rainfall, and elevations above 2500 m experienced more RoS events. Snowpacks between 2500–3100 m generated more snowmelt during RoS events, which was reflected in the watershed average. More high-intensity melt events can affect aquatic habitat, water quality, and the accuracy of streamflow forecasts across the region. • Warming scenario shows rainier, less snowy years with earlier snowmelt. • SnowModel results show seasonal, elevation-dependent declines in snow fraction. • Rainier springs led to more frequent rain-on-snow (RoS) events above 2500 m. • RoS events generated two times more melt across the entire watershed in a warming scenario.