Low snow fractions increase evapotranspiration efficiency across the Greater Yellowstone Ecosystem

Globally, mountainous headwaters are projected to experience less precipitation falling as snow. Changes in snowmelt rate and timing may affect streamflow generation by shifting the alignment of energy and water inputs which control the proportion of precipitation lost to the atmosphere, or evapotranspiration (ET) efficiency. Since changes in ET efficiency may make streamflow harder to predict in seasonally snow-covered watersheds, in this study, we investigate the impact of precipitation phase on ET efficiency in eight watersheds across the Greater Yellowstone Ecosystem. ET efficiency has significantly increased in four watersheds with greater land cover change (p < 0.05). Across all watersheds, mixed effects modeling shows that more available water goes to ET in low snow-fraction years. However, ET responses to snow conditions are locally moderated: groundwater-dominated watersheds are less responsive in the short term but retain a stronger memory of the previous year's conditions. These findings indicate vegetation water use and ET will more strongly affect streamflow generation in seasonally snow-covered regions during low snow years, potentially exacerbating existing droughts