Warmer Winters Drive Declines in Snowpack and Consequent Increases in Annual and Seasonal Runoff in a Headwater Region of the Northeastern United States

In montane, snow-affected regions of the United States, a warming climate threatens the timing and amount of future water delivery. It is expected that winter precipitation falling as snow will continue decreasing and the frequency of winter snowmelt events will continue increasing, with unknown impacts on the partitioning of water between evapotranspiration and runoff, water quality, flooding, and drought. The northeastern United States represents a humid climate with uniform precipitation seasonality and a transient snowpack. Limited research on changing winter conditions and water availability has been conducted in the region, in part due to scarce observations. An observational network has been recently established (2022) to span a Summit-to-Shore (S2S) continuum in Vermont for improved understanding and characterisation of snowpack variability across the landscape. We leverage the S2S network alongside available multi-decade records of meteorology, snow depth, and runoff to relate long-term snowpack characteristics in Vermont to seasonal and annual runoff within the high-elevation headwater Ranch Brook watershed (9.6 km²). In the last 57 years, average winter temperatures have increased by 2.6°C, snow season length has decreased by almost 3 weeks, average snow depth has decreased by 16%, and winter season rain-on-snow (ROS) event frequency has increased from 1 to 3.5 per year. In response, average daily winter runoff has increased, which is strongly related to increased annual runoff ratios (R² = 0.70). Separating the 22-year runoff record into water years with more versus less winter runoff revealed that years with more winter runoff corresponded to increased winter temperatures, 15% smaller snowpack, two times more ROS events, 52% more winter runoff, 31% larger annual runoff ratio, and increased summer rainfall variance. A steady decline in the regional snowpack and related impacts on downstream water resources may have implications for ecosystems and agricultural, industrial, and domestic water supply.