Warmer winters drive declines in snowpack and consequent increases in annual and seasonal runoff in a headwater region of the northeastern United States

Abstract

In montane, snow-affected regions of the United States (US), 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 partitioning of water between evapotranspiration and runoff, water quality, flooding, and drought. The Northeastern US represents a humid environment with relatively uniform precipitation seasonality and a transient snowpack, and the region has been largely overlooked with respect to changing winter conditions and water availability, in part due to scarce observations. An observational network has been recently established to span a Summit-to-Shore (S2S) continuum in Vermont for improved understanding and characterization 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 the Northeast 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 2.6oC, snow season length has decreased by almost 3 weeks, average snow water equivalent has decreased by 13%, and winter season rain-on-snow (ROS) event frequency has increased from 1 to 3.5 per year. In response, average daily winter season runoff has increased, which is strongly related to increased annual runoff ratios (R2 = 0.70). Separating the 22-year runoff record into years with more versus less winter season runoff, years with more winter runoff corresponded to significantly 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 implications for ecosystems and agricultural, industrial, and domestic water supply.