Effects of rain on snow events on runoff generation and nutrient export from forested and agricultural catchments in northern Vermont

The northeastern United States is predicted to experience changing winter conditions due to climate change, including reduced snowpack and increased frequency of rain-on-snow (ROS) events (National Climate Assessment 2014). These changes in the form of winter precipitation (rain vs. snow) have the potential to impact runoff generation pathways as well as the retention and export of biogeochemically reactive solutes (C, N, P). Furthermore, changes in the timing and magnitude of water and nutrient fluxes may impact water quality and ecosystem processes in downstream ecosystems.

In the winters of 2018 and 2019, we investigated the impact of these changing winter conditions on runoff generation and nutrient export in the Lake Champlain Basin in northern Vermont by intensively monitoring ROS events and spring snowmelt at two sites with contrasting land uses (agricultural vs. forested watersheds). Both watersheds were instrumented with a network of riparian groundwater wells, high-frequency soil sensors, and an in-stream monitoring station. During this period, we measured riparian groundwater and streamwater nutrient chemistry using grab samples and high-frequency water quality sensors to assess how solutes were mobilized during these dynamic periods.

We found that a significant portion of annual nitrate and dissolved organic carbon loads were mobilized during winter ROS events (~20%) and that disproportionately more nitrate was mobilized (relative to water export) during ROS events when compared to spring snowmelt, suggesting that these events are hot moments of nutrient export from catchments. Analysis of nitrate hysteresis patterns identified differences in runoff generation and solute mobilization pathways between ROS events and spring snowmelt. Furthermore, ROS events elevated terrestrial-aquatic connectivity during winter months, which may represent a shift in the degree of hydrologic and biogeochemical connectivity between uplands and the stream during this a period of the year that has historically been characterized by lower hydrologic connectivity. This substantial shift in the connectivity of terrestrial and aquatic systems suggests that changing winter precipitation patterns will have significant hydrobiogeochemical effects on forested and agricultural catchments.