Effects of changing winter snowmelt on watershed 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 events and freeze-thaw cycles (National Climate Assessment 2014). These changes will impact the retention and export of biogeochemically reactive solutes (C, N, P) because of the influence of melt events on the advective transfer of C, N, P from soils to streams. Furthermore, these changes in the timing and magnitude of nutrient fluxes may impact water quality and ecosystem processes in downstream receiving ecosystems.

We investigated the impact of these changing winter conditions on nutrient export in the Lake Champlain Basin in northern Vermont by intensively monitoring 8 rain on snow and winter thaw events at two field 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 these events, we sampled riparian groundwater and streamwater nutrient chemistry using high-frequency water quality sensors to assess how solutes were mobilized during these dynamic periods.

We found that a significant portion of the annual solute budget was mobilized during these winter events, presumably due to reduced biogeochemical processing in soils (due to cold temperatures) combined with higher winter-time connectivity between riparian wetlands and the stream. Furthermore, solute mobilization patterns and the magnitude of total export differed between forested and agricultural sites. This substantial shift in timing, modulated by land use, suggests that changing winter precipitation and temperature patterns will have significant hydrobiogeochemical effects on forested and agricultural catchments.