Winter Hydrology and NO₃^- Concentrations in a Forested Watershed: A Detailed Field Study in the Adirondack Mountains of New York

Abstract: More than 85% of NO₃^- losses from watersheds in the northeastern United States are exported during winter months (October 1 to May 30). Interannual variability in NO₃^- loads to individual streams is closely related to interannual climatic variations, particularly during the winter. The objective of our study was to understand how climatic and hydrogeological factors influence NO₃^- dynamics in small watersheds during the winter. Physical parameters including snow depth, soil temperature, stream discharge, and water table elevation were monitored during the 2007-2008 winter in two small catchments in the Adirondack Mountains, New York State. Snowpack persisted from mid-December to mid-April, insulating soils such that only two isolated instances of soil frost were observed during the study period. NO₃^- export during a mid-winter rain-on-snowmelt event comprised between 8 and 16% of the total stream NO₃^- load for the four-month winter study period. This can be compared with the NO₃^- exported during the final spring melt, which comprised between 38 and 45% of the total four-month winter NO₃^- load. Our findings indicate that minor melt events were detectable with changes in soil temperature, streamflow, groundwater level, and snow depth. But, based on loading, these events were relatively minor contributors to winter NO₃^- loss. A warmer climate and fluctuating snowpack may result in more major mid-winter melt events and greater NO₃^- export to surface waters.