The contribution of rain-on-snow events to annual NO3-N export from a forested catchment in south-central Ontario, Canada

The contribution of rain-on-snow (ROS) events to NO3-N levels in stream water has received relatively little research attention. However, individual ROS events during January and February contributed up to 40% of annual NO3-N export from a forested catchment in south-central Ontario between 1980 and 2000, but comprised less than 10% of annual precipitation. Nitrate-N concentrations in stream water increased rapidly following ROS events, and were similar to the concentrations in incident rainfall and the accumulated snow pack, likely due to limited contact of runoff with mineral soil under snow cover and low winter biological activity. Increased NO3-N associated with ROS events resulted in substantial depressions in stream pH and alkalinity, which may delay the biological recovery from acidification. The contribution of ROS events to annual or winter NO3-N export has been generally greater in recent years, although there is a considerable year-to-year variation. As a result, ROS events contribute to inter-annual variability in stream NO3-N concentrations and will have a strong effect on apparent temporal trends. The contribution of ROS events to annual NO3-N export should be considered when assessing surface water recovery from acidification and the N-status of forests, particularly if climate change projections for winter warming result in a greater proportion of winter precipitation occurring as rain.