Sources of nitrate export during rain-on-snow events at forested catchments

Abstract

Rain-on-snow (ROS) events are major drivers of nitrate (NO3-N) export from seasonally snow-covered forested catchments and may cause episodic declines in stream pH. High intensity monitoring of throughfall, snow pack and stream water draining two proximal catchments (Harp 3A and Harp 6A) with very different NO3-N export revealed that a very small percentage of ROS-induced stream discharge originates from throughfall and melting snow (new water; average = 6.4 %). However, this new water has a very high concentration of NO3-N (throughfall/snowmelt average = 498 μg/L) compared with baseflow (average = 7.3 μg/L in Harp 6A; average = 41 μg/L in Harp 3A) and as a result, throughfall and snowmelt contribute the majority of NO3-N export (average = 62 %) during ROS events. In contrast, concentrations of sulphate, dissolved organic carbon and calcium in rain and snowpack are similar to baseflow and therefore ROS-induced declines in pH (often to below pH 6.0) are attributed entirely to increases in NO3-N concentration. Differences in absolute magnitude of ROS NO3-N export between catchments are explained through differences in baseflow NO3-N concentrations. The frequency and magnitude of ROS events in this region are affected by both NO3-N deposition and winter temperature, and thus the impact of these events in the future depends on changes in both atmospheric deposition and winter climate.