Links between N Deposition and Export from a High-Elevation Watershed in the Colorado Front Range

Monday, 15 December 2014: 10:20 AM
Jill Baron1, Alisa Mast2, David W Clow2 and Gregory Alan Wetherbee3, (1)US Geological Survey, Fort Collins, CO, United States, (2)USGS Colorado Water Science Center Denver, Denver, CO, United States, (3)USGS CO, United States, Branch of Quality Systems, Denver, CO, United States
Long-term patterns of stream N export and atmospheric N deposition were evaluated over three decades in Loch Vale, a high-elevation watershed in the Colorado Front Range. Flow-normalized concentrations and fluxes were estimated using a regression model that removed the influence of interannual variability in streamflow to reveal underlying patterns of change. Stream N export increased in the early 1990s, peaked in the mid-2000s, and has since declined by over 30%. Similarities in the timing and magnitude of N deposition suggest the watershed is responding to changes in atmospheric deposition. Other possible explanations, including forest disturbance, snow depth, or permafrost or ice thaw could not explain stream N export although a climate signal is revealed in the analysis. High-elevation watersheds have responded rapidly to reductions in sulfur deposition and here we show the response to N deposition may be similar, at least in areas with high rates of deposition where surface-water nitrate concentrations are elevated. Our results show that stream N export responds rapidly to reductions in N deposition in high elevation watersheds, similar to patterns observed for reductions in sulfur deposition.