Streamwater nitrogen export in a developing mountain watershed in the Rocky Mountain West; a Bayesian analysis

Background/Question/Methods In recent decades, the Rocky Mountain West has been one of the fastest growing regions in the United States. Headwater streams in these mountain environments are particularly susceptible to nitrogen enrichment from residential and resort development yet poorly understood. We conducted six synoptic sampling events (50 sites) and weekly streamwater sampling (7 sites) across the mountainous, 212km2 West Fork of the Gallatin River watershed, located in Southwestern Montana. The West Fork stream network drains the rapidly developing Big Sky resort community. Synoptic sampling campaigns captured each season and a range of hydrological conditions and biologic activity. Streamwater chemistry was combined with land use/land cover (LULC) and terrain analysis to develop a new nutrient export model to examine the spatial and seasonal variability of LULC impacts on streamwater nitrogen. The export model was fitted using Bayesian statistical inference with an adaptive Markov chain Monte Carlo (MCMC) method for estimation of model parameters. The Bayesian approach provides a powerful framework for assessing parameter uncertainty and subsequent uncertainty in model simulations. Scientific knowledge and expert judgment were used to formulate prior probability distributions and characterize the uncertainty pertaining to 10 model parameters. We estimated export coefficients for nitrate and total nitrogen from different land use categories. Results/Conclusions Modeling results indicate seasonality in export coefficients. Nitrogen loading to the watershed was more conservatively transported through the uplands and stream network in the winter. In the late summer, however, transport and related biological parameters became more important. Spatial location of development along hydrological flowpaths was a major control of streamwater nitrogen export, suggesting the importance of the intersection of LULC spatial patterns and landscape structure. This study provides valuable insight into the spatial and seasonal influences of LULC impacts on streamwater nitrogen concentrations and variability in mountain streams.