Modelling the Impact of Future Climate on Mountainous Watershed Nitrogen Cycling

Terrestrial nitrogen cycling plays an important role in regulating the export of nitrogen to surface waters. Terrestrial nitrogen transformations, however, are sensitive to changes in climate and disturbances, including altered temperature and precipitation patterns, snowpack depth, or wildfires. Thus, understanding how changing climate impacts terrestrial nitrogen cycling and the corresponding export of N to surface waters is an important part of understanding the impact of climate change on Earth’s freshwater resources. In this study, we investigate the short-term impacts of changing temperature, water partitioning and wildfire in the mountainous East River Watershed (ERW) located in the Upper Colorado River Basin. We use the High Altitude Nitrogen Suite of Models (HAN-SoMo), a course resolution semi-distributed ensemble of process based models developed for the ERW, to model changes in N through the vadose zone, surface waters and groundwater. HAN-SoMo was developed and calibrated against over 1600 N concentration measurements in the ERW to explore equifinality in parameter combinations. Three calibration scenarios were developed which explore the importance of different N sources and sinks throughout the watershed. Results of the most probable calibration suggest that the N cycling in the ERW is dominated by instream transformations and recycling of cow and plant matter. Additionally, results suggest that geogenic N from weathering of Mancos shale accounts for about 12% of N sources in the watershed. Here we will use the most probable HAN-SoMo calibration to explore possible changes in nitrogen cycling (key sources and sinks, N fluxes, etc.) under perturbations in temperature, water sources and wildfire.