Evaluating relative sensitivity of SWAT‐simulated nitrogen discharge to projected climate and land cover changes for two watersheds in North Carolina,USA

We investigated how projected changes in land cover and climate affected simulated nitrate (NO₃^?) and organic nitrogen discharge for two watersheds within the Neuse River Basin, North Carolina, USA, for years 2010–2070. We applied the Soil and Water Assessment Tool watershed model to predict nitrogen discharge using (1) atmospheric carbon dioxide (CO₂) concentrations predicted by the Intergovernmental Panel on Climate Change, (2) land cover change predicted by the Integrated Climate and Land Use Change project and (3) precipitation and temperature simulated by two statistically downscaled and bias‐corrected Global Circulation Models. We determined the sensitivity of simulated nitrogen discharge to separate changes in each treatment (1) CO₂, (2) land cover and (3) precipitation and temperature (PT)] by comparing each treatment to a reference condition. Results showed that nitrogen discharges were most sensitive to changes in PT over the 60‐year simulation. Nitrogen discharges had similar sensitivities to the CO₂ and land cover treatments, which were only one‐tenth the influence of the PT treatment. Under the CO₂ treatment, nitrogen discharges increased with increasing ambient CO₂. NO₃^? discharge decreased with increased urbanization; however, organic nitrogen had a varied response. Under the PT treatment, there was high spatial variability in nitrogen discharges. In a single year, certain sub‐basins showed an 80% increase in nitrogen discharge relative to reference, while others showed a 400% decrease. With nitrogen discharge showing high sensitivity to PT change, we suggest that more emphasis should be placed on investigating impacts of PT on nutrient transport in the Neuse River Basin. Published 2015. This article is a U.S. Government work and is in the public domain in the USA