Modelling seasonal nitrate concentrations in runoff of a heathland catchment in SW Norway using the MAGIC model: I. Calibration and specification of nitrogen processes

MAGIC (the Model of Acidification of Groundwater In Catchments) has been widely applied on catchments all over the world. The model has been used with annual time resolution to simulate the long-term effects of acidic deposition on surface water chemistry. Here MAGIC was applied using a monthly time step. The purpose was to simulate observed seasonal nitrate (NO3) concentrations and fluxes at an upland heathland catchment in southwestern Norway during the period 1993–2004. The rates of the key ecosystem nitrogen (N) processes (mineralization, plant uptake, litterfall and immobilization) were assumed to be governed by temperature. A snow accumulation and melt routine was used. The rates were calibrated to obtain the best match between the observed and simulated NO3 patterns. The best fit was obtained with standard yearly cycles for deposition and N parameters. The results show that MAGIC can explain 68 and 88% of the variation in seasonal NO3 concentrations and fluxes, respectively. The calibrated model provides a tool for exploring the effects of future scenarios of climate change and N deposition on NO3 in streamwater.

Modelling future NO3 leaching from an upland headwater catchment in SW Norway using the MAGIC model: II. Simulation of future nitrate leaching given scenarios of climate change and nitrogen deposition

Future nitrate (NO3) leaching to surface water at an upland heathland catchment, Øygardsbekken, southwestern Norway, was simulated using the MAGIC model with monthly time steps. Øygardsbekken has high nitrogen (N) load and exhibits seasonally elevated NO3 leaching. Future estimates for temperature, precipitation and N deposition were implemented. The climate scenarios were based on dynamically downscaled data from the Rossby Centre Regional Climate model (RCAO) driven by two scenarios of greenhouse gas emissions, A2 and B2, and run with two global climate models, HadAM3 and ECHAM4/OPYC3 from the Hadley Center and Max Planck Institute, respectively. Estimates of future rates of nitrogen (N) and carbon (C) processes in the catchment were based on the downscaled temperature scenarios and two different storylines, one assuming changes only in soil processes (mineralisation N, decomposition C, plant N uptake, N immobilisation, litterfall N) due to future warming and N deposition (SLsoil), and the other assuming changes in both vegetation (plant N uptake and litterfall C/N ratio) and soil processes (SLsoil + veg). Compared to the present, MAGIC simulated higher future NO3 leaching for both storylines with much higher rates for SLsoil. The results suggest that differences between the two storylines were larger than differences between the different scenarios within each storyline. For the scenarios with the highest future leaching rates the pronounced seasonal NO3 pattern levelled out, while for the scenarios with moderate projected NO3 leaching the seasonal pattern prevailed but was skewed towards highest leaching during spring rather than in winter as at present.