Abstract
MAGIC-WAND (Model of Acidification of Groundwaters In Catchments—With Aggregated Nitrogen Dynamics) has been specifically developed for wide application and scenario assessment. It maintains the sulphur driven acid/base chemistry dynamics of MAGIC, and considers in addition the impacts of changes in nitrogen deposition from the atmosphere and changes in nitrogen utilisation within the catchment. The model uses estimates of nitrification, mineralisation, N fixation and denitrification and changes in these soil processes through time. Plant uptake is nonlinear and dependant upon inorganic nitrogen concentrations in soil solution. Calibration of the model requires specification of values for the soil N fluxes and for the parameters which describe the hyperbolic uptake function. Literature data can provide ranges for these values but specific catchment related values are not obtainable since the model is conceptual. Selection of uptake parameters must reflect current catchment vegetation and vegetation change through time, and this has been investigated at a catchment in SW Scotland. Changes in soil processes through time are also important to model functioning, but are not considered in this study. The model has been applied to 25 acid sensitive (Acid Neutralising Capacity (ANC)<50 μeq l −1) lochs in SW Scotland to examine the interaction of afforestation and changes in nitrogen deposition, assuming recently agreed reductions in sulphur deposition. The model results suggest that a 50% reduction in nitrogen deposition coupled with agreed sulphur reductions is not sufficient to promote reversibility of acidification at the most sensitive sites within 15 years. Increased nitrogen deposition in conjunction with agreed sulphur reductions will most likely cause continued acidification at afforested lochs in the region.
Published Version
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