AN ACIDIFICATION MODEL (MAGIC) WITH ORGANIC-ACIDS EVALUATED USING WHOLE-CATCHMENT MANIPULATIONS IN NORWAY

The results from two whole-catchment manipulation experiments in Norwa y are used to evaluate MAGIC (Model of Acidification of Groundwater In Catchments), a model of ecosystem biogeochemical response to acid dep osition. MAGIC is an aggregated catchment-scale model of acidification that has been widely used in assessment activities in Europe and Nort h America. The experiments involved artificial decrease (Reversing Aci dification In Norway, RAIN) and increases (Humid Lake Acidification Ex periment, HUMEX) in acidic deposition. Runoff from both sites is influ enced by moderate levels of organic acids. At each site the model was calibrated to the control catchment and then applied to the manipulate d catchments with only minor adjustments. The major responses in runof f chemistry to the manipulations were closely simulated by the model. Differences between simulated and observed volume-weighted annual aver age concentrations of all major ions were less than +/-6 mu equiv 1(-1 ) for the entire 4-8 year period of prediction. Trends in response to the manipulations were correctly simulated. Most of the residual error resulted from an inability of the model to reproduce the year-to-year variability (noise) around the trends. Although such model evaluation s cannot 'prove' the correctness of the model structure, good fits to experimental data increase confidence in model applications for assess ment and management purposes. Evaluations of this sort can also identi fy aspects of the model that need further development. For MAGIC, thes e are primarily a need for improvement in the calibration of aluminium solubility and a better process basis for nitrogen dynamics.