EVALUATING CRITICAL LOADS OF ACIDITY FOR SWISS FOREST SOILS - COMPARISON OF 2 CALCULATION METHODS

A variant of the European model Simple Mass Balance (SMB) and the regi onalized PROFILE model have been used to calculate critical loads of a cidity for Swiss forest soils. The single layer SMB has been applied t o 11,800 receptor points and the multi-layer PROFILE to 720 forest sit es. Weathering rates used in SMB calculations were assessed by means o f a modified de Vries soil classification, and calculated from physica l properties of the soil system with PROFILE. Cumulative frequency dis tributions of the results at the national resolution show that PROFILE predicts lower critical loads. Up to the 65-percentile critical load PROFILE percentile values are average 65% of the SMB percentile values . The upper percentiles of the PROFILE critical loads are merely 25% o f the respective SMB predictions. The analysis of the model prediction s on a regional scale implies that the multi-layer model should be use d for the assessment of critical loads for areas with potentially calc areous forest soils. The inherent inability of the SMB method to prope rly account for processes in the carbonate system and to estimate adeq uate weathering rates results in significantly higher critical loads c ompared to PROFILE predictions. Both models estimate critical loads in better agreement for low weathering forest soils in high-precipitatio n areas where hydrogen and aluminum leaching govern the model result.