Regional Calibration of a Steady-State Model to Assess Critical Acid Loads

The Model to Assess Critical Acid Loads (MACAL) has been developed for assessing and mapping critical acid loads on a national scale. MACAL simulates soil solution concentrations of major ions in a forest soil at any given depth at steady state for a given deposition level. The critical acid load is calculated from defined critical values for the A13+ concentration and the Al3+/Ca2+ ratio by inverse modelling. In order to minimize the uncertainty in the critical load computations, which is due to insufficient knowledge of parameter values, a multi-signal calibration of poorly defined important model parameters was performed using a data set on soil solution concentrations of 150 forest stands in the Netherlands. Since no detailed data was available on site scale (i.e. individual forest stands), a regional calibration was preferred. The cumulative distribution functions (CDF) of the model outputs for the 150 forest stands where fitted to those of the associated measurements. All model parameters could be identified with the objective function used except for forest filtering factors for nitrogen deposition. The calibration showed to be useful to reduce parameter ranges for some of the important model parameters, resulting in a lower uncertainty in model predictions.