UNCERTAINTY IN PREDICTING WEATHERING RATE AND ENVIRONMENTAL-STRESS FACTORS WITH THE PROFILE MODEL

The PROFILE model is a steady state soil chemistry model which is used to calculate soil weathering rate. The model has also been used to ca lculate critical loads of acidity and N to forest soils, using the rat io of Ca+Mg+K to total inorganic aluminium in the soil solution as cri terion, and to surface waters, using the ANC leached from the soil col umn as criterion. An uncertainty analysis of the PROFILE model was per formed by Monte Carlo analysis, varying input parameter errors individ ually and simultaneously in ranges of +/-10-100%, depending on paramet er. The uncretainty in calculation of weathering rate, ANC leaching an d ratio of Ca+Mg+K to inorganic Al in the soil solution was studied fo r three Nordic sites. Furthermore, the effect of uncertainty in estima tes of critical load for forest soils was assessed. The analysis shows that the weathering rate can be calculated with high precision, provi ded that the errors of input parameter are within the range that has b een reported in the literature. The model tend to be less sensitive to errors in input parameters for the range of conditions where forest d amage is most likely to occur. Critical loads of acid deposition for o ne site calculated on the basis of the model varies within a largest r ange of +/-40%. A study of one geographical grid included in the Swedi sh critical loads assessment shows that with the number of calculation points in the grid, the distribution of critical loads will stay stab le independently of stochastic errors.