Critical loads of acidity to surface waters - How important is the F-factor in the SSWC-model?

The critical load of acidity to surface water is based on the condition that the inputs of acids to a catchment do not exceed the weathering rate less a given amount of ANC Acid Neutralizing Capacity). The Steady State Water Chemistry (SSWC) Method is used to calculate critical loads of acidity, using present water chemistry. To calculate the weathering, the so-called F-factor is used to estimate the part of the base cation flux that is due to soil acidification. The F-factor has been estimated empirically from historical data comparisons from Norway, Sweden, U.S.A. and Canada and is considered to be a function of the base cation concentration by the formula: F = sin( BC*/S), where BC* is the present base cation concentration and S the base cation concentration at which F = 1. At higher values for BC* F is set to 1. For Norway, Sweden and Finland S has been set to 400 mu eq/l (ca. 8 mg Can), giving F-values in the range 0.05-0.2. The importance of the F-factor in the calculations of the critical loads of acidity for Nordic surface waters was tested by calculating the magnitude of the area where the critical load of acidity is exceeded in Norway for different values of S. Similar calculations were carried out for the Finnish and Swedish lake data Varying S from 100 mu eq/l to 1200 mu eq/l, the exceeded area in Norway decreases from 31,9 to 28,3%. For F = 0 (S = infinity, i.e. assuming no soil acidification), the exceeded area is reduced to 27,2%. For Finland and Sweden the the percent of lakes exceeded are reduced from 16,6 to 12,9% and 30 to 23,6%, respectively. For F = 0 the percent of lakes exceeded are reduced to 11,4 and 16,4, repectively. These results indicate that the F-factor is not of great importance for calculating critical load and critical load exceedances in Norway, Finland and Sweden.