Historical reconstructions and future forecasts of regional surface water acidification in southernmost Norway

A “regionalization” methodology is presented that uses a conceptual model of long‐term water quality responses to acidic deposition (MAGIC) in a Monte Carlo simulation framework to reproduce observed distributions of water quality variables derived from regional surveys. The methodology is applied to data from a 1974 survey of 700 lakes in southernmost Norway to produce a calibrated model for the region. Predicted water quality in 1986 agrees well with results from a resurvey of 300 lakes. The model indicates that significant acidification occurred in the decades preceding the survey. The simulated effects of the 5% deposition decrease over the period 1974–1986 are very small but the effects of a 30% decrease in 26 years are readily apparent. Simulations for a 30% reduction indicate little improvement in water quality in the future. Simulations for a 50% reduction and for a 70% reduction in deposition indicate substantial and continuing improvement in water quality. The regionalization approach is a new and potentially useful method for modeling impacts of atmospheric acidic deposition.