PREDICTING WATERSHED ACIDIFICATION UNDER ALTERNATE RAINFALL CONDITIONS

The effect of alternate rainfall scenarios on acidification of a fores ted watershed subjected to chronic acidic deposition was assessed usin g the model of acidification of groundwater in catchments (MAGIC). The model was calibrated at the Panola Mountain Research Watershed, near Atlanta, Georgia, U.S.A. using measured soil properties, wet and dry d eposition, and modeled hydrologic routing. Model forecast simulations were evaluated to compare alternate temporal averaging of rainfall inp uts and variations in rainfall amount and seasonal distribution. Soil water alkalinity was predicted to decrease to substantially lower conc entrations under lower rainfall compared with current or higher rainfa ll conditions. Soil water alkalinity was also predicted to decrease to lower levels when the majority of rainfall occurred during the growin g season compared with other rainfall distributions. Changes in rainfa ll distribution that result in decreases in net soil water flux will t emporarily delay acidification. Ultimately, however, decreased soil wa ter flux will result in larger increases in soil-adsorbed sulfur and s oil-water sulfate concentrations and decreases in alkalinity when comp ared to higher water flux conditions. Potential climate change resulti ng in significant changes in rainfall amounts, seasonal distribution o f rainfall, or evapotranspiration will change net soil water flux and, consequently, will affect the dynamics of the acidification response to continued sulfate loading.