This chapter focuses on the Model of Acidification of Groundwater in Catchments (MAGIC). Timothy J. Sullivan et al. presented data for Big Moose Lake and Constable Pond in the Adirondacks that showed diatom inferences of mean pH close to the mean of measured pH values that showed great seasonal variability. A number of models have been developed to simulate nitrogen (N) dynamics in forested ecosystems, and N has recently been added in various ways to MAGIC. Three primary models were used in Environmental Protection Agency’s Direct Delayed Response Project to project surface water acidification response: MAGIC, the Integrated Lake Watershed Acidification Study model, and the Trickle down Model. A major uncertainty in the modeling of ecosystem response to changing N deposition is specification of changes in the temporal dynamics and degree of N saturation. For some applications to forested watersheds, the MAGIC model has been structured to predict N saturation status from estimates of mineralization and nitrification.