The MAGIC (Model of Acidification of Groundwater In Catchments) model was developed to predict the long-term effects of acid deposition on soils and surface water chemistry (Cosby et al, 1985, 2001)  The model is used to simulate soil solution and surface water chemistry to predict annual average concentrations of major ions in lakes and streams.  The catchment is represented by aggregated, uniform soil compartments (one or two) and a water compartment of either a lake or stream.  It requires input data on:
Water chemistry
Catchment soil types
Soil bulk properties (eg, depth, bulk density, cation exchange capacity)
Soil C/N ratios
Soil pH
Catchment land cover
Forest uptake rates (ie, the amount of base cations and nitrogen removed by harvesting)
Present day N deposition
Hindcast deposition
Forecast deposition
Plus some fixed parameters (eg, denitrification, nitrogen immobilisation) based on soil type.

The model is calibrated using data from observations of soil and surface water chemistry for specified time periods.  The model generates predictions of pH, acid neutralising capacity (ANC), nitrate, sulphate, non-marine sulphate for each time step (year) for each site location.

Links to scientific papers that provide more detailed descriptions of the model can be found here.

Results of the application of MAGIC to surface water sites in the UK