The MAGIC dynamic model
Dynamic models offer the means to assess the future changes that might be expected in response to future emission reductions. A number of models have been developed and in the UK the Model of Acidification of Groundwaters In Catchments (MAGIC) has been extensively developed and tested. MAGIC contains mathematical representations of the key chemical processes linking the deposition of acidic pollutants and the acidification of soils and surface waters.
MAGIC is a process-oriented, intermediate-complexity dynamic model for the long term reconstruction and future prediction of soil and surface water acidification at the catchment scale. The model consists of:
- soil-soil solution equilibria equations in which the chemical composition of the soil solution is assumed to be governed by simultaneous reactions involving SO4 adsorption, cation exchange, dissolution and speciation of inorganic and organic carbon,
- mass balance equations in which the fluxes of major ions to and from soil and surface water are assumed to be governed by atmospheric inputs, mineral weathering, net uptake by biomass, and loss to streamwater (Figure 1)
MAGIC uses a lumped parameter approach to aggregate the complex chemical and biological processes active at the catchment scale into a few readily described processes and represent the spatial heterogeneity of soil properties throughout the catchment.
Dynamic simulation of soil and surface water chemistry is achieved by coupling the equilibria equations with the dynamic mass balance equations for each of the major cations (Ca, Mg, Na, K, and NH4) and anions (Cl, SO4, and NO3). Sulphate has an adsorbed phase in the soil compartment of the model which is concentration dependent and described by a langmuir isotherm. To maintain charge balance within each compartment of the model, the loss of strong acid anions to streamwater is accompanied by leaching of base cations. The model comprises a simplified version of nitrogen responses involving net catchment retention to calibrate simulated nitrogen against observed nitrogen in streamwater. This net retention is the integrated result of all the fluxes and processes of the terrestrial nitrogen cycle. Nitrogen uptake (nitrate and ammonium) is calculated from a first-order reaction with a rate constant determined by calibration.
MAGIC has recently been enhanced to include functions for N retention and loss. This more recent version (MAGIC7.77), incorporates a soil C and N pool. The changes in size of the carbon pool are specified external to the model. Nitrogen processes including leaching, uptake, nitrification, denitrification, and immobilization are represented in this new version of MAGIC.
The MAGIC model has been applied to a variety of catchments in the USA, Scandinavia and the UK and has proved to be a powerful tool for assessing the likely future consequences of changing acidic deposition and land-use on soil and surface water acidification status. The generic nature of the lumped approach combined with intermediate complexity, allows for a high degree of transferability to different catchments, and hence represents a tool for regional assessments.
Find out more about MAGIC and its application at the RECOVER 2010 web site.