MAGIC

MAGIC (Model for Acidification of Groundwater In Catchments) is a process-oriented intermediate-complexity dynamic model by which long-term trends in soil and water acidification can be reconstructed and predicted at the catchment scale (Cosby et al., 1985a,b). MAGIC consists of :

soil-soil solution equilibria equations in which the chemical composition of soil solution is assumed to be governed by simultaneous reactions involving sulphate adsorption, cation exchange, dissolution and precipitation of aluminium, and dissolution and speciation of inorganic and organic carbon (Figure 1);
mass balance equations in which the fluxes of major ions to and from the soil and surface waters are assumed to be governed by atmospheric inputs, mineral weathering, net uptake in biomass, and loss in runoff (Figure 1).

MAGIC produces long-term reconstructions and predictions of soil and streamwater chemistry in response to scenarios of acid deposition and land use. MAGIC uses a lumped approach in two ways:

a myriad of chemical and biological processes active in catchments are aggregated into a few readily described processes;
the spatial heterogeneity of soil properties within the catchment is lumped into one set of soil parameters.

Figure 1 Schematic representation of the flows and stores in MAGIC

Recent refinements to the MAGIC model include enhanced represenation of two species of inorganic nitrogen in soil and surface waters: nitrate and ammonium. The major processes affecting these ions in soils have been incorporated explicity or implicity into the model: atmospheric deposition, nitrification, denitrification, mineralisation, uptake by plants, immobilization into soil organic matter and export in discharge water. Two new state variables have been introduced into the model namely soil organic carbon and soil organic nitrogen. Atmospheric deposition of nitrate and ammonium, and denitrification are specified in the model. Mineralisation rates depends on carbon decomposition rates and the C:N ratio of the organic matter compartment (Figure 2).

Figure 2 Schematic illustration of the pools and fluxes included in MAGIC for use in simulating the dynamics of organic and inorganic nitrogen in soils

Immobilisation of inorganic nitrogen into the soil organic matter is controlled by the C:N ratio of the soil organic compartment. If the C:N ratio is above an upper threshold value immobilisation of inorganic N is complete. If the C:N ratio is below an lower threshold value there is no immobilisation of inorganic N. Any nitrate or ammonium remaining in the soil solution, after all the biotic demands (i.e. uptake from plants, immobilisation into the soil organic matter compartment) for inorganic nitrogen are satisfied, are leached from the soil with the soil water drainage. The new processes directly control both ammonium and nitrate ions in soil solution (directly affecting ANC and soil base saturation) and thus can have significant effects on both long and short-term simulation of acidification responses (Cosby et al., unpublished).

REFERENCE

Cosby, B.J., Hornberger, G.M., Galloway, J.N. and Wright, R.F., 1985a. Modelling the effects of acid deposition: assessment of a lumped-parameter model of soil water and streamwater chemistry. Water Resour.Res., 21:51-63.

Cosby, B.J., Wright, R.F., Hornberger, G.M. and Galloway, J.N. 1985b. Modelling the effect of acid deposition: estimation of long term water quality responses in a small forested catchment. Water Resour.Res., 21:1591-1601.

Cosby, B.J., Ferrier, R.C., Jenkins, A., and Wright, R.F. (unpublished). Modelling the effects of acid deposition: Fifteen years of MAGIC- refinements, adjustments and inclusion of nitrogen dynamics.

For more information contact: B.J.Cosby@virginia.edu

REGIONS

PREDICTING RECOVERY IN ACIDIFIED FRESHWATERS BY THE YEAR 2010, AND BEYOND

The James Hutton Institute

THE REGIONS

RESULTS

MAGIC