Journal of Conference Abstracts

Application of the First-Order Acidity Balance (FAB) Model for National Critical Loads Mapping in Great Britain: Sensitivity Analysis and the Prediction of Nitrate Leaching

C. J. Curtis¹ (Ccurtis@geog.ucl.ac.uk), T. E. H. Allott¹ (Tallott@geog.ucl.ac.uk),
M. Kernan¹ (Mkernan@geog.ucl.ac.uk), D. Bird¹ (Dbird@geog.ucl.ac.uk),
B. Reynolds² (BR@ite.ac.uk), R. Harriman³ (Harrimanr@marlab.ac.uk) & J. Hall⁴ (J.Hall@ite.ac.uk)

¹ Environmental Change Research Centre, University College London, 26 Bedford Way, London, WC1H 0AP, U.K.

² Institute of Terrestrial Ecology, Bangor Research Unit, Deiniol Road, Bangor, Gwynedd, LL57 2UP, U.K.

³ Freshwater Fisheries Laboratory, Faskally, Pitlochry, Perthshire, PH16 5LB, U.K.

⁴ Institute of Terrestrial Ecology, Monks Wood, Abbots Ripton, Huntingdon, PE17 2LS, U.K.

The relative contribution of nitrogen deposition to the acidification of freshwaters in Great Britain has increased over the last few years as sulphur deposition has fallen in line with reduced emissions, and the deposition of nitrogen species has increased in absolute terms with agricultural intensification and especially increasing car usage. In high deposition areas of Great Britain (e.g. North Wales, Lake District) nitrate based acidity can equal or exceed the contribution of sulphate based acidity in some upland waters. Since this problem appears to be increasing, and given the dynamic nature of N cycling processes in catchments, it is not sufficient to assume long-term steady-state conditions apply or that existing empirical models of critical loads (e.g. Henriksen et al., 1992, Battarbee et al., 1996) are adequate. The complexity of catchment processes which affect the retention or leaching of nitrogen species is such that only very detailed dynamic models (e.g. MAGIC-WAND, Ferrier et al., 1995) can predict the timing and magnitude of nitrate breakthrough. Such models cannot easily be applied for national scale mapping, however, because of the huge data requirements in order to calibrate and run them. A compromise can be provided by incorporating the major nitrogen sinks into the steady-state models in order to predict future nitrate leaching when N inputs are in steady state with sinks and outputs.

The first-order acidity balance model (FAB) is a simple process-oriented model which can be applied to predict the maximum nitrogen leaching from a catchment at steady state, and hence can be used to construct a critical load exceedance function for sulphur and nitrogen deposition for a site (Posch et al., 1995a, 1995b). The FAB model has been applied in several European countries, and here we suggest how the model might be modified to make it appropriate for national mapping of critical loads across Great Britain. Application of the model requires catchment soils and land cover data, and we examine the effects of using data of different types and resolution on the predicted nitrogen leaching from a catchment. Preliminary runs of the model using recommended default values for N sink terms have predicted nitrate leaching at much higher rates than are currently being measured in surface waters. Here we consider whether this may be due to inaccurate parameterisation of the model or to the continued short-term storage of N within catchment soils and vegetation, which might lead to much greater leaching of N at some time in the future as N saturation occurs and steady-state is achieved.

References

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