A MODELING STUDY OF LONG-TERM ACIDIFICATION IN AN UPLAND SCOTTISH CATCHMENT ALAN JENKINS*, R. C. FERRIER**, T. A. B. WALKER**, and P. G. WHITEHEAD* (Received December 26, 1987; revised June 30, 1988) Abstract. A modeling study of the Allt a Mharcaidh catchment in the Cairngorm region of Scotland has been undertaken to investigate long term trends in acidification and model sensitivity to soil physical and chemical characteristics. The MAGIC model (Model of Acidification of Groundwater In Catchments) is used to demonstrate that the sulphate adsorption ability of the soil and quality and quantity of rainfall inputs have significant effects on model output. Optimal weathering rates and predicted present day ion concentra- tions in streamwater compare well with measured and observed values. The analysis shows that the catchment has become progressively acidified since pre-industrial times but major changes in stream acidity have yet to occur. 1. Introduction The case for development and implementation of process-based models describing the response of surface waters to acidic deposition has been widely and eloquently stated in recent literature (e.g., Cosby et al., 1985a; Neal et al., 1986). For the purpose of such deterministic modelling exercises, acidification of surface water may be considered to be essentially a problem over two very different time scales. Short term fluctuations in surface water acidity caused by the flushing of near surface waters or snowmelt are generally driven by the hydrological processes operating in the catchment. The timescale of these events is of the order of hours, or at most days, and the level of acidity will be largely controlled by the ability of the catchment to buffer incoming acidity within the catchment hydrological response time. Models addressing this dynamic response have been developed, for example, by Christopherson et al. (1984) and Sehnoor et aI. (1984). On the other hand, the Model of Acidification of Groundwater in Catchments (MAGIC), developed by Cosby et al. (1985a, b) addresses the changes in buffeting capacity that occur over long periods (i.e., decades) where short term hydrological response is assumed to be negligible and yearly averages of deposition levels provide the principal driving force to the model. The MAGIC model is a relatively simple yet process-orientated model whereby long-term dynamic equations based on input-output mass balances for all major ions in atmospheric deposition are linked with equilibrium equations that describe soil processes. It has been applied to a variety of catchments in the U.S., Scandinavia (Wriht et aL, 1985), Scotland (Cosby et al., 1986a) and Wales (Whitehead et al., 1988) and has proved a useful tool for assessing future acidification levels in response to * Institute of Hydrology, Wallingford, Oxon, OX10 8BB, U.K. ** Macaulay Land Use Research Institute, Aberdeen, U.K. Water, Air, and Soil Pollution 40 (1988) 275-291. © 1988 by Kluwer Academic Publishers.