LONG-TERM SULFATE DYNAMICS AT LANGE BRAMKE (HARZ) USED FOR TESTING TWO ACIDIFICATION MODELS H. LANGE and M. HAUHS BITOK, University of Bayreuth, 95440 Bayreuth, Germany and S. SCHMIDT Institute for Soil Science and Forest Nutrition, Biisgenweg 2, 37077 GOttingen, Germany (Received October 7, 1993; accepted September 19, 1994) Abstract. At Lange Bramke (Harz) soil solution and runoff concentrations of major elements were observed over 16 yr. During this period acid deposition was high but showed a marked decrease of H + and SO2- both in concentrations and fluxes over the last five years. Among others, this record reveals the following patterns: seasonality in the signals for SO] + and NO 3 in runoff which are synchronous; an accumulation of SOl- in the soil, initially up to 50% of the deposition fluxes; apparently no correlation between runoff and SO2- concentration, and no long-term trend in runoff concentration of SO]-. In this paper we use these patterns in the data set from Lange Bramke to test two established acidification models. The test criterion is that the algorithms employed by the SO]- modules of these models must be able to reproduce these features. To that end, both models need not to be run as it can be shown that even with completely unrestricted parameter values the two algorithms are unable to match the observed SO2- dynamics. The MAGIC model (Cosby et al., 1985) is unable to reproduce, given the existence of net SOl- accumulation, the constant SOl- concentration in runoff during the last 16 years. The second model, BEM (Prenzel, 1986), is succesful in reconstructing the constant SO]- levels in runoff. However, on a monthly time scale BEM predicts a shift between the periodic maximum concentrations of SO42- and NO3 which is not observed in the data. 1. Introduction The catchment Lange Bramke in the Harz Mountains (Germany) is situated in a region where surface water acidification and forest decline have been documented. Runoff from the Bramke catchments has been monitored hydrologically since 1949 and chemically since 1976 (Liebscher, 1988; Hauhs, 1989). The site is covered by a 45-year old Norway spruce stand and is impacted substantially by acid deposition, but during the last five years the deposition of sulfuric acids has declined. A full description of the site and the instrumentation can be found in (Hauhs, 1989). Biota in the upper parts of the Lange Bramke stream, as well as surface waters through- out the Harz Mountains, show clear effects of acidification (Matschullat et al., 1994). Despite empirical evidence linking atmospheric loadings with streamwater responses, the soil processes that govern such responses are still only partly under- stood in any of the sites for which they have been documented (Christophersen et al. 1990, 1993). Yet an extrapolation of empirical models outside the range Water, Air and Soil Pollution 79: 339-351, 1995. (~) 1995 Kluwer Academic Publishers. Printed in the Netherlands.