A model for streamwater chemistry at Birkenes, Norway

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Abstract
A simple two‐reservoir model incorporating a small number of physically realistic processes accounts for the major short‐term variations in Streamwater chemistry during the snow free season at Birkenes, a 0.41‐km2 gaged catchment in coniferous forest on granite bedrock in southernmost Norway. This area is in the zone of maximum deposition of acidic compounds from the atmosphere (weighted‐average pH of precipitation 4.24), and the Streamwater is acidic (pH 4.48). The complete model builds on the hydrologic and sulfate submodels previously described. The cation submodel includes H+, Al, Ca, and Mg and is based on the mobile anion concept. As input data series the model requires only precipitation volume and sulfate concentration, estimates of sulfur dry deposition, and daily mean temperature. The model simulates volume and concentrations of major ions in stream‐water. Chemical processes incorporated in the model include cation exchange, weathering, dissolution/precipitation of gibbsite, sulfate adsorption/desorption, and sulfate mineralization. Typical characteristics of Streamwater chemistry at Birkenes which are simulated by the model are (1) elevated sulfate concentrations in the first autumn stormflow following dry summers and the general decrease later in the autumn and (2) positive correlations between the H+ and Al concentrations and discharge and negative correlations between these factors and the Ca and Mg concentrations. The simplicity of the model and the fact that it is based on known, physically realistic processes make it particularly suitable as a tool in clarifying important mechanisms in catchments.