Sulfate budget and a model for sulfate concentrations in stream water at Birkenes, a Small forested catchment in southernmost Norway

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Abstract
As part of the Norwegian SNSF project, the volume and chemical composition of precipitation and stream water have been measured for 6 years at Birkenes, a small (0.41 km2), gaged, coniferous‐forested catchment on granitic bedrock in southernmost Norway. Daily measurements of SO2 gas and SO4 aerosol have also been collected and provide the basis for estimation of dry deposition. The sulfate budget for the period November 1972 to October 1978 indicates that about 15% more sulfate left the catchment in runoff (7880 mg SO4 m−2 yr−1) than entered in dry (1870 mg SO4 m−2 yr−1) and wet (4970 mg SO4 m−2 yr−1) deposition. This difference is probably not significantly different from zero, given the difficulties inherent in the measurements. The input‐output budget reveals a marked seasonal pattern (accumulation during summer and winter; washout during spring and autumn). The annual sulfate flux is perhaps 5–10 times larger than natural. The result is an acidification of stream water and an increased leaching of base cations from the soil. The concentration of sulfate in stream water at Birkenes shows a characteristic pattern of high levels (10–13 mg l−1) in the first stormflow following a dry summer period and low levels (∼ 5 mg l−1) in baseflow. For the summer half‐year we have simulated sulfate concentrations using a simple two‐compartment model. Sulfate is assumed to accumulate in the uppermost soil layers because of dry deposition and evapotranspiration of rainwater. Mineralization of other sulfur compounds is included indirectly. In the lower mineral soil layers, adsorption/desorption of sulfate is assumed to occur. The ability of this simple model to simulate sulfate concentrations in stream water suggests that the model quantitively accounts for these processes.