Modelling long-term stream acidification in the chemically heterogeneous Upper Severn catchment, Mid-Wales

A two-box version of the long-term acidification model MAGIC is applied to the Upper Severn catchment, Mid-Wales. Comparison between modelled output and the observed stream- and groundwater chemistry points to the limitations of modelling, due to the inherent complexity and variability in the catchment hydrology, soils, geology and chemistry. The MAGIC model is used to produce long-term hind- and forecast predictions of average stream-, soil- and groundwater chemistry, and to simulate long-term changes in sulfate and nitrate deposition, in line with current proposals of reduction. Changes in flow-routing pathways between soil- and groundwater are simulated and the long-term effects on streamwater quality noted. The use of a long-term acidification model enabled the simulation of streamwater quality under these different case scenarios. However, the modelled output is insensitive to depositional and flow routing changes, indicating that catchment processes are not being represented to a sufficient degree. Changes in simulated output as a result of increased acidic deposition are not statistically significant, lying within the variance of long-term observed data. Simulated changes in flow routing suggest a lack of model sensitivity, in terms of the effect on stream chemistry. The need for large amounts of measured data to ensure correct model representation of the hydrology, chemistry and the heterogeneous/variable nature of upland catchments is outlined. It is vital that these long-term data are available to ensure that problems do not arise due to over-reliance by catchment managers on potentially unreliable modelled output.