Jens Fölster

Assessment of Uncertainty in Long-Term Mass Balances for Acidification Assessments: A MAGIC Model Exercise

Long-term (1860–2010) catchment mass balance calculations rely on models and assumptions which are sources of uncertainty in acidification assessments. In this article, we report on an application of MAGIC to model acidification at the four Swedish IM forested catchments that have been subject to differing degrees of acidification stress. Uncertainties in the modeled mass balances were mainly associated with the deposition scenario and assumptions about sulfate adsorption and soil mass. Estimated base cation (BC) release rates (weathering) varied in a relatively narrow range of 47–62 or 42–47xa0meqxa0m−2xa0year−1, depending on assumptions made about soil cation exchange capacity and base saturation. By varying aluminum solubility or introducing a dynamic weathering feedback that allowed BC release to increase at more acidic pHs, a systematic effect on predicted changes in acid neutralizing capacity (ΔANC ca. 10–41xa0μeqxa0l−1) and pH (ca. ΔpHxa0=xa00.1–0.6) at all sites was observed. More robust projections of future changes in pH and ANC are dependent on reducing uncertainties in BC release rates, the timing, and extent of natural acidification through BC uptake by plants, temporal changes in soil element pools, and fluxes of Al between compartments.

A metamodel based on MAGIC to predict the pre-industrial acidity status of surface waters

Abstract.When assessing the acidification status for a body of water, a reference value that describes the natural state is often estimated. One way to define this reference is to use the biogeochemical model MAGIC. However, MAGIC requires too many input parameters and modelling skills to use for operational application at a regional scale where hundreds or thousands of objects need to be assessed. Here we explore the possibility of complementing MAGIC with a simpler metamodel. The metamodel, metaMAGIC, was calibrated to 95 lakes with pre-industrial ANC predicted by MAGIC. The lakes constituted a diverse population, spread across a region stretching 1360xa0km in a N-S direction. MetaMAGIC proved capable of estimating the pre-industrial ANC calculated by MAGIC with a validated median error of 13.2xa0μeq/L, using contemporary concentrations of base cations, SO42- and Cl- as predictors. In comparison with the commonly used SSWC-model, MetaMAGIC reproduces the pre-industrial buffering capacity reconstructed with MAGIC more accurately. An advantageous feature of the metamodel is that it allows the reference value to vary in response to short-term hydroclimatological influences. Hence it provides a more stable assessment when few contemporary measurements of water chemistry are available. This metamodelling approach is suggested as a useful tool for acidification assessment purposes, and more generally, for extending the information from complex biogeochemical models.