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 1360 km in a N-S direction. MetaMAGIC proved capable of estimating the pre-industrial ANC calculated by MAGIC with a validated median error of 13.2 μeq/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.