Simulation of soil and surface water acidification: Divergence between one-box and two-box models?

One soil-layer and two soil-layer model structures within the model of acidification of groundwater in catchments (MAGIC) were calibrated to 3 sites from the United Kingdom (UK) Acid Waters Monitoring Network. Catchment soil properties were segregated into two layers based on their hydrologic characteristics as defined by the hydrology of soil types (HOST) classification. Both model structures produced similar results for surface waters and soils. In general, simulated historic pH and acid neutralising capacity were slightly lower for the two-box structure. Similarly, weighted base saturation for both structures were comparable. Although both structures are highly consistent, the two-box simulations reflect the vertical differentiation in acid sensitivity, with the upper layers showing greater response to changes in acid deposition. The HOST classification appears to be a robust methodology to segregate soils and estimate flow-routing, providing a consistent methodology for application of the two-box protocol across the UK. The approach provides improved resolution for soils by representing their natural horizonation (acid-sensitive upper layers and higher-weathering lower layers), and a more appropriate tool for the integrated assessment of land use and climate impacts on catchments.