A comparison of methods for estimating soil characteristics in regional acidification models; an application of the MAGIC model to Scotland

The results from an application of MAGIC (Model of Acidification of Groundw ater In Catchments) to 733 Scottish catchments are presented. The availabil ity of representative, good quality soil data is frequently a limiting fact or for biogeochemical modelling, particularly those involving modelling at various spatial scales. This study tests the sensitivity of MAGIC to soil i nput data derived from two different methodologies; the 'nearest neighbour method' considers the closest representative soil profile to a catchment, a nd the 'spatial weighting method' of all soil types identified ina catchmen t, based on a soil physico-chemical classification of Scotland. Soil data ( soil depth, density, cation exchange capacity and base saturation) calculat ed using the 'nearest neighbour method' and the 'spatial weighting method' were highly variable, although the range of upper and lower limits were gre ater for soil data produced using the nearest neighbour method. In contrast to the predominantly organic soil data calculated by the nearest neighbour method, the spatially weighted soil parameters included a greater proporti on of mineral soils. With regard to simulated surface water Acid Neutralisi ng Capacity (ANC) for 1851, 1997 and 2050, MAGIC predictions were similar i rrespective of the methodology used to determine soil input parameters. How ever, soil input data derived from both methods resulted in variable base s aturation predictions. It is concluded that the 'nearest neighbour' methodo logy is most appropriate if the objective is to determine the predicted res ponse of the most acid-sensitive sites within a region in line with the app roach used in Critical Load mapping. On the other hand,'spatial weighting' integrates catchment soils and represents a more robust methodology by whic h to determine changes in median soil and water response in a regional cont ext. The anticipated reductions in S emissions associated with the Second S Protocol are predicted to have a marginal beneficial effect on the recover y of soils and surface waters of Scotland.