Systematic comparison of ILWAS, MAGIC, and ETD watershed acidification models: 3. Mass balance budgets for acid...

Three watershed acidification models-ILWAS, MAGIC, and ETD-were quantitatively compared to determine model structural differences by using a combination of input mapping and ANC mass balance budgets. Input mapping is a set of rules and algorithms to ensure that consistent input values were simultaneously derived for all three models. ANC budget analysis under current SO4(2-) deposition and a 70% reduction in SO4(2-) deposition allows examination of the relative importance of biogeochemical processes in affecting predictions of ANC or predicted changes in ANC. Model inputs were based on two dissimilar watersheds having characteristics typical of watersheds in the northeastern US. After mapping inputs, the three models predicted values of outflow ANC fluxes that were similar among the models for each watershed and deposition scenario. Within each watershed, the changes in outflow ANC fluxes between the scenarios were similar for the three models. Terrestrial weathering was the major source of ANC for all three models for both watersheds and deposition scenarios. The contributions of other processes to the ANC of the two watersheds were, under certain conditions, model-specific. Cation exchange was responsible for changes in ANC when deposition decreased for the three models. Other processes responsible for changes in ANC between scenarios were SO4(2-) sorption (for MAGIC) and in-lake weathering (for ETD). The processes responsible for the change in ANC from a change in deposition (cation exchange, SO4(2-) sorption, and in-lake weathering) were different from the processes contributing to the absolute ANC for a given deposition scenario (terrestrial weathering). The budget analysis complements an earlier Monte Carlo analysis that showed that the three models are structurally different and that predictions viewed on a relative scale are more similar than absolute scale predictions.