SIMULATING ACIDIFICATION AND RECOVERY PROCESSES IN EXPERIMENTAL CATCHMENTS WITH THE ILWAS MODEL

The Integrated Lake Watershed Acidification Study (ILWAS) model was used to simulate soil discharge chemistry at two neighboring experimental catchments. One catchment underwent deacidification because of the artificial application of deacidified precipitation whereas the other catchment received unaltered acidic precipitation. Simulated results reproduce the observed seasonal dynamics in the concentrations of base cations, NO3-, Al, and H4SiO40 in soil discharges for both catchments. Simulated results also indicate that the export flux of base cations was decreased by 30% at the deacidification catchment in response to the decrease in acid deposition. However, simulated SO42-concentrations show decreases that are about 40% more rapid than were observed. Simulated organic acid concentrations were also substantially lower than those observed at the deacidification catchment, indicating that organic matter decomposition processes were not correctly simulated. Acid-base budgets for both 5 and 50-yr simulations indicate that acid displacement by base cations through ion exchange is the principal process delaying recovery of runoff alkalinity, whereas SO42- desorption has a minor role. Silicate weathering is the dominant acid-consuming process at both catchments. Criteria proposed here for assessing forecast reliability include reproducing seasonal dynamics in discharge chemistry, providing numerically accurate chemical concentrations when compared to monitoring data, and correctly predicting deacidification rate and extent. The ILWAS model generally meets these criteria, indicating that the model can produce a reliable forecast of the effects of acid deposition on the acid-base chemistry of surface waters given sufficient temporal data for confident optimization of the calibrated variables in the model.