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

The Integrated Lake Watershed Acidification Study (ILWAS) model was us ed to simulate soil discharge chemistry at two neighboring experimenta l catchments. One catchment underwent deacidification because of the a rtificial application of deacidified precipitation whereas the other c atchment received unaltered acidic precipitation. Simulated results re produce the observed seasonal dynamics in the concentrations of base c ations, NO3-, Al, and H4SiO40 in soil discharges for both catchments. Simulated results also indicate that the export flux of base cations w as decreased by 30% at the deacidification catchment in response to th e decrease in acid deposition. However, simulated SO42-concentrations show decreases that are about 40% more rapid than were observed. Simul ated organic acid concentrations were also substantially lower than th ose observed at the deacidification catchment, indicating that organic matter decomposition processes were not correctly simulated. Acid-bas e budgets for both 5 and 50-yr simulations indicate that acid displace ment by base cations through ion exchange is the principal process del aying recovery of runoff alkalinity, whereas SO42- desorption has a mi nor role. Silicate weathering is the dominant acid-consuming process a t both catchments. Criteria proposed here for assessing forecast relia bility include reproducing seasonal dynamics in discharge chemistry, p roviding numerically accurate chemical concentrations when compared to monitoring data, and correctly predicting deacidification rate and ex tent. The ILWAS model generally meets these criteria, indicating that the model can produce a reliable forecast of the effects of acid depos ition on the acid-base chemistry of surface waters given sufficient te mporal data for confident optimization of the calibrated variables in the model.