INVESTIGATING THE UNCERTAINTY IN PREDICTING RESPONSES TO ATMOSPHERIC DEPOSITION USING THE MODEL OF ACIDIFICATION OF GROUNDWATER IN CATCHMENTS (MAGIC) WITHIN A GENERALISED LIKELIHOOD UNCERTAINTY ESTIMATION (GLUE) FRAMEWORK

摘要

This study investigates the uncertainty associated with the modelled response of a catchment to historic and predicted future acidic deposition for the period 1851 -2041. The MAGIC model is applied within a GLUE framework to the 3.88 km~2 Afon Gwy catchment at Plynlimon, Wales. Nine million Monte Carlo simulations resulted in 5700 being accepted as behavioural as defined by a fuzzy measure comparing observed to simulated variables. Model output and parameter sensitivity analysis indicate that, for this example where weathering rates are low, model dynamics are limited compared to control exerted by model initial conditions and by the specified acidic deposition boundary conditions. The results show that despite the small number of behavioural simulations, they are widely spread across the ranges for most of the parameters varied. The GLUE methodology allows simulated prediction ranges for important variables to be presented as quantitative likelihood weighted uncertainty estimates rather than a single prediction for each variable over time.