MODELING THE HYDROCHEMISTRY OF CATCHMENTS - A CHALLENGE FOR THE SCIENTIFIC METHOD

The development and testing of coupled hydrological and chemical models for describing the impact of acid deposition on soil water and surface water chemistry are reviewed. Two problems fundamental to the modelling of environmental systems are identified. First, calibration data generally do not contain enough information uniquely to determine model parameters; this leads to an apparently good fit between observations and predictions, but provides only a weak test of the hypothesized processes. Second, state variables contained within the model are often difficult to relate to field observations, because of spatial heterogeneity or a 'conceptual' model structure being imposed on the system. This difficulty can prevent application of the scientific method to model development. Within hydrochemistry, more testable and thus better posed models ran be built by using chemical signals to constrain the hydrological structure. More generally for environmental systems, the use of synthetic data analysis is suggested as a means to determine the minimal field observations necessary to identify the model parameters and to test the model. Still, given the measurements that can be performed, there may be fundamental limitations to the modelling of environmental systems that cannot be overcome. Probing such questions is vital to the future of environmental modelling.