FUTURE CHALLENGES IN WATER QUALITY MODELLING WITH PARTICULAR REFERENCE TO OPERATIONALLY CLOSED CATCHMENTS

SUMMARY As a result of the declining quality of many water sources, the use of water quality models in planning is increasing. To date applications have involved the simulation of total dissolved salts and nutrients, largely in catchments affected by urban and industrial development. Whilst such applications are certain to continue, the future problems requiring this type of analysis are likely to be far more complex. Additionally, the questions asked of modellers will become more detailed and specific. By far the most disturbing water quality problems facing water managers is that of future effluent management in catchments which have no direct outlet. Two such catchments are the Modder-Riet catchment of the lower Vaal River and the Middle Buffalo catchment in the Eastern Cape. Both these river systems are impeded by dams from which it is either undesirable, or extremely difficult, to make downstream releases. Given that both catchments are certain to experience rapid urban expansion in the coming years, the possibility of eventual water quality problems is high. In order to predict the effects on water quality of various effluent management strategies in these and other catchments, future models will have to be capable of simulating; water quality parameters in entire systems, pollutant loads from diffuse sources, and the hydrodynamic and limnological processes within reservoirs.