The DWA Water Quality Model is a comprehensive and powerful tool for the dynamic simulation of quantitative and qualitative conditions and processes in rivers and streams. It was developed by a DWA working group in cooperation with the Institute for Applied Mathematics and Statistics of the Technical University of Munich and several other research institutions. The main objective in developing the DWA Water Quality Model was to create a widely available tool for the analysis and solution of water quality problems, which should be adapted predominantly to the practical day by day activities of public agencies, private companies or research institutions. The model has been available since 1998 and has in this time gained remarkable acceptance in water resources practice. Special attention of the DWA water quality model is focused on the applicability for smaller watercourses, which usually react with high sensitivity to changes in morphological and physico-chemical conditions.

Fields of application extend from data and system analysis, to the analysis of alternatives in water pollution control planning, to the implementation of alarm plans.

The model structure is strictly modular and consists of six parts (Figure 1). The manual input of all spatial data is based on a digitised topographic map. The 18 separate modules comprise the flow simulation and the important physical, chemical and biological processes which are relevant to overall water quality in water bodies. The modules are highly interrelated. For each simulation problem a suitable combination has to be composed.

The water flow simulation module enables calculation of steady and unsteady flow for simple and branched river systems. To solve the transport equation, the method of characteristics is adopted. This method is particularly appropriate for describing highly dynamic processes, so the procedure is also suitable for simulation of accidental pollution events.

The example of the Swist river is used to present the results of water quality simulations with a view to assessing current conditions and forecasting the future development of water quality. The model has proven to be an effective way to obtain useful information on Swist river water quality in various circumstances.

The river-specific geo-elements relevant to establishing the Swist water quality model are:

• Climatic zones

• River reaches with lateral inflow

• Municipal wastewater treatment plants

• Direct discharges

• Combined sewer overflows

• Storm water discharges

The DWA water quality model makes it possible to calculate time-variant flows and concentrations. The model is thus suitable for describing effects in the water body stemming from unsteady discharges and dynamic concentration gradients of combined sewer overflows.