Implementing river water quality modelling issues in mesoscale watershed models for water policy demands -: an overview on current concepts, deficits, and future tasks

The political awareness of river water quality issues has increased substantially in the US and the EU during the last decade. New or enhanced environmental policies (e.g. European Union Water Framework Directive, US Total Maximum Daily Load Concept) require improved methods for investigation and evaluation of river water quality as well as derivation and assessment of management practices. Policy makers consider mesoscale watershed models as important support tools in this process if they provide the required functionality, i.e. they show capable of deriving reliable indicators for biological, hydromorphological and physicochemical water quality. The aim of this study is to give an overview of the current state of integration of water quality issues in watershed models and the application experience with the approaches. It is pointed out that the suitability of current models for the projected tasks is limited. Some of the models do not have water quality routines, and for those that have respective routines integrated there often exists minor application experience and the range of available water quality parameters is insufficient. Increasing efforts regarding the integration of river water quality issues in watershed models and more contributions to the application experience of these models are therefore recommended. In particular, watershed modellers will have to develop tools capable of linking physico-chemical variables already predicted by present watershed models with additional hydromorphological and biological quality elements demanded by policy programs such as EU-WFD. It is outlined that the derivation of expert systems instead of process-based model routines should be considered for the purpose of relating abiotic and biotic quality elements. Expert systems are expected to limit the increase of model complexity, which inevitably occurs in the course of model extension. Habitat evaluation procedures used in impact assessment and current river assessment methods provide directions and a sound database for establishing biotic-abiotic relationships. (C) 2004 Elsevier Ltd. All rights reserved.