Abstract

Abstract. The design and evaluation of solutions for integrated surface water quality management requires an integrated modelling approach. Integrated models have to be comprehensive enough to cover the aspects relevant for management decisions, allowing for mapping of larger-scale processes such as climate change to the regional and local contexts. Besides this, models have to be sufficiently simple and fast to apply proper methods of uncertainty analysis, covering model structure deficits and error propagation through the chain of sub-models. Here, we present a new integrated catchment model satisfying both conditions. The conceptual iWaQa model was developed to support the integrated management of small streams. It can be used to predict traditional water quality parameters, such as nutrients and a wide set of organic micropollutants (plant and material protection products), by considering all major pollutant pathways in urban and agricultural environments. Due to its simplicity, the model allows for a full, propagative analysis of predictive uncertainty, including certain structural and input errors. The usefulness of the model is demonstrated by predicting future surface water quality in a small catchment with mixed land use in the Swiss Plateau. We consider climate change, population growth or decline, socio-economic development, and the implementation of management strategies to tackle urban and agricultural point and non-point sources of pollution. Our results indicate that input and model structure uncertainties are the most influential factors for certain water quality parameters. In these cases model uncertainty is already high for present conditions. Nevertheless, accounting for today's uncertainty makes management fairly robust to the foreseen range of potential changes in the next decades. The assessment of total predictive uncertainty allows for selecting management strategies that show small sensitivity to poorly known boundary conditions. The identification of important sources of uncertainty helps to guide future monitoring efforts and pinpoints key indicators, whose evolution should be closely followed to adapt management. The possible impact of climate change is clearly demonstrated by water quality substantially changing depending on single climate model chains. However, when all climate trajectories are combined, the human land use and management decisions have a larger influence on water quality against a time horizon of 2050 in the study.

Highlights

  • Catchments are complex systems where water quantity, quality, and the ecological services provided are determined by interacting physical, chemical, biological, economic, and social factors

  • We found that for some water quality parameters predicted future changes are much smaller than the observed quantile variability and predictive uncertainty together

  • The objectives of this paper were to integrate the current state of knowledge to analyse how water quality of catchments may develop over the decades under the influence of climate change, socio-economic development, and the implementation of different management strategies to improve water quality and to develop a model structure that makes such a comprehensive analysis possible

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Summary

Introduction

Catchments are complex systems where water quantity, quality, and the ecological services provided are determined by interacting physical, chemical, biological, economic, and social factors The awareness of these interactions led to the prevailing catchment management paradigm of Integrated Water Resources Management (IWRM) (GWP, 2000). The reduction of suspended solid loads increases water transparency, which, given enough residence time in large rivers such as the Danube, can lead to advanced eutrophication (ICPDR, 1999) This could have been forecasted by an integrated modelling approach, but side effects can come from outside the simulation domain of catchment models; the revitalization of a section of the Aare River in Switzerland has improved recreational attraction, which caused an unexpected increase in disturbance and waste load (Witschi and Käufeler, 2014)

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