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Experiences in water quality simulations of a medium-sized agricultural temporary river basin

Author(s): Application of the tempQsim - Stream model to the catchment of the Vène
Congress: 2008
Division of Water Resources Management Institute for Water Quality and Waste Management (ISAH) Gottfried Wilhelm Leibniz Universität Hannover Am Kleinen Felde 30 D - 30167 Hannover Germany

Keyword(s): temporary river, water quality simulation, first flush, Mediterranean climate
Article: Poster:
AbstractKnowledge about the nature of droughts and the functioning of ecosystems which are affected by extended dry periods is still rudimentary. There is a rising awareness, that droughts are important events in many natural systems and that their removal can be even more harmful than their occurrence. However, the majority of rivers in Europe is already heavily altered and far away from their natural state, which is often unknown or difficult to define. In regions where anthropogenic pressures are high, where intensive agriculture, industry and the existence of sewage water inflows exist, there is a need for measures to mitigate the impact in these sensitive ecosystems, especially in the transition period just before or during the early rainy season. The implementation of the EU Water Framework Directive requests a more adapted understanding of processes in water quality and ecosystem functioning, an adequate monitoring and the specific design of management options. It is well known, that temporary streams form an important part of Mediterranean basins, but the occurrence of first flush effects in such water bodies is still poorly understood. The comparison of effects is hindered, as long as no suitable methods and criteria are available. This work shows the experiences in the improvement and application of a water quality model, built to simulate the important fluctuations of nutrients flows in these systems. For this purpose, common approaches have been reviewed and tested during real applications on Mediterranean basins. The tempQsim – STREAM model was developed as part of the MOHID Water Modelling System. The model is capable of simulating water quality in the aquatic and benthic phase and the development was especially focused on the reproduction of processes occurring in temporary rivers. It is discretised on finite volumes and the timestep adapts to the actual flow conditions to guarantee stability. Further, unlike many other models, it allows the quantification of settled materials at the channel bed also over periods, when the river falls dry. These features are very important to secure mass conservation in highly varying flows of temporary rivers. The water quality part of the model was based on WASP and was further modified. This model was applied to the Vène river at the France Mediterranean coast and it shows good coincidence with the measurements on a medium term perspective. Important processes of instream accumulation of organic matter during the dry phase could be reproduced correctly. The simulations showed the importance of small summer events, which are negligible in terms of maximum flow rate and flow volume, but are entering the system in a state of high pollution potential. The a highly contaminated discharge of an event was estimated to bring up to 10% of the annual load of VSS (volatile suspended solids).
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