MODELLING THE EFFECTS OF THERMAL STRATIFICATION ON THE OXYGEN BUDGET OF AN IMPOUNDED RIVER ANNETTE BECKER, a * VOLKER KIRCHESCH, a HELMUT Z. BAUMERT, b HELMUT FISCHER a and ANDREAS SCHO ¨ L a a Federal Institute of Hydrology, Koblenz, Germany b IAMARIS (Institute for Applied Marine and Limnic Studies), Hamburg, Germany ABSTRACT The River Saar is a heavily impounded river with an average discharge of 80 m 3 s 1 . The German reach of the River Saar, i.e. the lower 90 km, was gradually impounded from 1977–2000, resulting in a doubled average water depth (today 4.2 m). In parallel to river development, water pollution was decreased strongly, relieving the critical oxygen budget in the river. Nowadays, oxygen concentrations may still fall below 4 mg O 2 l 1 during low-flow periods in summer, when thermal stratification and depth gradients of oxygen occur. In August 2005, high-resolution measurements of temperature, conductivity, turbulence, as well as oxygen-levels were carried out over 48 hours. These data were used to develop and validate a ‘quasi-two-dimensional’, depth- resolving modelling approach with the deterministic water quality model QSim. This model includes the mathematical description of the influence of flow velocity and solar radiation on thermal stratification, on which the exchange between water layers depends. Three data sets of continuous measurements were compared with the model outputs. Thermal stratification shows diurnal rhythms and longitudinal variability depending on solar radiation, water depth, and flow velocity. In the course of the day, measurements and model outputs showed best agreement during the strongest stratification in the evening. The modelled effects of thermal stratification on oxygen-budget rates were quantified and showed that the reduction of atmospheric re-aeration is partly compensated by an increase in algal oxygen production. For the River Saar, the high temporal and spatial variability of oxygen concentrations documented here is of major ecological significance. Copyright # 2009 John Wiley & Sons, Ltd. key words: impounded river; Saar; oxygen budget; thermal stratification; water quality model; 2D modelling; turbulence; atmosopheric re-aeration Received 28 December 2008; Accepted 6 March 2009 INTRODUCTION Oxygen problems in many Western-European large rivers have been strongly reduced by improved wastewater treatment during the last 20 years (EEA, 2003). However, some lowland rivers and heavily regulated rivers still suffer severe oxygen deficits. These rivers are of special interest in water management, particularly in the context of the European Water Framework Directive (EU-WFD). The importance of dissolved oxygen for ecosystem processes, nutrient budgets and riverine organisms makes the oxygen budget a primary subject of numerical modelling in engineering ecology (Chapra, 1997, Williams, 2006). Major activities are devoted to the quantification of oxygen-consuming and producing processes, aiming to predict oxygen concentrations in running waters for management purposes. These studies are often implemented in process-oriented, 1D water-quality models that are used as a tool in waste–water and river-basin management (Billen et al., 1994; Garnier et al., 2001; Scho ¨l et al., 1999, 2002, 2006; Lindenschmidt et al., 2005; Scho ¨l, 2006). In contrast to freely flowing rivers, impounded rivers are more likely to become thermally stratified during summer periods due to their reduced flow velocities, with direct consequences for oxygen levels, plankton and solutes. Regarding the German part of the River Saar, the potential significance of thermal stratification for water quality has been shown earlier through measurements and modelling (Scho ¨l et al., 1999; Scho ¨l, 2006). In the RIVER RESEARCH AND APPLICATIONS River Res. Applic. 26: 572–588 (2010) Published online 22 April 2009 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rra.1260 *Correspondence to: Annette Becker, Federal Institute of Hydrology, Department of Ecology, Am MainzerTor 1, 56068, Koblenz, Germany. E-mail: becker@bafg.de Copyright # 2009 John Wiley & Sons, Ltd.