The formulations of SWITCH, a model for prediction of nutrient fluxes across the sediment-water interface, are presented. Results of the application to data on the sediment of Lake Veluwe are presented and discussed.SWITCH calculates the thicknesses of the aerobic and denitrifying layers on the basis of a step-wise steady state approach. The concentrations of detritus, ammonium, nitrate and phosphate in the sediments and the pore water are simulated dynamically using mass balance equations.Analysis of the data for Lake Veluwe show large spatial heterogeneity. This presents a major drawback for the calibration of SWITCH, which focused on the silty part of the lake. The results show that the model simulates realistically and consistently layer thicknesses, concentrations and mass fluxes connected with the transport and conversion processes. The model appears to have potential for describing both seasonal patterns and developments on the long term.SWITCH calculates strongly increased phosphate return fluxes, following total reduction of the top sediments. An important hypothesis in the model is that phosphate precipitated in reduced sediment layers is transferred to the oxidized layer and dissolves instantaneously. This results in a decrease of the phosphorus content of the sediment, but also maintains high release rates of phosphorus after the reduction of the external phosphorus loading of Lake Veluwe. Model results and mass balance studies for the overlying water indicate that the removal of phosphorus to deeper sediment layers is underestimated or that dilution of the sediments occurs as the result of sedimentation.

The content and calibration of the comprehensive generic 3D eutrophication model ECO for water and sediment quality is presented. Based on a computational grid for water and sediment, ECO is used as a tool for water quality management to simulate concentrations and mass fluxes of nutrients (N, P, Si), phytoplankton species, detrital organic matter, electron acceptors and related substances. ECO combines integral simulation of water and sediment quality with sediment diagenesis and closed mass balances. Its advanced process formulations for substances in the water column and the bed sediment were developed to allow for a much more dynamic calculation of the sediment-water exchange fluxes of nutrients as resulting from steep concentration gradients across the sediment-water interface than is possible with other eutrophication models. ECO is to more accurately calculate the accumulation of organic matter and nutrients in the sediment, and to allow for more accurate prediction of phytoplankton biomass and water quality in response to mitigative measures such as nutrient load reduction. ECO was calibrated for shallow Lake Veluwe (The Netherlands). Due to restoration measures this lake underwent a transition from hypertrophic conditions to moderately eutrophic conditions, leading to the extensive colonization by submerged macrophytes. ECO reproduces observed water quality well for the transition period of ten years. The values of its process coefficients are in line with ranges derived from literature. ECO’s calculation results underline the importance of redox processes and phosphate speciation for the nutrient return fluxes. Among other things, the results suggest that authigenic formation of a stable apatite-like mineral in the sediment can contribute significantly to oligotrophication of a lake after a phosphorus load reduction.

No abstract

No abstract

Hydrodynamic and water quality 3D modelling of the Nam Theun 2 Reservoir (Lao PDR): predictions and results of scenarios related to reservoir management, hydrometeorology and nutrient input Modélisation 3D de l'hydrodynamique et de la qualité d'eau du réservoir de Nam Theun 2 : prédictions et résultats de scénarios liés à la gestion du réservoir, à l'hydrométéorologie et à l'apport de nutrimentsV. Chanudet (1) , J. Smits (2) , J. Van Beek (2) , P. Boderie (3) , F. Guérin (4,5,6) , D. Serça (7) , C. Deshmukh (7 Abstract -A 3D water quality model has been applied to predict medium term evolution of the water quality in the Nam Theun 2 Reservoir and also to quantify the effect of various scenarios. 15-year simulations show that the oxygen concentration will continue to increase in the water column although the hypolimnion will remain anoxic in some areas of the Reservoir. In parallel, the concentration of reduced compounds will decrease with time. The significance of the hydrodynamics in water quality evolution is pointed out with two scenarios in which natural or human forcings have been modified. The comparison of simulations made for years with contrasted hydrometeorological conditions shows that and duration of major hydrometeorological related events (rainfall, flood and air temperature drop) have a major influence on the seasonal evolution of water quality in the whole Reservoir. Simulations also show that the physico-chemical quality of the water released downstream of the power house would have been different if the commissioning had been carried out immediately after the impoundment. Finally, the model has been used to quantify the impact of an increase of the NO 3 -and PO 4 3-incoming flux consecutive to potential changes in the watershed land use. The fluxes have been multiplied by a factor two separately (2 scenarios) and together. While the additional load of NO 3 -has almost no impact on physico-chemistry and phytoplankton activity, the INTRODUCTIONWater quality in reservoirs depends on many parameters and processes, including hydrodynamics. Interactions are so numerous (Stumm & Morgan, 1996) that it is difficult to predict or assess precisely the effect on water quality of a modification of forcing data (from change in the watershed to global change). A possible approach to overcome this difficulty is numerical modelling. Since the 1970s many water quality and ecological processbased models have been developed (Jorgensen et al., 1996) and the studies of water quality and ecosystem problems with numerical models are increasing rapidly (Cerco & Cole, 1993;Jorgensen, 1999;Gin et al., 2001;Arhonditsis & Brett, 2005;Chao et al., 2007). Some well-established threedimensional water quality models have been successfully applied to simulate water quality variables in rivers, lakes, estuaries and ocean environments (Cerco & Cole, 1995;Luyten et al., 1999;Antenucci et al., 2000;Wool et al., 2001;Hydroqual, 2004;Romero et al., 2004; Danish Hydraulic Institute, 2001;Deltares, 2013). This type of model has be...