Development of sixth generation model schematisation 3D D-Water Quality Veerse Meer : model set-up and calibration

Recent water quality problems in the Veerse Meer have renewed efforts to develop modelling tools for the lake. In the context of setting up the new sixth-generation models for Rijkswaterstaat, a 3D D-Water Quality model of the Veerse Meer including eutrophication parameters, dissolved oxygen, and related water quality processes has been developed. During this first phase of model development an initial water quality model was set up with the purposes of contributing to understanding of the system dynamics and biochemical processes which underly system functioning in the Veerse Meer. The model schematisation uses the latest advances in online coupling to the D-HYDRO hydrodynamic model and shares the same geometry. Two full years (2011-2012) of observations were used for initial calibration of the model parameters. After calibration, the model reproduced spatial and seasonal dynamics in algae growth, spring bloom behaviour, nutrient depletion and low oxygen in deep water resulting from stratification during summer months in agreement with the current understanding and observation data. The model simulated winter concentrations well, however, summer concentrations of chlorophyll-a are generally overestimated and nutrient and dissolved oxygen concentrations underestimated. During calibration of the eutrophication parameters it became apparent that grazing of algae by higher trophic organisms likely plays an important role in Veerse Meer. Observed chlorophyll-a concentrations are lower than predicted without grazing using typical algae growth and mortality processes, suggesting that algae populations are limited by predation. During winter primary production is limited by light, so adjustment of the specific coefficient for light extinction, and especially background extinction, were most effective in calibration of winter concentrations. Analysis of the model results for 2011 and 2012 shows that transport in deeper sections of the Veerse Meer is highly dynamic and oxygen processes in the summer are dominated by sediment oxygen demand and mixing with inflow from the Eastern Scheldt via the Katse Heule. Inflow from Katse Heule is effective in consistently bringing oxygenated water to the eastern half of the lake but is less effective in the western half where dissolved oxygen concentrations decrease quickly during periods of low inflow which can last up to several days. After initial calibration, the model was shown to accurately reflect the general system dynamics of the Veerse Meer related to eutrophication and dissolved oxygen. It can be used for qualitative assessments to investigate the research questions into system functioning of the system analysis including the effect of potential measures, i.e. to predict the direction and order of magnitude of change. A model validation should be carried out, before a judgement on the application for quantitative assessments, i.e. prediction of concentrations and effect of potential measures with an accuracy still to be defined, can be given. Validation of the model on more recent years is foreseen, to be executed in the first quarter of 2022, in parallel with and contributing to a system analysis including analysis of the additional monitoring data collected in 2021. The validation result and an extended assessment of model performance and model applicability will be added to an updated version of this report.