Modelling of dissolved oxygen and biochemical oxygen demand in river water using a detailed and a simplified model

Abstract

Different model types are available to model catchment surface water quantity and quality. They vary from detailed physically‐based models to simplified conceptual and empirical models. The most appropriate model type for a certain application depends on the project objectives and the data availability. The detailed models are very useful for short‐term simulations of representative events. For long‐term statistical information and as a management tool they cannot be used. For that purpose more simplified (conceptual or meta) models have to be used. In this study, dissolved oxygen (DO) and biochemical oxygen demand (BOD) dynamics are modelled in a river in Flanders. BOD sources from agricultural leaching and domestic point sources are considered. Based on this input, concentrations of DO and BOD in the river water are modelled in Mike11 (river modelling software from DHI Water & Environment). Advection and dispersion were taken into consideration, together with the most important biological and chemical processes. Model calibration was done on the basis of available measured water quality data. A more simplified model was calibrated to this detailed model, with the objective to yield more easily long‐term simulation results which can be used in a statistical analysis. Two aspects of adequacy of model results are highlighted, namely accuracy and model speed. The conceptual simplified model is 1800 times faster than the Mike11 model. Moreover, the two models have almost the same accuracy. The construction of the simplified model is, however, only possible using simulations with the detailed model. The detailed and the simplified model have to be used in a complementary way.

Keywords:

• Water quality modelling
• simplified model
• sensitivity analysis