Global sensitivity analysis of transformation processes in a river water quality model by means of conceptualization

Ingrid Keupers, Patrick Willems

Thursday 2 july 2015

14:20 - 14:35h at Asia (level 0)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology

Parallel session: 12G. Environment - Impact

The European Water Framework Directive commits member states to achieve a good water quality status of all their river bodies by 2015. To assess whether this goal will be reached and, if not reached, which measures can be undertaken to remediate the situation, an integrated hydrological-hydrodynamic-water quality river model is required that can simulate the concentrations of relevant water quality state variables in rivers under given boundary conditions. The use of integrated, detailed physically-based models poses restrictions on the calculation time which complicates the model calibration. A solution is proposed to complement the detailed physically-based model by a simplified, conceptual model based on reservoir-type modeling. We show that such an approach allows reducing the model computational times with a factor 5000, without significant loss in accuracy. The proposed approach was applied to the Grote Laak, a river of 13 km situated in the North East of Belgium. The resulting conceptual model is used for sensitivity analysis by means of Monte Carlo simulations to determine the most sensitive parameters. Latin hypercube sampling was used to select 5000 random parameter sets from their probability distribution as determined by expert judgment. A multiple linear regression is performed on the standardized inputs and outputs of the model to obtain the standardized regression coefficient (SRC) for each of the parameters and allow for a ranking of the parameters. The global sensitivity analysis indicates that from the 23 model parameters that define the water quality transformation processes, only 9 have a significant contribution to the uncertainty in the model output result. Next steps are to perform the model calibration, focusing on the identified sensitive parameters, and application of the integrated hydrological – hydrodynamic – water quality conceptual model for water management decision support.