Uncertainty Propagation In Integrated Urban Water QualityModelling

Sensitivity and uncertainty assessment of integrated urban drainage water qualitymodels are crucial steps in the evaluation of the reliability of model results. Indeed, theassessment of the reliability of the results of complex water quality models is crucial inunderstanding their significance. In the case of integrated urban drainage water qualitymodels, due to the fact that integrated approaches are basically a cascade of sub-models(simulating the sewer system, wastewater treatment plant and receiving water body),uncertainty produced in one sub-model propagates to the following ones in a mannerdependent on the model structure, the estimation of parameters and the availability anduncertainty of measurements in the different parts of the system. Uncertainty basicallypropagates throughout a chain of models in which the simulation output from upstreammodels is transferred to the downstream ones as input. The paper presents the uncertaintyassessment of an integrated urban drainage model developed in previous studies by meansof the Generalized Likelihood Uncertainty Estimation (GLUE) methodology. Astraightforward approach based on the analysis of the coefficient of variation (Rxy). Rxy isdefined as the ratio between the standard deviation (α) and the average (μ) value of themodel output of reference taken into account. The analysis has been applied to anexperimental catchment in Bologna (Italy) which consists of a part of the Bologna sewernetwork and a reach of the Savena river. The results showed that the method can be a usefultool for uncertainty analysis and for guiding the operator in the choice of the modellingapproach.