Comparison of a one-at-a-step and variance-based global sensitivity analysis applied to a parsimonious urban hydrological model

Abstract:

A sensitivity analysis was conducted on an existing parsimonious model aiming to reproduce flow in engineered urban catchments and sewer networks. The model is characterized by his parsimonious feature and is limited to seven calibration parameters. The objective of this study is to demonstrate how different levels of sensitivity analysis can have an influence on the interpretation of input parameter relevance in urban hydrology, even for light structure models. In this perspective, we applied a one-at-a-time (OAT) sensitivity analysis (SA) as well as a variance-based global and model independent method; the calculation of Sobol indexes. Sobol’s first and total effect indexes were estimated using a Monte-Carlo approach. We present evidences of the irrelevance of calculating Sobol’s second order indexes when uncertainty on index estimation is too high. Sobol’s method results showed that two parameters drive model performance: the subsurface discharge rate and the root zone drainage coefficient (Clapp exponent). Interestingly, the surface discharge rate responsible flow in impervious area has no significant relevance, contrarily to what was expected considering only the one-at-a-time sensitivity analysis. This last statement is clearly not straightforward. It highlights the utility of carrying variance-based sensitivity analysis in the domain of urban hydrology, even when using a parsimonious model, in order to prevent misunderstandings in the system dynamics and consequent management mistakes.