A comparison between the uncertainties in model parameters and in forcing functions: its application to a 3D water-quality model

This paper presents the application of both local and global sensitivity analysis techniques to an estimation of the uncertainty of the output of a 3D reaction-diffusion ecological model, which describes the seasonal dynamics of dissolved Nitrogen and Phosphorous, and that of the phytoplanktonic and zooplanktonic communities in the lagoon of Venice. Two sources of uncertainty were taken into account and compared: 1) uncertainty concerning the parameters of the governing equation; 2) uncertainty concerning the forcing functions. The mean annual concentrations of Dissolved Inorganic Nitrogen, DIN, was regarded as model output, as the current Italian legislation sets a quality target for Total Dissolved Nitrogen in the lagoon of Venice. Local sensitivity analysis was initially used, so as to rank the parameters and provide an initial estimation of the uncertainty, which is due to an imperfect knowledge of the dynamic of the system. This uncertainty was compared with that induced by an imperfect knowledge of the loads of Nitrogen, which represent the main forcing functions. On the basis of the results of the local analysis, the most important parameters and loads were then taken as the sources of uncertainty, in an attempt to assess their relative contribution. The global uncertainty and sensitivity analyses were carried out by means of a sampling-based Monte Carlo method. The results of the subsequent input-output regression analysis suggest that the variance in model output could be partitioned among the sources of uncertainty in accordance with a linear model. Based on this model, 87.8% is due to the uncertainty in the parameters which specify the dynamics of phytoplankton and zooplankton only % of the variance in DIN mean annual concentration is accounted for by the uncertainties in the three main source, while.