Unravelling the output uncertainty of a tree water flow and storage model using several global sensitivity analysis methods

The output uncertainty of two variables (stem sap flow rate and stem diameter variations) of a tree water flow and storage model is broken down into its different constituents using two global sensitivity analysis methods: extended Morris screening and an extended Fourier amplitude sensitivity test. Using these methods, quantitative and qualitative information about the parameters contributing most to the model output uncertainty was obtained. It was shown that only three parameters (out of the 11 considered) were responsible for the uncertainty on the stem sap flow rate and that three other parameters were accountable for most of the uncertainty in stem diameter variations. Furthermore, the parameters influencing the stem diameter variations were involved in significant interactions. An investigation into the effect of the magnitude of the parameter uncertainty on the results of the sensitivity analysis showed that the contribution of the parameter interactions increased with increasing parameter uncertainty. By combining several global sensitivity analysis methods the results were not only verified but more confidence was gained in the accuracy of the methods used and complementary information was obtained. This allowed a more detailed picture to be constructed of how the individual parameters interact and contribute to the output uncertainty of the model.