IAHR and China Institute of Water Resources and Hydropower Research (IWHR) jointly organized a webinar on Roles of Uncertainty in the Assessment of Subsurface Flow and Transport on 2 March 2023, with the keynote lecture given by Prof. Alberto Guadagnini, Politecnico di Milano, Italy, also the chair of IAHR’s Technical Committee on Groundwater Hydraulics and Management. The webinar was chaired by Dr. Xin He, an IWHR professor and also a member of the technical committee.

In his report, Prof. Guadagnini pointed out that modern models of flow and transport across environmental and industrial porous media have reached a remarkable level of complexity, with the aim of capturing the nature of target phenomena and processes. The level of complexity of these models could hamper unambiguous understanding of relationships among model inputs and outputs of interest. This is a critical aspect in a variety of scenarios, including flow and reactive transport in heterogenous aquifers and sustainable use of underground resources. All of these settings are associated with multiple sources of uncertainty. These are typically linked to (i) our conceptual understanding of how a natural system functions and the possibility of depicting its key features through various modeling approaches/formulations, (ii) our knowledge of model parameters and/or initial and boundary conditions, and (iii) the amount of available data/information and the scale with which these are associated. In this broad context, emphasis is here devoted to model diagnosis through (moment- and distribution- based) local and global sensitivity analyses embedding model, process, and parameter uncertainty. Exemplary applications encompass a variety of scales (from pore- to field-scale) and processes, including reactive chemical transport and contaminants of emerging concern.

The panel discussion following the lecture was joined by Prof. Mohaddeseh Mousavi Nezhad from University of Warwick, UK and Prof. Felipe de Barros from University of Southern California.

Regarding the scale effect of parameters, Prof. Guadagnini believed that most of the parameters of groundwater models are actually difficult to measure directly, and it is necessary to link the inversion process of model parameters to state variables, and indirectly reflect the scale effect of parameters through the observation of different state variables.

Regarding the practical application of the concept of uncertainty in management, Prof. Nezhad believed that the acceptance of model uncertainty in the industrial circles of Western countries has increased significantly, and managers do not want to get the only definite answer but hope to get a set of possible answers and the risks corresponding to these answers.

Regarding the impact of new machine learning algorithms on the uncertainty of groundwater simulation, Prof. De Baros believed that although this is a hot research direction, machine learning algorithms cannot essentially improve our understanding of physical processes, and the current more feasible way is to make some black-box models gradually clear through the method of proxy models, but at the same time, we should pay attention to the general lack of groundwater observation data, so excessive extrapolation should be avoided when using proxy models.

Also discussed were topics including the impact of climate change extremes on groundwater modeling, and the uniformity of mathematical formula expression of different sources of uncertainty.

For the past years, IAHR's Technical Committee on Groundwater Hydraulics and Management has been organized webinars related to various topics:

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