Abstract

Global sensitivity analysis (GSA) is increasingly used for interpreting complex hydrogeological models and iden tifying the relative importance of different sources of uncertainty. Although classical methods focus primarily on parameter uncertainty, the role of geological uncertainty, arising from incomplete knowledge of the spatial arrangement of the hydrogeological facies, may be equally relevant. In this study, we propose an extension of the Morris' Elementary Effects Test to incorporate geological uncertainty represented through multiple geostatistical facies realizations in a GSA framework. This general approach enables a combined assessment of the influence of parametric and geological uncertainty within a computationally parsimonious diagnostic framework, iden tifying the factors that most strongly affect different model outputs. The methodology is applied to a complex regional-scale aquifer system located in northern Italy, with integrated modeling of river-aquifer interactions. Across all investigated outputs, the saturated hydraulic conductivity of the gravelly aquifers consistently emerges as the dominant source of uncertainty. However, the spatial arrangement of the facies and the hydraulic prop erties of the confining units can locally influence hydraulic heads and advective particle tracking as much as aquifer parameters. We also show that parameters controlling river-aquifer exchange processes can affect advec tive transport pathways even in deep confined aquifers. The proposed approach provides actionable guidance for model refinement, data prioritization, and sensitivity-informed parameter selection.