Uncertainties In Saltwater Intrusion Due To Model Physics And Parameter Decisions
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Saltwater intrusion (SWI) driven by rising sea levels threatens coastal infrastructure and ecosystems. Modeling this process requires numerous decisions about model physics that introduce uncertainties in our estimation of SWI, but most modelling studies only quantify uncertainty due to parameter choices (for example, permeability) once the model physics has been constrained. Our goal was to evaluate both model and parameter uncertainty in estimating freshwater–saltwater interface movement due to sea level rise and climate-induced changes in groundwater recharge. We evaluate a 1D analytical (sharp-interface) solution, 2D and 3D sharp interface models, and fully coupled 2D flow and salt transport models in a synthetic coastal landscape that experiences 1 m of sea level rise and various changes in groundwater recharge. Sharp interface models use an experimental version of a new SWI package implemented in MODFLOW 6. The salt transport model simulates variable-density groundwater flow coupled with dispersive salt transport. Preliminary results indicate that hydraulic conductivity is the dominant source of uncertainty across all recharge conditions. Model selection uncertainty becomes more pronounced under specific recharge scenarios, including strongly drying and stable scenarios. These findings highlight the importance of constraining hydraulic conductivity to improve SWI predictions. Additionally, we identify parameter and data ranges where analytical and sharp interface models significantly deviate from the full numerical model, offering guidance for future SWI assessments.