Manual Sensitivity Analysis to Enhance a Previously PEST-Calibrated Shallow Aquifer and Aquitard Breach Model
Publication: Journal of Hydrologic Engineering
Volume 28, Issue 12
Abstract
Numerical groundwater modelers often encounter challenges in computing and implementing regional unconfined aquifer models that match real-world water table undulations and local features. A commonly-practiced simplifying assumption in groundwater models states that incorporating extensive real-world detail does not correlate to better model results. This study discusses how this assumption may lead to unrepresentative models due to oversimplification. Sometimes, when determining the appropriate complexity of regional models, important elements of the conceptual model may be oversimplified, resulting in a nonrepresentative model or part of a model. This study tests the hypothesis that, by incorporating specific areas of greater hydrogeologic complexity into modeling a regional-sized unconfined aquifer, simulated heads will significantly better match observed conditions, especially in areas where unconfined groundwater discharges through aquitard breaches and where dry cells are a natural (realistic) condition of the system. Three surface processes (additional recharge, stream density, and evapotranspiration) were tested in multiple sets of impact models. Model outcomes were compared against observed conditions of the shallow unconfined aquifer beneath Memphis, TN, US, and against the parent fully calibrated groundwater model of the aquifer systems beneath Shelby County, TN. Results indicate that an increase in stream detail better mimics observed water table undulations. These undulations occur at the same scale as aquitard breaches and improve appropriate gradients and flow surrounding the breaches. Results further indicate that the addition of greater hydrogeologic complexity to the shallow unconfined aquifer improves models like that of Shelby County, TN, by better representing realistic gradients, conveying appropriate quantities of water through breaches, and offering an opportunity for improved contaminant transport modeling from the shallow unconfined aquifer to the deeper semiconfined Memphis aquifer via aquitard breaches.
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Data Availability Statement
Some or all data, models, or codes that support the findings of this study are available from the corresponding author upon reasonable request, including raster files for all reported model results; Excel files for dry cell counts; Excel files for model flow budgets and error reports; and Model.gpr files.
Acknowledgments
Great appreciation and gratitude go to Memphis Light Gas and Water for their interest in the world of groundwater and for funding these endeavors. Furthermore, we would like to extend our gratitude to the ASCE reviewers, and whose comments contributed significantly to the improvement of the article.
Author contributions: Drs. Waldron and Jazaei guided the study at several stages. Mr. Pierce developed all numerical models and conducted the analyses. All authors discussed and revised several drafts of this manuscript, approving this final version.
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Published In
Journal of Hydrologic Engineering
Volume 28 • Issue 12 • December 2023
Copyright
© 2023 American Society of Civil Engineers.
History
Received: May 16, 2022
Accepted: Apr 24, 2023
Published online: Sep 28, 2023
Published in print: Dec 1, 2023
Discussion open until: Feb 28, 2024
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