Date of Award

August 2026

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Engineering

First Advisor

Taufique Mahmood

Abstract

Shallow unconfined aquifers in cold agricultural regions represent critical freshwater resources that sustain food production, ecosystems, and rural communities. In the Northern Great Plains (NGP) of North America, these aquifers are strongly influenced by seasonal snow accumulation, soil freezing and thawing cycles, rainfall variability, surface water connectivity, and groundwater pumping. Despite their importance, key uncertainties persist regarding how recharge processes are partitioned between snowmelt and rainfall, how frozen soil dynamics regulate infiltration, and how interacting climatic and anthropogenic drivers govern long-term groundwater storage. Addressing these gaps is essential for improving predictions of aquifer response under evolving hydroclimatic conditions.This dissertation develops and applies an integrated, process-based modeling framework to quantify recharge mechanisms, groundwater storage dynamics, and flux redistribution in the Oakes aquifer of southeastern North Dakota over the 1990–2024 period. The framework combines the Cold Regions Hydrological Model (CRHM) and HYDRUS-1D to simulate physically based recharge under snow-dominated conditions, which is subsequently coupled with a groundwater flow model to represent aquifer storage changes, river and wetland exchanges, drainage, and seasonal pumping. To further elucidate dominant controls on groundwater storage variability, a machine-learning–based attribution analysis is incorporated to quantify the relative influence of recharge, surface water connectivity, and pumping across distinct climatic phases. Results demonstrate that groundwater recharge in this cold-region system is primarily driven by spring and summer rainfall, while snowmelt contributions vary depending on winter temperature regimes, soil freezing duration, and melt intensity. Prolonged frozen soil conditions restrict infiltration and enhance surface runoff, whereas warmer winters with intermittent thaw events facilitate greater infiltration potential. Recharge exhibits pronounced spatial heterogeneity controlled by soil texture, elevation gradients, land cover, and surface depressions, with coarser-textured zones contributing disproportionately to aquifer replenishment. These findings underscore the importance of explicitly representing snow processes and freeze–thaw dynamics in recharge estimation for cold regions. Long-term groundwater simulations reveal that, despite significant seasonal drawdowns during peak irrigation pumping, the aquifer experienced a net cumulative storage gain over the 35-year study period, corresponding to a prolonged regional wet climatic phase. However, flux partitioning analysis indicates substantial redistribution of groundwater exchanges, including shifts in river–aquifer and wetland–aquifer connectivity during wetter intervals. Machine-learning attribution identifies a temporal evolution in dominant storage controls, transitioning from a recharge-controlled regime in earlier decades to increasing influence of pumping and hydraulic connectivity in later years. This shift suggests emerging structural changes in aquifer response under combined climatic variability and human water use. Collectively, this research advances understanding of cold-region groundwater systems by linking physically based recharge processes to basin-scale storage dynamics within a unified modeling framework. The dissertation provides new insight into how snowmelt, rainfall, frozen soil processes, landscape heterogeneity, and groundwater extraction interact to shape long-term aquifer resilience. Beyond the study region, the integrated methodology and findings offer transferable tools and conceptual understanding for sustainable groundwater management in cold agricultural landscapes facing intensifying hydroclimatic variability and evolving water demands.

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Available for download on Friday, September 08, 2028

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