Integrated hydrologic modeling of groundwater flow dynamics and recharge in the San Joaquin Valley

Authors: CASILLAS-TRASVINA, ALBERTO - University Of California, Davis; Meles, Menberu; CUI, WENYI - University Of California, Davis; HATCH, TYLER - Intera, Inc; Bradford, Scott; HARTER, THOMAS - University Of California, Davis

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/21/2025
Publication Date: 4/22/2025
Citation: Casillas-Trasvina, A., Meles, M.B., Cui, W., Hatch, T., Bradford, S.A., Harter, T. 2025. Integrated hydrologic modeling of groundwater flow dynamics and recharge in the San Joaquin Valley. Journal of Hydrology. 660(A). Article 133377. https://doi.org/10.1016/j.jhydrol.2025.133377.
DOI: https://doi.org/10.1016/j.jhydrol.2025.133377

Interpretive Summary: Groundwater resources in California’s Central Valley, particularly in the Modesto, Turlock, and Merced subbasins, are critically stressed due to historical overdraft and recurring droughts. To address these challenges, this study developed a high-resolution, integrated hydrologic model to better understand groundwater recharge and flow dynamics in these areas. By combining two advanced tools—HYDRUS-1D for simulating water movement in the unsaturated zone and MODFLOW-2005 for three-dimensional groundwater flow—the model provides detailed insights into how groundwater moves through the landscape and how much water recharges the aquifer. The model revealed significant variability in recharge rates across the subbasins, ranging from 0.5 mm to 1460 mm per year. Higher recharge rates were observed in areas dominated by almond, walnut, and grain crops, while lower rates occurred in grasslands and urban zones. The study also highlighted the severe impact of groundwater pumping in East Turlock, where excessive extraction has caused a significant drop in groundwater levels, creating a large "cone of depression"—a low point that pulls water from surrounding areas. The findings show that groundwater can take anywhere from 2 to 5000 years to reach the cone of depression, with faster replenishment occurring when recharge happens near the center of the cone in areas of intensive pumping. This variability in recharge rates and residence times underscores the need for targeted recharge efforts, particularly in high-recharge areas, to effectively replenish depleted groundwater. In conclusion, this study offers a comprehensive tool for understanding groundwater dynamics and provides a detailed modeling approach that can help formulate effective groundwater management plans. These insights are crucial for addressing the long-term sustainability of groundwater resources in California’s Central Valley.

Technical Abstract: The Turlock and Modesto subbasins in California’s Central Valley face critical groundwater sustainability challenges exacerbated by historical overdraft and recurrent droughts. This study develops a high-resolution, integrated hydrologic model to enhance understanding of groundwater dynamics, recharge processes, and sustainable management strategies under the Sustainable Groundwater Management Act (SGMA). The model integrates spatially distributed Hydrogeologic Response Units (HGRUs) using HYDRUS-1D for variably saturated flow and MODFLOW-2005 for 3D groundwater flow, all at a high spatial resolution of 300 by 300 meters. Key findings from this integrated method include spatially varying recharge rates ranging from 0.5 mm to 1460 mm per year across the model domain, with higher rates observed in cultivated areas dominated by almond, walnut and diverse grain crops. The study identifies intensive groundwater pumping in East Turlock as a significant driver of a regional cone of depression that impacts groundwater levels throughout the area. Advective particle tracking reveals faster flow paths from elevated recharge zones towards the cone of depression and groundwater residence times that range from 2 to 5000 years, with an average of 636 years. This research enhances the understanding of groundwater dynamics crucial for formulating effective management strategies under SGMA. By delineating recharge patterns and groundwater flow paths, this study provides water managers with actionable insights to optimize recharge efforts, prioritize areas for managed aquifer recharge, and develop targeted interventions to mitigate overdraft impacts and preserve aquifer resources in the Central Valley of California.