UC Davis

Abstract In typical clastic sedimentary aquifer systems, interbedded coarse‐ and fine‐grained deposits impede vertical flow and promote horizontal flow. This effect, known as vertical anisotropy, is often underestimated in regional groundwater models, leading to overestimated vertical aquifer communication. The issue stems partly from dependence on vertically insensitive calibration targets, such as laterally distributed observations of groundwater levels. As a result, models may appear well‐calibrated while differing in their representation of key processes, including surface water–groundwater exchange, recharge, and solute transport pathways. Yet few studies have demonstrated how poorly constrained vertical flow parameters can bias model outcomes. California's Sacramento Valley (19,800 km 2 ) illustrates the value of observed vertical hydraulic gradients (VGs—representing head change with depth) for constraining aquifer parameters during model calibration. Of three publicly available models developed for this region, only one uses VGs from 81 continuously monitored well nests to calibrate aquifer parameters. Although all three models reproduce observed heads comparably well, only this model also reproduced observed VGs, exhibited stronger vertical anisotropy than the other two, and simulated markedly different water budgets. This stronger anisotropy corresponds with a vertical flow field that better matches expected recharge and discharge patterns. In addition, observed VGs appear to be increasing at most well nests (56 of 81), reflecting strengthening downward (or weakening upward) flow potential. Continued increases could intensify surface water losses and expose deeper aquifers to contamination. Our findings highlight the importance of VG‐informed monitoring and model development for detecting and managing emerging risks in sedimentary aquifer systems. Key Points Regional groundwater models reproduce heads but diverge in vertical flow and budgets, revealing key model contrasts Calibrating to vertical head gradients increased vertical anisotropy and yielded a more defensible vertical flow field Observed vertical head gradients are increasing across California's Sacramento Valley, which signals emerging groundwater risk