Review of integrating thermal, electrical, and seismic geophysical methods for shallow groundwater modeling in the critical zone

Groundwater systems are components of the Critical Zone, in dynamic balance with the climate and human pressure. Water and heat fluxes control biogeochemical processes and regulate both resource potential and system vulnerability. Evaluating system responses to climate and land use changes requires an estimation of subsurface structure, flow dynamics, and the spatial distribution of recharge and discharge zones. Classical hydrological data, such as water levels and river discharge, are commonly coupled with unconventional methodologies, including heat tracing, electrical resistivity tomography, and surface wave analysis. Their coupling via time-lapse monitoring and inversion frameworks enables the characterization of thermal, electrical, and mechanical interactions, thus encouraging a transition from static structure description to transient representations of groundwater. We identify key challenges, including data limitations, model uncertainty propagation, and the integration of transient variables and parameters into inversion workflows. Petrophysical and geostatistical approaches help overcome these issues by coupling geophysical and groundwater models, quantifying spatio-temporal uncertainties, and addressing scale change. Finally, choices regarding experimental design, parameter reduction, dimensionality, model hypotheses, and inversion scale must be assessed to balance parsimony with model accuracy. These developments underscore the central role of hydrogeophysics in advancing Critical Zone science and sustainable groundwater management. Emphasizing transient processes is essential for capturing how subsurface systems evolve over time in response to environmental changes.