Exploring the interplay between infiltration dynamics and Critical Zone structures with multiscale geophysical imaging: A review

Infiltration is the process of water entering into, and routings through, the subsurface. It has a profound impact on water flux in the Earth's Critical Zone (CZ), influencing phenomena such as overland flow, stream generation, plant available water, and groundwater recharge, as well as the associated CZ processes and functions of weathering, erosion, slope stability, and food production. However, reliable measurement of infiltration in structured geomedia (soils and regolith) with hidden subsurface features remain challenging. The increasing availability of geophysical techniques has helped them emerge as viable options for minimally invasive determination of infiltration at the pedon to hillslope or catchment scales. Geophysical methods replace the educated guesswork of interpreting field data from traditional infiltrometers with direct visualization and quantification. This paper provides a comprehensive literature review of various applications of geophysical techniques used to (1) visualize infiltration-relevant structures, (2) monitor non-uniform (or preferential) infiltration patterns, and (3) map soil moisture distribution after infiltration. It then proposes a new conceptualization (form-and-function dualism) for subsurface hydrology to link preferential infiltration to CZ structures, with the goal of formulating more realistic infiltration theories related to subsurface structural heterogeneity. The characterization of both subsurface structures (form) and infiltration dynamics (function) via geophysical imaging should help infiltration models, currently emphasizing macroscale behaviors rooted in the continuum concept, expand to accommodate the microscale nature of preferential flow. Integrating multiple geophysical approaches and developments into the new generation of geophysical techniques will further enhance our ability to see into the fabric of the subsurface and capture the dynamics of non-uniform infiltration through the hidden half of the CZ. This approach offers exciting opportunities for understanding the complex interactions between CZ structures and CZ processes.