Earth’s “critical zone” — the zone of the planet from treetops to base of groundwater — is critical because it is a sensitive region and open to impacts from human activities, while providing water necessary for human consumption and food production. Quantifying water movement and stores in the subsurface are fundamental to predicting how water-driven critical zone (CZ) processes will respond to changes in climate and human perturbation.  While shallow soils and above-ground parts of the CZ can be easy to instrument and explore, the deeper parts of the CZ—through the soils and into rock—are harder to access, leaving many open questions about the role of water in this environment.

Here, we will explore the role of geophysics as a “macroscope” for providing information on a number of key CZ properties and processes: developing links between tree-water use and subsurface water stores, exploring controls on water infiltration and exchanges in 3-D, constraining geochemical reactions, and testing hypotheses about subsurface weathering and the subsequent controls on CZ processes.  Geophysical tools are central to the quantitative study of processes where we don’t have easy access for observation.