Observing the critical zone (CZ) below the top few meters of readily excavated soil is challenging yet crucial to understanding Earth surface processes. Near‐surface geophysical methods can overcome this challenge by imaging the CZ in three dimensions (3‐D) over hundreds of meters, thus revealing lateral heterogeneity in subsurface properties across scales relevant to understanding hillslope erosion, weathering, and biogeochemical cycling. We imaged the CZ under a soil‐mantled ridge developed in granitic terrain of the Laramie Range, Wyoming, using data from five boreholes and a 3‐D volume (970 by 600 by 80 m) of seismic velocities generated by ordinary kriging of 25 two‐dimensional seismic refraction transects. The observed CZ structure under the ridge broadly matches predictions of two recently proposed hypotheses: the uppermost surface of weathered bedrock is consistent with subsurface weathering driven by bedrock drainage and subsurface topography defining the top of unweathered protolith is consistent with fracturing predicted from topographic and regional stresses. In contrast, differences in slope aspect along the ridge are too subtle to explain observed variations in regolith structure. Our observations suggest that multiple processes, each of which may dominate at different depths, work in concert to regulate deep CZ structure.

Enhanced understanding of subsurface water storage will improve prediction of future impacts of climate change, including drought, forest mortality, wildland fire, and strained water security. Previous research has examined the importance of plant‐accessible water in soil, but in upland landscapes within Mediterranean climates, soil often accounts for only a fraction of subsurface water storage. We draw insights from previous research and a case study of the Southern Sierra Critical Zone Observatory to define attributes of subsurface storage; review observed patterns in their distribution; highlight nested methods for estimating them across scales; and showcase the fundamental processes controlling their formation. We review observations that highlight how forest ecosystems subsist on lasting plant‐accessible stores of subsurface water during the summer dry period and during multiyear droughts. The data suggest that trees in these forest ecosystems are rooted deeply in the weathered, highly porous saprolite or saprock, which reaches up to 10–20 m beneath the surface. This review confirms that the system harbors large volumes of subsurface water and shows that they are vital to supporting the ecosystem through the summer dry season and extended droughts. This research enhances understanding of deep subsurface water storage across landscapes and identifies key remaining challenges in predicting and managing response to climate and land use change in mountain ecosystems of the Sierra Nevada and in other Mediterranean climates worldwide. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water Extremes Water and Life > Nature of Freshwater Ecosystems

As bedrock weathers to regolith – defined here as weathered rock, saprolite, and soil – porosity grows, guides fluid flow, and liberates nutrients from minerals. Though vital to terrestrial life, the processes that transform bedrock into soil are poorly understood, especially in deep regolith, where direct observations are difficult. A 65-m-deep borehole in the Calhoun Critical Zone Observatory, South Carolina, provides unusual access to a complete weathering profile in an Appalachian granitoid. Co-located geophysical and geochemical datasets in the borehole show a remarkably consistent picture of linked chemical and physical weathering processes, acting over a 38-m-thick regolith divided into three layers: soil; porous, highly weathered saprolite; and weathered, fractured bedrock. The data document that major minerals (plagioclase and biotite) commence to weather at 38 m depth, 20 m below the base of saprolite, in deep, weathered rock where physical, chemical and optical properties abruptly change. The transition from saprolite to weathered bedrock is more gradational, over a depth range of 11–18 m. Chemical weathering increases steadily upward in the weathered bedrock, with intervals of more intense weathering along fractures, documenting the combined influence of time, reactive fluid transport, and the opening of fractures as rock is exhumed and transformed near Earth’s surface.

In high-mountain watersheds, the critical zone holds crucial life-sustaining water stores in the form of shallow groundwater aquifers. To better understand the role that the critical zone plays in moderating hydrologic response to fluxes at the surface and in the subsurface, the hydrologic properties must be characterized over large scales (i.e., that of the watershed). In this study, we estimate porosity from geophysical measurements across a 58-ha area to depths of~80 m. Our observations include velocities from seismic refraction, downhole nuclear magnetic resonance logs, downhole sonic logs, and samples acquired by push coring. We use a petrophysical approach by combining two rock physics models, a porous medium for the saprolite and a differential effective medium for the fractured rock, into a Bayesian inversion. The inverted geophysical porosities show a positive correlation with measured values (R 2 = 0.93). We extrapolate the porosity estimates from 30 individual seismic refraction lines to a 3D volume below our study area using ordinary kriging to quantify the water holding capacity of our study area. Our results reveal that the critical zone in our study area holds 2.9 × 10 6 ± 9.6 × 10 5 m 3 of water, where 34% of this storage is in the saprolite, 55% is in the fractured rock, and the remaining 11% is in the bedrock.