The structure of the critical zone is a product of feedbacks between hydrologic, climatic, biotic, and chemical processes. Ample research within snow-dominated systems has shown that aspect-dependent solar radiation inputs can produce striking differences in vegetation composition, topography, and soil depth between opposing hillslopes. However, more research is needed to understand the role of microclimates on critical zone development within rain-dominated systems, especially below the soil and into weathered bedrock. To address this need, we characterized the critical zone of a north-facing and south-facing slope within a first-order headwater catchment located in central coastal California. We combined terrain analysis of vegetation distribution and topography with field-based soil pit characterization, geophysical surveys and hydrologic measurements between slope-aspects. We observed thicker soil profiles, higher shallow soil moisture, and denser vegetation on north facing slopes, which matched previously documented observations in snow-dominated sites. However, average topographic gradient and saprolite thickness were uniform across our study hillslopes, which did not match common observations from the literature. These results suggest dominant processes for critical zone evolution are not necessarily transferable across regions. Thus, there is a continued need to expand critical zone research, especially in rain-dominated systems. Here, we present four non-exclusive, testable hypotheses of mechanisms that may explain these unexpected similarities in slope and saprolite thickness between hillslopes with opposing aspects. Specifically, we propose both past and present ecohydrologic processes must be taken into account to understand what shaped the present day critical zone.

Bedrock vadose zone water storage (i.e., rock moisture) dynamics are sparsely observed but potentially key to understanding drought responses. Exploiting a borehole network at a Mediterranean blue oak savanna site-Rancho Venada-we document how water storage capacity in a deeply weathered bedrock profile regulates woody plant water availability and groundwater recharge. The site is in the Northern California Coast Range within steeply dipping turbidites. In a wet year (water year 2019; 647 mm of precipitation), rock moisture was quickly replenished to a characteristic storage capacity, recharging groundwater that emerged at springs to generate streamflow. In the subsequent rainless summer growing season, rock moisture was depleted by about 93 mm. In two drought years that followed (212 and 121 mm of precipitation) the total amount of rock moisture gained each winter was about 54 and 20 mm, respectively, and declines were observed exceeding these amounts, resulting in progressively lower rock moisture content. Oaks, which are rooted into bedrock, demonstrated signs of water stress in drought, including reduced transpiration rates and extremely low water potentials. In the 2020-2021 drought, precipitation did not exceed storage capacity, resulting in variable belowground water storage, increased plant water stress, and no recharge or runoff. Rock moisture deficits (rather than soil moisture deficits) explain these responses.

Understanding how soil thickness and bedrock weathering vary across ridge and valley topography is needed to constrain the flowpaths of water and sediment within a landscape. Here, we investigate how soil and weathered bedrock properties vary across a ridge-valley system in the Northern California Coast Ranges where topography varies with slope aspect such that north facing slopes, which are more densely vegetated, are steeper. In this study, we use seismic refraction surveys to extend observations made in boreholes and soil pits to the hillslope scale and identify that while soils are thicker on north facing slopes, the thickness of weathered bedrock does not vary with slope aspect. We estimate the porosity of the weathered bedrock and find that it is several times the annual rainfall, indicating that water storage is not limited by the available pore space, but rather the amount of precipitation delivered. Bedding-parallel and bedding-perpendicular seismic refraction surveys reveal weathering profiles that are thickest upslope and taper downslope to channels. We do not find a clear linear scaling relationship between depth to bedrock and hillslope length, which may be due to local variation in incision rate or bedrock hydraulic conductivity. Together, these findings, which suggest that the aspect-independent weathering profile structure is a legacy of past climate and vegetation conditions and that weathering varies strongly with hillslope position, have implications for hydrologic processes across this landscape.