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Abstract The creation of fractures in bedrock dictates water movement through the critical zone, controlling weathering, vadose zone water storage, and groundwater recharge. However, quantifying connections between fracturing, water flow, and chemical weathering remains challenging because of limited access to the deep critical zone. Here we overcome this challenge by coupling measurements from borehole drilling, groundwater monitoring, and seismic refraction surveys in the central California Coast Range. Our results show that the subsurface is highly fractured, which may be driven by the regional geologic and tectonic setting. The pervasively fractured rock facilitates infiltration of meteoric water down to a water table that aligns with oxidation in exhumed rock cores and is coincident with the adjacent intermittent first‐order stream channel. This work highlights the need to incorporate deep water flow and weathering due to pervasive fracturing into models of catchment water balances and critical zone weathering, especially in tectonically active landscapes. 

Abstract Quantifying evapotranspiration (ET) is critical to accurately predict vegetation health, groundwater recharge, and streamflow generation. Hillslope aspect, the direction a hillslope faces, results in variable incoming solar radiation and subsequent vegetation water use that drive ET. Previous work in watersheds with a single dominant vegetation type (e.g., trees) have shown that equator‐facing slopes (EFS) have higher ET compared to pole‐facing slopes (PFS) due to higher evaporative demand. However, it remains unclear how differences in vegetation type (i.e., grasses and trees) influence ET and water partitioning between hillslopes with opposing aspects. Here, we quantified ET and root‐zone water storage deficits between a PFS and EFS with contrasting vegetation types within central coastal California. Our results suggest that the cooler PFS with oak trees has higher ET than the warmer EFS with grasses, which is counter to previous work in landscapes with a singule dominant vegetation type. Our root‐zone water storage deficit calculations indicate that the PFS has a higher subsurface storage deficit and a larger seasonal dry down than the EFS. This aspect difference in subsurface water storage deficits may influence the subsequent replenishment of dynamic water storage, groundwater recharge and streamflow generation. In addition, larger subsurface water deficits on PFS may reduce their ability to serve as hydrologic refugia for oaks during multi‐year droughts. This research provides a novel integration of field‐based and remotely‐sensed estimates of ET required to properly quantify hillslope‐scale water balances. These findings emphasize the importance of resolving hillslope‐scale vegetation structure within Earth system models, especially in landscapes with diverse vegetation types.