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Abstract The spatiotemporal dynamics of plant water sources are hidden and poorly understood. We document water source use ofQuercus garryanagrowing in Northern California on a profile of approximately 50 cm of soil underlain by 2–4 m of weathered bedrock (sheared shale mélange) that completely saturates in winter, when the oaks lack leaves, and progressively dries over the summer. We determined oak water sources by combining observations of water stable isotope composition, vadose zone moisture and groundwater dynamics, and metrics of tree water status (potential) and use (sapflow). During the spring, oak xylem water is isotopically similar to the seasonal groundwater and shallow, evaporatively enriched soil moisture pools. However, as soils dry and the water table recedes to the permanently saturated, anoxic, low‐conductivity fresh bedrock boundary,Q. garryanashifts to using a water source with a depleted isotopic composition that matches residual moisture in the deep soil and underlying weathered bedrock vadose zone. Sapflow rates remain high as late‐summer predawn water potentials drop below−2.5 MPa. Neutron probe surveys reveal late‐summer rock moisture declines under the oaks in contrast to constant rock moisture levels under grass‐dominated areas. We therefore conclude that the oaks temporarily use seasonal groundwater when it occupies the weathered profile but otherwise use deep unsaturated zone moisture after seasonal groundwater recedes. The ample moisture, connected porosity, and oxygenated conditions of the weathered bedrock vadose zone make it a key tree water resource during the long summer dry season of the local Mediterranean climate.
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This content will become publicly available on March 1, 2026
Self‐potential signals of soil columns experiencing evaporation and transpiration
Abstract The self‐potential (SP) method has been used in hydrological sciences to monitor many hydrologic processes thanks to the electrokinetic coupling between water flow and streaming current in geological materials. Despite many useful applications, quantitative interpretations are still rare, in particular for unsaturated soils where the water fluxes are of orders lower than that in saturated conditions. In this study, we used laboratory soil column tests to simulate vadose zone hydrologic processes (drainage, evaporation, and transpiration) and to generate SP data in low water flow conditions. The measured water fluxes and SP signals in different hydrologic stages of the tests are used to study if electrokinetic coupling is still the dominant mechanism for the SP signals in unsaturated, low‐flow conditions. Theoretical models of electrokinetic and electrodiffusion couplings are also used to guide the analysis. It is shown that the SP signals measured during soil evaporation and plant transpiration in this soil column test were not only caused by unsaturated water flows in the soil column through electrokinetic coupling. Instead, they are likely related to the ion concentration gradient in the soil column, which creates an electrical current of a diffusive nature. The ion concentration gradient is likely related to the different reaction rates of mineral–water interactions in saturated and unsaturated soils. This study, therefore, highlighted the importance of considering the electrodiffusion coupling in interpreting the measured SP signals in vadose zone hydrology.
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- PAR ID:
- 10657003
- Publisher / Repository:
- Soil Science Society of America
- Date Published:
- Journal Name:
- Vadose Zone Journal
- Volume:
- 24
- Issue:
- 2
- ISSN:
- 1539-1663
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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