Subsurface hydrologic dynamics in hillslopes remain poorly understood, in part because it is not directly observable, and in part because existing techniques for measuring the movement of water in rock fractures are not easily or effectively deployed in subsurface environments. in effect, the subsurface profile in hillslopes—which links atmospheric, hydrologic, geochemical, ecological, and geomorphic processes, and which may help to regulate the impact of climate change—remains unmeasured and poorly understood. An understanding of the physical makeup of this fractured continuum, and how water is stored and transmitted through it during and between precipitation events, will lead to improved predictive capabilities of trends in water quality and quantity, as dictated by changes in climate.<br>Along a hillslope in the Angelo Coast Range Reserve in northern California we are exploring the use of Electrical Resistivity Tomography (ERT) to reveal the hydrologic response, in the vadose zone of the subsurface domain, to seasonal precipitation. Surface ERT is a non-invasive geophysical technique that is sensitive to rock properties, which are related (directly or indirectly) to hydrological processes.<br>We have instrumented the 4000 m2 hillslope with numerous sensors to continuously track saturation changes in shallow soil and the underlying, fractured argillite. in addition, over a period of 18 months, which encompasses both a prolonged wet and dry season, we have conducted six ERT surveys along two transects laid across the hillslope. Results from time-lapse ERT Inversions agree with other measurements we have made of the subsurface hydrologic response. they imply that changes in resistivity are influenced by temperature, amount of rain, evaporation, and fluid movement into deeper sections of the subsurface profile. these Initial preliminary observations suggest that ERT could be a viable technique for exploring moisture dynamics in fractured rock underlying hillslopes.


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