The description of catchment response is a fundamental issues in geoscience. However, some of the basic governing processes, particularly the role of the subsurface, remain poorly understood. In this paper, we present the results of non invasive time-lapse monitoring of a controlled infiltration test at a site in the Italian Alps. The key hydrologic question is whether rainfall infiltrates mainly into the underlying bedrock or flows in the soil layer towards the stream channel a few hundred meters downgradient. We applied 1000 mm of artificial rain on a 3 m x 3 m slope box over about 18 hours. The soil moisture variation and the underlying bedrock was monitored via a combination of electrical resistivity tomography (ERT), TDR probes and tensiometers. A small-scale 3D cross-hole ERT was performed via 2 m deep borehole purposely drilled and completed with electrodes in the irrigated plot. As a result, we observed a fast vertical infiltration through the soil cover followed by infiltration into the fractured bedrock. The lateral transmission of infiltrating water was negligible. This experiment confirms that the fractured bedrock has a key role in controlling the fast hydrological dynamics of the small catchment system under study.


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