Geophysical Investigation of Slope Aspect Effect on Soil and Rock Moisture Interactions

Seldom observed, rock moisture greatly influences plant water availability and is an important component of the terrestrial water cycle. However, its spatiotemporal dynamics and major influencing factors in a watershed are still unclear. Here, we present the results of a year-long time-lapse electrical resistivity tomography (ERT) survey at a semi-arid watershed, the Dry Creek Experimental Watershed (DCEW) in Idaho, USA. The ERT monitoring was conducted across a ridge at the Treeline site of DCEW, and the results show a clear aspect effect on the dynamics of the subsurface water storage. The northeast-facing slope exhibits an increased weekly sensitivity to precipitation and evapotranspiration compared to the southwest slope, which has a smooth response. It also shows that the thicker regolith on the northeast slope holds more water than the thinner regolith on the southwest slope. Regarding the interaction between soil and rock moisture, the results show that there is an approximately two-week delay for the rock moisture to reach its lowest storage after the soil reaches its minimum storage in mid-August. The same delay is also observed for rock moisture during the wetting process occurring in later spring and early summer. Further work is suggested to develop a conceptual model for soil moisture/rock moisture interactions at the hillslope scale.