In mountain regions, subsurface water flow and storage are important hydrologic processes that significantly affect the function of a watershed. On the hillslope scale, water in the subsurface may be stored in the soil/regolith layer (i.e., soil moisture) or in the matrix and fractures of weathered bedrock (i.e., rock moisture). While soil moisture has been extensively studied, the importance of rock moisture in ecohydrology has only been recognized recently. A comprehensive understanding of subsurface water dynamics, including soil moisture, rock moisture, and their interactions, is still lacking. In this talk, we introduce recent field studies from a semi-arid watershed near Boise, Idaho.

Electrical resistivity monitoring results show that the shallow soil moisture and deep rock moisture respond differently to the hydrologic events such as precipitation and evapotranspiration. In general, the dynamics of rock moisture (recharge and depletion) are associated with the changes in soil moisture, surface water input, and plant water use, but their temporal responses are desynchronized. In addition, the geophysical results show that the slope aspect also plays a role in affecting the seasonal rock moisture dynamics. These field data improve our understanding of the transport and storage of subsurface water and the factors that control these processes.