Subsurface Water Storage Dynamics of a Semi-Arid Hillslope Revealed by Time-Lapse Electrical Resistivity Tomography

Abstract

Weathering in the Critical Zone (CZ) not only transforms bedrock into regolith but also increases the moisture storage potential of the subsurface by producing porosity. In the subsurface, water 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). Examining the seasonal variation of subsurface water storage will develop a better understanding of water partitioning in the CZ, specifically under a warming climate with shifting precipitation patterns. In this thesis, electrical resistivity tomography (ERT) is used to monitor subsurface water storage dynamics in a semi-arid snow dominated hillslope from late summer to early winter to answer the research question: are shallow soil moisture and deep rock moisture synchronized in their responses to hydrologic events (precipitation and evapotranspiration)? Thanks to the strong correlation between resistivity and moisture content, the ERT method allows us to examine subsurface water storage changes in the region of the critical zone that is difficult to monitor with other costly methods. The seismic refraction imaging was used to determine the subsurface CZ structure of the hillslope, and by combining two geophysical methods, it is possible to accurately distinguish resistivity trends of shallow and deep storage reservoirs. To evaluate storage responses to hydrologic events, measured meteorologic data are used to estimate subsurface storage, which are then used with a petrophysical model to estimate resistivity changes for evaluation. It is found that shallow (~5 m deep in subsurface) and deep resistivities ( \u3e 6 m deep) of the hillslope are asynchronous, with time lag during seasonal wetting and drying. During wetting, soil and rock moisture are asynchronous across the hillslope with a substantial lag time (~15 weeks); whereas during dying soil and rock moisture have a smaller lag time (~4 weeks). Soil moisture is constantly recharged during precipitation events; whereas rock moisture is only recharged after soil moisture, requiring longer time scales for recharge. After soil moisture is depleted, with continuing ET demands, rock moisture diminishes quickly. Different wetting and drying trends indicate the vulnerability of rock moisture to drought, and further demonstrate it is a vital resource when soil moisture is depleted. This research advances our understanding of subsurface water storage dynamics, highlighting the utility of hillslope-scale geophysical investigations to help better predict how mountainous hydrologic systems store and release water for ecohydrological use