Title: Using Self-Potential to Monitor Evapotranspiration: A Soil Column Experiment
Program: Master of Science in Geosciences
Advisor: Dr. Qifei Niu, Geosciences
Committee Members: Lee Liberty, Geosciences; and Dr. James McNamara, Geosciences
The critical zone (CZ), which extends from the canopy of the trees to the bottom of groundwater circulation, provides life-sustaining resources to humans. To better understand the CZ functions, it is of vital importance to characterize various CZ structures and processes. In this study, the focus is on the measurement of water fluxes in CZs induced by evapotranspiration (ET). Traditional methods like time domain reflectometry (TDR) and tensiometers have been frequently used to measure the hydrologic states from which the ET-induced water fluxes could be estimated. However, this estimation is based on water balance calculations, so significant uncertainties could be involved. Recently, the Self-Potential (SP) method has emerged as a useful tool in monitoring the subsurface water movement. However, for ET-induced water flux, the SP method can only give qualitative results. The research question addressed in this study is if SP signals can be used to quantify ET-induced water flux within the vadose zone. A laboratory soil column was designed and constructed for this purpose. Evaporation and ET were simulated in the column tests, and these processes were monitored with TDR, matric suction sensors, and electrodes installed at different levels of the soil column. In addition, the water level was be monitored with a pressure sensor. Four tests were planned with different combinations of soil types (sand or loam) and water flux drivers (evaporation or ET). The soil column experiment conducted in the study showed that drainage-, evaporation- and transpiration-driven unsaturated water fluxes in soils could induce measurable SP signals. The experimental results confirm that the SP method can be used in vadose zone hydrology for monitoring hydrologic processes in the unsaturated critical zone of the Earth.