The electrical resistivity survey, traditionally realized by the direct current (DC) resistivity method, has demonstrated great value in advancing the measurement of vadose zone dynamics. Compared with the DC resistivity method, the capacitively coupled (CC) resistivity method has a higher ratio of measurement speed to data density, and thus is economically preferred for resistivity surveys that require high data density, e.g., hydrological characterizations. To test the applicability of the CC resistivity method to the study of vadose zone dynamics, we conducted time-lapse resistivity surveys using a commercial CC resistivity (line antenna) system, the OhmMapper, to monitor the water content change in an unsaturated zone due to artificial rainfall infiltration The experimental...[ Read more ]

The electrical resistivity survey, traditionally realized by the direct current (DC) resistivity method, has demonstrated great value in advancing the measurement of vadose zone dynamics. Compared with the DC resistivity method, the capacitively coupled (CC) resistivity method has a higher ratio of measurement speed to data density, and thus is economically preferred for resistivity surveys that require high data density, e.g., hydrological characterizations. To test the applicability of the CC resistivity method to the study of vadose zone dynamics, we conducted time-lapse resistivity surveys using a commercial CC resistivity (line antenna) system, the OhmMapper, to monitor the water content change in an unsaturated zone due to artificial rainfall infiltration The experimental results clearly capture the water movement in the vadose zone and therefore prove the applicability of the method. However, several practical problems were encountered. When the ground surface became extremely wet, the OhmMapper falsely interpreted the current level. When the whole ground became less resistive as a result of continued rainfall infiltration, one should be cautious about the overestimation of the measured resistivity, especially when the measurement is made at a short dipole distance (i.e., for the investigation at a shallower depth).

The hydrological effect of vegetation on slope stability is complex under changing climate conditions. Beneficial effects include canopy interception and evapotranspiration (ET). Detrimental effects are mainly due to the increased infiltration capacity because of existence of vegetation roots. For geotechnical engineers who consider vegetation as an ecotechnological solution to slope stability, it is necessary to identify climate conditions under which beneficial effects of vegetation dominate. In this study, we developed a new capacitive resistivity array for measuring soil moisture in slopes. This capacitive resistivity array, i.e., the equatorial dipole-dipole line electrode array, is applied in monitoring soil moisture variations in both bare and vegetated slopes. The temporal soil moisture variations are analyzed to study the combined effect of vegetation on slope hydrology. Experimental results show that (1) soil moisture at shallow depth is relatively higher in the vegetated slope than in the bare slope; and (2) the difference in soil moisture at deep depth between bare and vegetated slopes is negligible. Temporal soil moisture variations reveal that detrimental effects of vegetation dominate when the rainfall is adequate (monthly larger than 250 mm). It is because vegetation increases the ground infiltration capacity, and thus more water is retained in the vegetated slope. While precipitation is low (monthly lower than 100 mm) and potential ET is relatively high (monthly around 100 mm), vegetation has an obvious beneficial effect on slope stability. However, the transpiration of vegetation could be prohibited during extremely dry periods (monthly rainfall amount less than 20 mm), resulting in a conversion of the vegetation impact from beneficial to detrimental. When heavy rainfall appears frequently (monthly larger than 200 mm), those detrimental effects could be “erased”, and the difference in soil moisture between bare and vegetated slopes declines gradually over time. When the antecedent rainfall exceeds 1000 mm, the influence of vegetation on soil moisture in slopes completely disappears.