Seasonal Shifts in Soil Moisture throughout a Semiarid Hillslope Ecotone during Drought: A Geoelectrical View

Abstract

Core Ideas Resistivity imaging revealed dynamic subsurface response to drought across an ecotone. Resistivity responses restricted to rooting zones of vegetation type on each slope. Drought led to a deepening of interpreted soil moisture losses. Rewetting after drought did not immediately return to pre‐drought conditions. This study investigated the spatial distribution and seasonal variation of soil moisture during a drought year throughout a first‐order drainage basin whose hillslopes have different vegetation and soil. The north‐facing slope has juniper trees [Juniperus monosperma (Engelm.) Sarg.] and finer soil textures, while the south‐facing slope has creosote bushes [Larrea tridentata (DC.) Coville]. Time‐lapse, two‐dimensional electrical resistivity measurements, which can detect changes due to soil moisture, were conducted to capture subsurface moisture changes across the ecotone, from one hillslope to the other and along the ecotone down the valley axis. After the onset of the drought, the upper 3 m of the entire hillslope was mostly drying (resistivity was increasing). As the drought progressed and when there was less available shallow soil moisture, the resistivity profiles suggested that both juniper and creosote bushes sequestered moisture stored from increasingly greater depths, eventually reaching their respective average maximum rooting depths of ∼6 m for juniper and ∼3 m for creosote. However, this was also accompanied by decreases in resistivity (increases in soil moisture) at shallower depths, suggesting hydraulic redistribution. With shallow rewetting of the hillslope following storm events near the end of the study period, shallow areas could once again support vegetation on both hillslopes. Electrical resistivity imaging is effective for studying the spatiotemporal dynamics of soil moisture throughout the critical zone in response to water stress and can be used to inform ecohydrologic connections.