Technical note: Monitoring diel soil water variations below trees using high-resolution electrical resistivity tomography

Abstract. Urban trees provide many important ecosystem services like cooling or CO₂-sequestration. For planting, maintenance and long-term health it is essential to understand their water system and supply. While soil moisture sensors and soil sampling provide only point-scale information and are often invasive, geophysical methods provide spatially continuous information, capture subsurface structures non-invasively and thereby improve process understanding of water dynamics below trees. In this study, electrical resistivity tomography (ERT) was used to monitor subsurface moisture dynamics at a tree site in an urban park over a period of one month continuously and at a high temporal resolution. The ERT measurements were complemented by soil water content (SWC) and sap flow measurements. We focus our analysis on precipitation-free periods to capture small SWC variations caused by soil water redistribution. During these periods, both the SWC data and the ERT data showed a clear diel pattern, with zones of decreasing SWC during the day and increasing SWC at night. At the same time, sap flow measurements show increased tree activity during the daytime drying phase. We explain these observations with root water uptake during the day and a recovery of subsurface moisture during the night following the redistribution of water in the subsurface. In addition, we found indications for an additional conductivity signal that is linked to transpiration and not captured by the soil moisture sensors. Our results show that high-resolution ERT, in combination with SWC and sap flow measurements, is a useful approach to investigate tree–soil water interactions in urban environments given that artefacts and temperature effects are carefully handled.