Time-lapse ERT inversion for the monitoring of soil water content at urban tree sites

Urban trees play a crucial role in mitigating climate impacts through their cooling effects while also contributing other ecosystem services, like carbon sequestration. However, growth conditions for urban trees are challenging to begin with. Due to climate change, there is an urgent demand for understanding how variations in precipitation and temperature affect soil moisture. We present monitoring data of soil moisture at two urban tree sites using Electrical Resistivity Tomography (ERT). Monthly ERT measurements were conducted along rows of trees over 1.5 years at one sealed and one unsealed site. For comparison and calibration, soil moisture sensors capable of also measuring temperature were installed at five depths up to 2 m. To eliminate temperature effects on ERT data, a method was used to model subsurface temperature variations down to 20 m using temperature data provided by the soil moisture sensors. A comparison of different time-lapse inversion techniques highlights their advantages and disadvantages and indicates that a full time-lapse inversion is most suitable for our application. However, at the sealed site, variable electrode coupling conditions made the inversion results more prone to noise and artifacts. Due to small targeted soil moisture and, thus, resistivity changes, careful filtering and error analysis are crucial. There is greater dynamic in the changes of resistivity at the unsealed site compared to the sealed site, where most changes occur in the areas of the tree pits. Nonetheless, the setup allowed for the identification of periods with changes in groundwater levels, evaporation, and root water uptake.