Geoelectrical monitoring of tree-soil water interactions at urban sites

The impact of climate change is increasingly restricting water availability in the soil, posing a significant challenge in urban areas where plants have to deal with limited space and sealed surfaces hinder rainwater infiltration. However, the amount of plant-available soil moisture plays a crucial role in plant vitality and is therefore important for ecosystem health. In urban environments, obtaining information on soil moisture is challenging. Commonly used methods, such as soil-moisture sensors, are often not applicable or do not provide a spatially resolved picture of soil moisture. Within the context of the interdisciplinary project CliMax, we explore the applicability of geophysical methods to characterize soil moisture in the rhizosphere in urban areas. Over the past year, monthly monitoring Electrical Resistivity Tomography (ERT) measurements were conducted at nine different tree locations in Braunschweig, Germany, characterized by diverse tree species and degrees of sealing of the surface. Additionally, temporally and spatially higher-resolution measurements were selectively taken. Various time-lapse inversion approaches implemented in the open geophysical inversion library pyGIMLi were tested and applied. Furthermore, Time-Domain Reflectometry (TDR) soil-moisture sensor data from different depths are available at each site, allowing calibration of ERT results with respect to soil moisture. The time-lapse inversion reveals well-resolved variation in soil moisture over the observed period, distinguishing between weather fluctuations and the influence of trees on the water balance. The water uptake is evident through increased resistivity values directly beneath the trees. Our study indicates that, depending on tree species and degree of sealing, the investigated urban trees predominantly extract water from the upper 1-4 meters. This finding is substantiated at specific locations by the data from deployed soil-moisture sensors.