Densely populated urban areas are threatened by global change. Blue-green solutions are therefore a prominent strategy to increase resilience and life quality. To design blue-green infrastructure and to supply sufficient water to urban green, accurate tree transpiration measurements and drought stress detection are crucial. Hence, ecosystem services can successfully be safeguarded. The major sources of error in tree transpiration measurements with sap flow sensors are dynamics in stem water content, sapwood area and radial and azimuthal variability in sap flux density. This PhD research proposal will address these challenges by complementing state-of-the-art sap flow sensors with promising electrical resistivity tomography (ERT). ERT will be used to assess spatio-temporal dynamics in stem water content and to gain novel insights into its diurnal and seasonal patterns. Observations of the dynamic behaviour of stem water content with ERT at such fine resolution represent an untapped resource and will deepen our fundamental knowledge about tree hydraulic functioning and drought stress responses. Electrical resistivity patterns will also be related to radial and azimuthal sap flux density variation and sapwood area. The outcome of this project will therefore lead to more accurate estimates of tree transpiration, insight into drought stress responses, and a mechanistic better understanding of tree water relations in an urban environment.