Electrical resistivity tomography as a tool for assessing the spatial variability of soil water content and root water uptake under precision irrigation of kiwifruit orchard

This study evaluates the use of electrical resistivity tomography (ERT) as a non-invasive tool to monitor soil water content (SWC) dynamics and root water uptake (RWU) in a commercial kiwifruit orchard where a deficit-irrigation regime was applied. Time-lapse ERT surveys were performed around a representative kiwifruit orchard following a controlled, full irrigation event, and resistivity changes were spatially analyzed using a systematic cell-grid framework. The empirical calibration derived from FDR sensors confirmed a strong logarithmic relationship between resistivity and SWC, validating the use of ERT for quantifying soil wetting and drying processes. Time-lapse maps revealed that the most pronounced changes occurred within approximately 1.5 m of the trunk, where drying progressed rapidly both laterally and vertically, indicating that RWU dominated over physical drainage. The positive change in electrical resistivity (ΔE), calculated as the normalized variation of the electrical resistivity, near the trunk across the entire profile suggests a sequential activation of shallow and deep roots as the upper layers dried. A multivariate analysis further identified the near-trunk region as the main contributor to cumulative sap-flow-derived transpiration, while transitional zones reflected mixed effects of RWU and water redistribution from upper layers. Overall, the integration of resistivity imaging, time-lapse analysis, and sap-flow measurements proved effective in capturing the spatial structure of RWU and provides a robust framework to support precision-irrigation strategies in kiwifruit orchards.