Abstract. Mass and energy exchanges between soil, plants and atmosphere control a number of key environmental processes involving hydrology, biota and climate. The understanding of these exchanges also play a critical role for practical purposes e.g. in precision agriculture. In this paper we present a methodology based on coupling innovative data collection and models in order to obtain quantitative estimates of the key parameters of such complex flow system. In particular we propose the use of hydro-geophysical monitoring via "time-lapse" electrical resistivity tomography (ERT) in conjunction with measurements of plant transpiration via sap flow and evapotranspiration (ET) from eddy covariance (EC). This abundance of data is fed to spatially distributed soil models in order to characterize the distribution of active roots. We conducted experiments in an orange orchard in eastern Sicily (Italy), characterized by the typical Mediterranean semi-arid climate. The subsoil dynamics, particularly influenced by irrigation and root uptake, were characterized mainly by the ERT set-up, consisting of 48 buried electrodes on 4 instrumented micro-boreholes (about 1.2 m deep) placed at the corners of a square (with about 1.3 m long sides) surrounding the orange tree, plus 24 mini-electrodes on the surface spaced 0.1 m on a square grid. During the monitoring, we collected repeated ERT and time domain reflectometry (TDR) soil moisture measurements, soil water sampling, sap flow measurements from the orange tree and EC data. We conducted a laboratory calibration of the soil electrical properties as a function of moisture content and porewater electrical conductivity. Irrigation, precipitation, sap flow and ET data are available allowing for knowledge of the system's long-term forcing conditions on the system. This information was used to calibrate a 1-D Richards' equation model representing the dynamics of the volume monitored via 3-D ERT. Information on the soil hydraulic properties was collected from laboratory and field experiments. The successful results of the calibrated modelling exercise allow for the quantification of the soil volume interested by root water uptake (RWU). This volume is much smaller (with a surface area less than 2 m 2 , and about 40 cm thick) than expected and assumed in the design of classical drip irrigation schemes that prove to be losing at least half of the irrigated water which is not taken up by the plants.

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Plant roots affect the exchanges of mass and energy between the soil and atmosphere. However, it is challenging to monitor the activity of the root-zone because roots are not visible from the soil surface, and root systems undergo spatial and temporal variations in response to internal and external conditions. Therefore, measurements of the activity of root systems are interesting to plant biologists in general, and are especially important for specific applications, such as precision agriculture. This study demonstrates the use of small scale three-dimensional (3-D) electrical resistivity tomography (ERT) to monitor the root-zone of orange trees irrigated by two different regimes: (i) full rate, in which 100% of the crop evapotranspiration (ET c) is provided, and (ii) partial root-zone drying (PRD), in which 50% of ET c is supplied to alternate sides of the tree. We performed time-lapse 3-D ERT measurements on these trees from 5 June to 24 September 2015, and compared the long-term and short-term changes before, during, and after irrigation events. Given the small changes in soil temperature and pore water electrical conductivity, we interpreted changes of soil electrical resistivity from 3-D ERT data as proxies for changes in soil water content. The ERT results are consistent with measurements of transpiration flux and soil temperature. The changes in electrical resistivity obtained from ERT measurements in this case study indicate that *Revised Manuscript with no changes marked Click here to view linked References 2 root water uptake (RWU) processes occur at the 0.1 m scale, and highlight the impact of different irrigation schemes.

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