Non-invasive monitoring and numerical modeling of the Soil-Plant continuum

The Earth’s critical zone (ECZ) is the near-surface domain that regulates the availability of most life-sustaining resources. Among all the subdomains of the ECZ, a crucial role is played by the Soil-Plant continuum (SP), as it is a major performer of the exchanges of mass and energy between soil and plants (and then atmosphere). However, despite its importance and its strong interconnection with human activity, the characterization of this subdomain is still in an early stage, mainly because of the lack of spatial and temporal information regarding the occurring processes. To overcome this issue, we present the combination of geophysical measurements and hydrological modeling in the framework of a hydrogeophysical approach, with the aim of characterizing the active root zone, i.e. the portion of the root system involved in the water uptake. In fact, the water uptake is performed by root hair, the microscopic cell outgrowths whose location is difficult (if not impossible) also after the removal of the root system from the soil. Nevertheless, determining its position is fundamental not only for merely scientific purposes, but most of all for practical applications, as it affects the performing of precision irrigation. Therefore, in this work I propose the identification of the active root zone on the basis of its main effect, i.e. the reduction of soil water content over time. This is achieved by means of 3-D small-scale electrical resistivity tomography (ERT) carried out combining superficial and borehole electrodes. We monitored the processes occur-ring in the root zone of three orange trees located in south-eastern Sicily. These trees are drip irrigated according to different deficit irrigation techniques to improve the exploitation of the water resource, while the plants’ transpiration is monitored thanks to sap flow and eddy covariance measurements. More in detail, in the first case study ERT measurements before and after the cut of the tree are compared, while in the second case study the ERT monitoring is focused on two orange trees drip irrigated with different treatments (i.e. full irrigation and partial root drying). The datasets thus obtained provide interesting insights into the root system activity, given their abundance of information regarding both atmospheric and underground phenomena (i.e. transpiration and root water uptake, respectively). In particular, the ERT time-lapse approach well highlights the portions subject to a decrease in water content, which can be related to the water uptake put in place by the plants. Nevertheless, the interpretation of the resistivity patterns, although combined with agronomic information, can be rather intricate. A proper hydrological modeling provides a solution to this problem, even if choosing the most suitable approach requires a specific mathematical analysis. To do this, we developed a synthetic case study with two identical hydrological models, one of which describing also the activity of an orange tree. These models resemble the real datasets provided by the ERT measurements, without all the uncertainties introduced by the geophysical acquisition and the model calibration. The location of the active root zone is reconstructed from the combination of these two models by means of Taylor series expansion, with particular reference to the effects of the approximation thus introduced. The final aim is to evaluate the proposed numerical procedure for a future application on one of real case study presented in this work.