Geophysical Methods for Field-Scale Monitoring of Flow and Transport In the Root Zone.

Geophysical methods offer the possibility to image non-invasively 3-D subsurface structures of soil properties and associated flow and transport processes at spatial scales ranging from soil columns to whole field plots. This offers the opportunity to address exciting research questions dealing with the feedback between vegetation, soil heterogeneity and soil moisture patterns, such as row-inter row variations and effects of variations in root systems in mixed cropping systems. In this invited contribution, we focus on three geoelectrical methods (Electrical Resistivity Tomography (ERT), Electrical Impedance Spectroscopy (EIS) and Ground Penetrating Radar (GPR)), which can be used to non-invasively image soil electrical properties such bulk electrical conductivity, complex bulk electrical conductivity and dielectric permittivity. First, we will give an overview of relations between these geoelectrical properties and soil state variables, such as soil moisture and salt concentration, and root properties, such as root mass and surface area. In a second part, we give examples of studies in which ERT and GPR were used to monitor spatial patterns of processes such as water flow, root water uptake, and solute transport in soils. In particular, we present results from monitoring campaigns in which soil water distributions in lysimeters grown with barley and in transects of a field plot with a mixed cropping system of maize, chilli and leucaena were imaged and monitored using ERT. In addition to real monitoring data, we demonstrate the potential and limitations of ERT for imaging and monitoring root zone processes using synthetic experiments. Finally, we present some new developments in the interpretation of GPR data using full waveform inversion, which increases the resolution of the inverted images. These developments are promising for the investigation of small scale variations in soil moisture resulting from soil heterogeneity and root distributions.