Hydro-geophysical monitoring of orchard root zone dynamics in semi-arid region

Monitoring the moisture patterns at the root zone is necessary for agricultural, hydrological, and environmental applications. Conventional monitoring methods are usually invasive, destructive, and only sample at a small spatial scale. Electrical resistivity tomography (ERT) can set an alternative or be complementary to common traditional methods in evaluating the moisture content and its spatiotemporal patterns. In this study, we used the ERT method to monitor the hydro-geophysical dynamics under a drip-irrigated citrus orchard in a semi-arid region. Geophysical surveys were performed monthly for over a year. The obtained data from the electrical measurements were inverted to produce 2D tomograms of the bulk electrical conductivity. Calibrations of the petrophysical relations were conducted using both laboratory and field procedures. The obtained electrical results, and especially their temporal dynamics, cannot always be explained using the common assumption of uniform spatiotemporal distribution of the pore water electrical conductivity. To separate the two main components of the petrophysical relations, namely water content and pore water conductivity, we used a modeling approach. A coupled flow and transport model was calibrated using the electrical conductivity measurements, allowing separation of the contribution of the water content and pore water electrical conductivity to the bulk electrical conductivity. This allowed explaining the temporal dynamics of the measured electrical signal and a better understanding of the water and solute dynamics in the root zone.