Monitoring of root-zone water content in the laboratory by 2D geoelectrical tomography

Root water uptake is one of the essential processes within the soil-plant-atmosphere continuum. We present a method for monitoring soil-water redistributions due to water uptake by roots. Our aim is to image and monitor diurnal soil-water redistribution during a small-scale (centimeter-to-decimeter range) indoor experiment and to correlate water content determined by applied geoelectrical time-lapse imaging techniques with values from single-point time domain reflectometry (TDR) measurements. This includes establishing pedophysical relationships within the root zone and deriving the water-content distribution from the electrical-resistivity model. Using DC geoelectrics of high resolution (970 data points for 220 cm²), we monitored significant spatial heterogeneity of soil moisture with time, whereas no irrigation was applied. Thus, we imaged the high heterogeneity of fluid movements within the soil. We found diurnal variations with high spatial variability of soil water content during the morning and afternoon hours. The water content continuously increased from dawn to noon, whereas the increase started in the near-surface zone from 1 cm to 3 cm above the main root zone. Between 8:00 a.m. and 10:00 a.m., water content decreases along most of the sections. Water content irregularly decreases and increases during the afternoon. During night time, we observed nearly no changes in soil water content due to the absence of transpiration and subsequently soil-water redistribution. Most of these diurnal variations in soil water content lie within the intensive root zone, as measurements showed on soil samples excavated from these areas after the experiment.Furthermore, we quantified water content derived from geoelectrical tomography of the monitored area before and after an irrigation event using a geophysical pedotransfer function of Archie, established specifically for the used lupine and the applied physico-chemical boundary conditions of the experiment. The resulting average water content from 2D geoelectrical tomography agreed well with the values determined by the TDR measurements.