The most significant biotic and abiotic stress agents of water extremity, salinity, and infection lead to wood decay and modifications of moisture and ion content, and density. This strongly influences the (di-)electrical and mechanical properties and justifies the application of geophysical imaging techniques. These are less invasive and have high resolution in contrast to classical methods of destructive, single-point measurements for inspecting stresses in trees and soils. This review presents some in situ and in vivo applications of electric, radar, and seismic methods for studying water status and movement in soils, roots, and tree trunks. The electrical properties of a root-zone are a consequence of their moisture content. Electrical imaging discriminates resistive, woody roots from conductive, soft roots. Both types are recognized by low radar velocities and high attenuation. Single roots can generate diffraction hyperbolas in radargrams. Pedophysical relationships of water content to electrical resistivity and radar velocity are established by diverse infiltration experiments in the field, laboratory, and in the full-scale 'GeoModel' at Kiel University. Subsurface moisture distributions are derived from geophysical attribute models. The ring electrode technique around trunks images the growth ring structure of concentric resistivity, which is inversely proportional to the fluid content. Healthy trees show a central high resistivity within the dry heartwood that strongly decreases towards the peripheral wet sapwood. Observed structural deviations are caused by infection, decay, shooting, or predominant light and/or wind directions. Seismic trunk tomography also differentiates between decayed and healthy woods.

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.

Data from an infiltration and drainage experiment into and from a sand tank model of 5 by 3 m and 2 m deep were reevaluated. The uniform sand overlaid a 0.2‐m‐thick gravel layer from which five perforated pipes collected drainage flow. The data include time series of capillary potentials, ψ, volumetric water contents, θ, both from nine levels, and drainage flow, q, that resulted from sprinkler infiltration with a constant rate of 15.6 mm h−1 that lasted >16 h. The wetting and the pressure fronts (i.e., the increases of water content and of capillary potential) moved with the same constant velocity of 3.25 × 10−5 m s−1 The capillary potentials about 0.2 to 0.4 m behind the wetting front remained constant at about −2.5 kPa. The maximum degree of saturation during infiltration was <0.6 and the mobile water content involved in flow was about 0.15 m3 m−3 The constant velocity indicates a continuous and dynamic balance of forces acting on the mobile water. Thus, viscosity is considered to have balanced gravity as the driving force, and a momentum dissipation approach to infiltration evolved. Capillary and Bond numbers indicate that flow was subjected to considerable capillary potential, yet its gradient was only active throughout a depth range of 0.2 to 0.4 m behind the wetting front. The constant velocity prevailed despite local variations in the mobile water content. Constant velocity of wetting is proposed as the global key parameter during infiltration on which local features, like volume flux density and variation of water content, may depend.

An electrical resistivity technique using a ring array of needle electrodes is applied to image the internal electrical structures of the trunks of living standing trees and trunk disks. Measured electrical resistance data are inverted using a 2D iterative finite-element algorithm which incorporates the cylindrical geometry of the trunk. The technique is successfully tested using synthetic models showing that the resolution obtained when mapping anomalous zones inside the trunk is higher for the dipole-dipole configuration than for the Wenner configuration. Measurements on a trunk column show a well-established inverse relationship between the imaged resistivity and the moisture content determined from wood cores.Healthy tree trunks always show a concentric ring structure with a central maximum of resistivity, which decreases towards the outside, corresponding to an increase in moisture from the inner dry heartwood to the outer wet ring of sapwood. Asymmetry in the ring structure is interpreted as the influence of branching or the direction of sunshine and wind. Strong local resistivity anomalies are related to infections causing wet, rotting or dry cavities. The resistivity of the heartwood is a maximum in olive and oak trunks, intermediate in young fruit trees and a minimum in cork oak trunks that are considered to be wet. The resistivity imaging technique can also monitor the dynamic process of sapflow if adequate tracers are used.

The main goal of a joint project undertaken by the geophysical and hydrological research units of Kiel University is to study preferential flow in a large open‐air, full‐scale model, looking in particular at near‐surface penetration and flow of water through the unsaturated vadose zone. An artificial irrigation device is installed in place of natural rain, and a homogeneous sand body is used instead of natural soil. This provides a reference model for future field experiments. Inside the sand body there are a large number of geophysical and hydrological sensors to measure DC resistivity (using various electrode configurations), water content and water potential (using TDR and tensiometer instruments, respectively). A ground‐penetrating radar (GPR) system is installed at the surface, whereas at the bottom several containers and a thin gravel layer are embedded to measure the flow arrival and the discharge of water. Irrigation is varied in intensity, time, area, and salt content (tracer).  Results of the first six experiments show that the percolation of intruding water can be followed by all techniques and percolation is finally controlled by the discharge measurements. These display some undulations and variations of the water ‘front’ and agree with the measurements of all other sensors. The redundancy achieved by the use of multiple methods was intended to enable an assessment of the reliability of the techniques used. The true values of electrical resistivity before and after irrigation reflect the distribution of water saturation within the sand body. A numerical 3‐D inversion of the apparent resistivity provides information regarding future field experiments, in which it will be possible to install only some of the sensors in order to preserve the natural structure of the soil.