A nondestructive and spatially integrated multielectrode method for measuring soil electrical resistivity was tested in the Beauce region of France during a period of corn crop irrigation to monitor soil water flow over time and in two‐dimensional (2‐D) with simultaneous measurements of soil moisture and thermal profiles. The results suggested the potential of surface electrical resistivity tomography (ERT) for improving soil science and agronomy studies. The method was able to produce a 2‐D delimitation of soil horizons as well as to monitor soil water movement. Soil drainage through water uptake by the roots, the progression of the infiltration front with preferential flow zones, and the drainage of the plowed horizon were well identified. At the studied stage of corn development (3 months) the soil zones where infiltration and drainage occurred were mainly located under the corn rows. The structural soil characteristics resulting from agricultural practices or the passage of agricultural equipment were also shown. Two‐dimensional sections of soil moisture content were calculated using ERT. The estimates were made by using independently established “in situ” calibration relationships between the moisture and electrical resistivity of typical soil horizons. The thermal soil profile was also considered in the modeling. The results showed a reliable linear relationship between the calculated and measured water contents in the crop horizon. The precision of the calculation of the specific soil water content, quantified by the root mean square error (RMSE), was 3.63% with a bias corresponding to an overestimation of 1.45%. The analysis and monitoring of the spatial variability of the soil moisture content with ERT represent two components of a significant tool for better management of soil water reserves and rational irrigation practices.

Characterizing water processes at the field scale requires an accurate and high-resolution description of the variability of representative state variables, such as water content. Obtaining these data, however, is unrealistic with conventional soil water sensors; we therefore propose an empirical approach to measure water content based on electrical resistivity. This geophysical method provides exhaustive and time-repetitive information on the resistivity of a DC signal. Resistivity can be used to estimate water content, although the interpretation is not straightforward. As the approach is inexpensive and allows for repeated high-resolution measurements, its potential benefits for the monitoring of soil water processes at the field scale warrant greater investigation. Soil electrical resistivity and soil water content were monitored at the field scale on four dates in 2006, by MuCEP (MultiContinous Electrical Profiling), which gives measurements at high spatial resolution, and by precise measurements of soil water content. The spatial organization of soil water dynamics was extrapolated from the geostatistical analysis of the spatial and temporal variability of electrical resistivity, and then improved data were obtained for the whole of the studied area. Homogeneous zones were then delineated and compared with soil units defined on a soil map. We demonstrated that the zones that had less than average soil water content over time for the whole area corresponded to those zones that had greater than average electrical resistivity values over time. The spatial similarity between these zones and the soil units defined on the soil map suggested that water flows were mainly vertical in the field. Nevertheless, our method could be used to characterize some specific hydrodynamic processes, such as lateral flows or upward capillary flows, that are usually difficult to characterize from the information derived from the soil map.

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sol / conductivité thermique / in situ / sonde à choc thermique / méthode de mesure Summary — Estimating thermal conductivity of soil in situ, with a new thermal probe. A simple system, using a thermal probe was designed to measure the thermal conductivity of a soil in situ. The operating conditions were investigated. The system was shown to be quite adapted for the field experiments. Measurements were made in a siltyclay-loam soil for different humidity and density conditions. As expected, the thermal conductivity increased with gravimetric water content and density of the soil. In the deeper layers, with homogeneous and larger densities, the thermal conductivity had high, steady values even at low water content, which may be due to the shrinkage of clay as the soil dries, compensating the diminution of thermal conductivity due to water losses. This behavior was quite different from that of the tilled surface layers with a granular structure. The measured values of thermal conductivity were higher than those found by laboratory methods using reconstituted soil samples. The absence of a reference method for in situ determination of the thermal conductivity did not allow us to completely validate this thermal probe.soil / thermal conductivity / in situ / thermal probe / measuring method