Contribution of Electrical Resistivity Tomography (ERT) measurements for characterizing hydrological behaviour of an experimental plot in relation to pedo-geological factors (AOC Gaillac, SW France)

Electrical Resistivity Tomography (ERT) measurements have been performed by the Wenner method on an experimental plot situated in Gaillac region. They have been carried out during two highly contrasted hydric periods: (i) dry (spring 2006), (ii) humid (spring 2007) with soils close to field capacity. Results are compared to evaluate the hydrological behavior of the plot in relation with its main pedo-geological characteristics. The three reiterated transects (North-Median-South) give a general view of the plot configuration in agreement with the pedo-geologic observation trenches data. All the resistivity profiles show the superposition of two highly contrasted sequences. The first sequence, at the bottom, is a very low resistivity values sequence (up to 40 Ω.m) which coincides with the argillaceous-dominated molassic bed-rock. The second is a high to very high resistivity values sequence (from 300 Ω.m to more than 1500 Ω.m at the very top) which coincides with a silty-sandy and gravels soil complex of about 2 m thick. Resistivity of the molassic clayed-dominated geological basement does not depend on climatic conditions and stays at a very low value independently of dry or humid periods. Resistivity values of the silty-sandy/gravels horizons vary with a factor 2, from 300 to 750 Ω.m in humid conditions and from 750 Ω.m to 1500 Ω.m under dry conditions. Furthermore, the invariant location in the resistivity profiles of the two sequences, implies that the water runoff at the molassic bed rock/gravels interface is short-lived and most probably of low amplitude.
The hydric behavior of the experimental plot evidences a high risk of drought stress during summer. The choice of a rootstock with a hemi-plunging habit (Gravesac) will allow roots to attain the moisture at the molasse/gravels boundary and protect them from excess of drought.