Friday, 14 July 2006

Spatial and Temporal Monitoring of Water Flow Using 2d Electrical Resistivity Tomography in a Cultivated Soil : A Decimeter Scale Study.

Didier Michot, UMR «Sol, Agronomie, Spatialisation», Agrocampus - INRA, 65, rue de Saint-Brieuc, BP 35042, Rennes cedex, Rennes, France

Tools actually available to study water flow in soil are limited by their point-to-point measurement. They are also sometimes intrusive or destructive. Since geophysical methods are non invasive, they can be of great help in soil studies because they disturb neither the structure nor the water dynamics of the soil. The aim of this work was to monitor over time by geophysical methods soil water fluxes, and to relate the results to the spatial variability of structure, bulk density, porosity and hydraulic conductivity induced by agricultural practices. The electrical resistivity tomography method was tested on a cultivated sandy-loamy soil during a period of irrigation to monitor soil water flow over time and in two dimensional at a decimeter scale. Simultaneous monitoring of soil moisture and soil electrical resistivity in the soil profile was performed over a 6 days period. Before, during and after water sprinkling, soil water flow was monitored over time by 2D electrical resistivity tomography using a pole-dipole array and conventional soil moisture recording methods (i.e. buried tensiometers). Simultaneously, soil water electrical conductivity, soil temperature, precipitation and irrigation were measured. Physic and hydrodynamic properties were also characterized by cultural and soil profiles description, bulk density and hydraulic conductivity measurements. Results show that electrical resistivity method allowed good delineation of soil horizons, but non-accurate delineation of the different tilled-horizon structures observed on the cultural profil. After irrigation, resistivity decreased as soil water content increased. Electrical resistivity changes showed water infiltration according to hydric potentiel gradients and defined three preferential water flow pathways. These ones were developed between no-compacted and compacted tilled-horizon structures. One preferential water flow pathway detected by the geophysical method corresponds to a significant higher hydraulic conductivity near saturation (h= -0,05kPa). Overall electrical resistivity tomography can trace the preferential flow paths continuously along the plane of measurements according to the spatial variability of soil structure and soil hydraulic conductivity. The results of the individual applied methods complement and confirm each other. Keywords: electrical resistivity tomography ; preferential flow ; water ; soil structure.

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