Evaluation of soil moisture by electrical resistivity in Oxisols of the central Brazilian savanna.

Abstract

The determination of the spatial distribution of soil moisture in the field is relevant for many purposes in agriculture and engineering, but it is still a challenge, since standard determination methods have limitations and problems of representativeness. Electrical resistivity may be used with this finality, but this method is still poorly studied for highly weathered tropical soils, which present structural and textural peculiarities. This work aims to present the calibration of the moisture/electrical resistivity relationships for a typical tropical soil, and to evaluate its robustness as a soil moisture prediction method. A highly weathered clay soil (Oxisol), representative of the Brazilian savanna (“Cerrado”), was sampled and the resistivity tests of the A, Bw and C horizons were conducted in a four-electrode cell array. The A and Bw horizons of clay Oxisols usually present resistivity values greater than temperate soils counterparts, which can be attributed to the lower content of highly active clays and higher macroporosity. The soil electrical resistivity/volumetric moisture relationship present a nonlinear and inverse trend and the generalized form of Archie’s law has the potential to be applied to determine the water content of these soils. The electrical resistivity/volumetric soil moisture semi-log plot showed a two-step pattern,

with a remarkable change in the behavior around the moisture associated to the wilting point. The field vali- dation shows that the use of the electrical resistivity is possible for the measurement of soil moisture and for the

subsurface mapping of its horizons.