The time-domain reflectometry (TDR) technique has been one of the most used techniques to determine soil volumetric water content (theta) owing to some favorable characteristics such as high precision, non-ionizing radiation and little influence of soil salinity, bulk density and texture. However, most of the TDR equipment and probes available do not allow a detailed measurement along the soil profile, as for instance the neutron probe does. For conventional TDR probes holes or trenches must be opened in the soil to install the probes, which limits the number of points measured in the soil profile. Therefore, the aim of the present study was to test a TDR probe system that allows a detailed description of the water content in the soil profile with no need of installing individual probes. The system is a two-rod coiled probe fixed to the end of a metal rod, linked to a impact penetrometer used for the insertion of the probe to the desired depths. The probes were calibrated in the laboratory with 7 soils of different textures and validated at field conditions for the same soils. The dielectric constant (theta) measured with the coiled probe as a function of the water content was best fitted with the mathematical function theta = a - bepsilon + c/epsilon2. The field experiment for validation of the calibration equations presented a relative root mean square error of 6% considering one equation for all soils together, and 5% when considering two different equations for two set of soils grouped according to their texture.
time-domain reflectometry; soil water content; texture
