Operative Calibration Methodology of a TDR Sensor for Soil Moisture Monitoring under Irrigated Crops

Time domain reflectometry (TDR) is more and more frequently used for soil water content measurements in replacement of other techniques as neutron probe. Such observations that can be continuously collected on dataloggers are convenient for monitoring water fluxes under irrigated crops. Though relationships to calculate volumetric water content from analysis of TDR signal and collected data are published in literature or given by manufacturers for different commercialized devices, the results are not satisfactory for many soils and specific calibrations are required. In replacement of the traditional calibration method, two rapid operative computer assisted methods are proposed. The first one is based on drip moistening of a cylindrical sample of disturbed dry soil in which a TDR sensor is vertically embedded. The second one uses the same cylindrical sample wetted close to saturation in which a TDR sensor is inserted step by step during measurements. The data from the two methods were fitted using second degree models. These results are in good agreement with conventional calibration method or gravimetric field measurements of soil water content. Utilization of short time step TDR measurements for monitoring soil water storage under a furrow irrigated corn shows consistent observations with water applications or uptakes by crop. Field comparison with neutron probe and gravimetric measurements corroborate the need of a specific calibration for the soil studied in this paper though its clay content is about 20%.