A general heat dissipation sensor calibration equation and estimation of soil water content

Ten heat dissipation sensors (HDS) were calibrated in the laboratory in order to produce a simple, generalized linear equation that could be applied to a large number of uncalibrated sensors of like manufacture. This generalized equation was based on temperature rise of the sensor and was compared with Reece's (1996) equation, which was based on thermal conductivity. Both of the calibration equations produced similar matric potential output, indicating that an equation developed from a small number of HDS can be applied to uncalibrated sensors. Matric potentials from both generalized equations were converted into estimates of volumetric water content and compared with measured values at three rangeland sites in southwestern Oklahoma. At two sites, volumetric water content from the HDS compared well, within the operating limits of the sensor, with volumetric water content determined from soil samples. The third site was characterized by a sandy loam soil, and during drying cycles, the HDS-derived values of water content did not compare well with the soil sample values. It is suspected that the sensor lost hydraulic contact with the soil, which drained faster than the sensor indicated.