Effects of soil type and temperature on soil water content measurement using a soil dielectric sensor

Authors: Seyfried, Mark; Murdock, Mark

Submitted to: First International Symposium on Soil Water Measurement Using Capacitance Impedance and Time Domain Transmission
Publication Type: Proceedings
Publication Acceptance Date: 11/20/2002
Publication Date: 11/20/2002
Citation: Seyfried, M.S., Murdock, M.D. 2002. Effects of soil type and temperature on soil water measurement jusing a soil dielectric sensor In: First International Symposium on Soil Water Measurement using Capactance and Impedance. Nov 6-8, Beltsville, MD.

Interpretive Summary: In recent years a number of relatively inexpensive soil water sensors have been developed. These instruments may potentially be applied to a variety of research questions regarding soil water movement and plant water use as well as commercial uses such as irrigation scheduling. Most of these sensors have not been rigorously tested, which hinders their application. We tested the Hydra soil water sensor for accuracy of calibration and temperature sensitivity on four different kinds of soil, different salt solutions and other fluids with known properties. We found that the instruments were accurate and precise in known fluids. In soils we found that calibration by soil type was important. We also found that, for some soils, there was a marked effect of temperature. We suggest that it may be possible to correct for these effects using the electrical conductivity measured by the instrument. Further demonstration of this approach requires testing on a wider variety of soil conditions.

Technical Abstract: We tested the Hydra soil water sensor for accuracy of calibration and temperature sensitivity. We found that the sensors are very precise. Measurements in fluids of known dielectric constant, ethanol, water and air, were accurate when the solution concentration was less than 0.01M. The sensor calibration for soil water content was evaluated using four different soils, the Summit, Sheep Creek and Foothill soils and sand. For the sand, the Topp equation agreed closely with measured data and the manufacturer's calibration. For the other three soils tested, there were large discrepancies between both the Topp and manufacturer's equations and measured data. Temperature effects were minimal in air and oven-dry soil. In saturated soils, two of the four soils tested were practically insensitive to temperature and two indicated apparent increase in water content over a 40 °C temperature change of about 0.10 m3/m3.