Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/29/2007
Publication Date: 11/15/2007
Citation: Seyfried, M.S., Grant, S. 2007. Temperature Effects on Soil Dielectric Properties Measured at 50 MHz. Vadose Zone Journal, 6:759-765.
Interpretive Summary: With the recent developments in electronics there have been a number of new instruments designed to measure soil water content electronically. Aside from the benefits for scientific research, these instruments have many practical applications related to water management. For example, irrigation of crops or golf courses, monitoring of waste containment or prediction of water supply, are all enhanced with accurate soil water information. One difficulty in using these new instruments that has emerged is that the less expensive, and thus more practical, instruments tend to highly temperature sensitive in some types of soil. This means that soil water content may appear to change when only the temperature has changed. The effect can be dramatic in some cases. In this study we set out to quantify how much that change is for a wide range of soils (19) taken from around the USA. We also wanted to dissect the electronic signal used to determine soil water content into its components to establish what is causing the temperature sensitivity. Our results apply to one such commercial sensor, the Hydra Probe, which is unique in its ability to separate different components of the signal. We found that there is a large range of temperature response among soils, some negative and some positive. The direction and amount of the response is related to the soil electrical conductivity, which is highly sensitive to temperature. Since most sensors do not separate out the electrical conductivity portion of the signal, those sensors may be highly temperature sensitive in soils with relatively high electrical conductivity. The Hydra Probe, which does separate the components, is much less sensitive. These results have important implications for the application and future design of soil water sensors. They also show that the Hydra Probe temperature sensitivity, while clearly measurable, is constrained to a manageable amount. This will support work on the SNOTEL and SCAN networks currently in place to aid in water resource management and in ongoing commercial applications of the Hydra Probe.
Technical Abstract: In recent years a number of soil water monitoring instruments have been developed and made commercially available. These instruments determine soil water content from a sensor signal from which soil dielectric permittivity is resolved, which is a complex number and composed of real and imaginary components, representing energy storage (real) and energy losses (imaginary). Although it is known that these instruments are sensitive to temperature change in some soils, little empirical data exists describing the degree of this sensitivity or the two components. We quantified temperature effects on both the real and imaginary components of the permittivity for 19 soils collected around the USA using the Hydra Probe soil water sensor, which operates at 50 MHz. We found that the real component response ranged from positive to negative such that the effect of a 40 °C temperature change resulted in a maximum apparent water content change of ± 0.028 m3m-3. The imaginary component was much more sensitive to temperature than the real. We also found that this response could be closely approximated by assuming that it is due to electrical conductivity. These results explain the extreme temperature sensitivity observed with some instruments. In addition, we found a good correlation between the effect of temperature on the real component and the magnitude of the imaginary component. It is therefore possible to estimate temperature effects on water content estimation from measured data only. This result, however, applies only to saturated conditions. Further research is required to incorporate water content into the temperature response.