Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 30, 2008
Publication Date: May 1, 2009
Citation: Schwartz, R.C., Evett, S.R., Pelletier, M.G., Bell, J.M. 2009. Complex permittivity model for time domain reflectometry soil water content sensing: I. Theory. Soil Science Society of America Journal. 73(3):896-897. Interpretive Summary: Time-Domain Reflectometry (TDR) is the most widely used and accurate electromagnetic technique for estimating water content in the soil. Because of its ability to automatically acquire measurements at multiple locations and times, TDR is a promising technique for monitoring soil water. However, there are serious difficulties in estimating accurate soil water contents using TDR under field conditions. The greatest obstacles are its sensitivity to soil clay content, electrical conductivity, and to fluctuations in temperature. We developed a physically-based calibration model to remove soil clay, electrical conductivity, and temperature effects from water contents estimated using TDR. The model was able to correctly describe the behavior of apparent permittivity in response to temperature for two soils. Ignoring the effects of clay and electrical conductivity in the calibration equation would cause large (5%) errors in field estimated water contents.
Technical Abstract: Despite numerous applications of time-domain reflectometry (TDR), serious difficulties exist in estimating accurate soil water contents under field conditions remain, especially in fine-textured soils. We developed a physically-based calibration model to predict the frequency and temperature dependent complex dielectric response of soils. The model was used to predict frequency dependent attenuation and a single “effective” frequency approximation of apparent permittivity of the soil. Bandwidth was predicted to decline from 450 MHz to 160 MHz as water contents increased from air dry to saturation. Predicted frequency decline was small for an input bandwidth of 130 MHz reflecting that modeled polarization mechanisms associated with relaxation frequencies above 100 MHz were responsible for most of the frequency dependent attenuation. For specific surface areas ranging from 150 to 300 m2 g-1, simulations indicate that ignoring dielectric and conductive losses or the associated decline in effective frequency results in a 5 to 22% underestimation of the apparent permittivity. Both the power law and de Loor–Dobson mixing models gave a reasonable approximation to the measured apparent permittivity for a silty clay loam (34% clay) across the entire water content range. Moreover, modeled results were able to describe the behavior of apparent permittivity in response to temperature for two soils with contrasting bulk electrical conductivity contributions to losses. These results demonstrate that loss mechanisms and declines in effective frequency will need to be considered to accurately predict soil water content of fine-textured soils.