Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 26, 2004
Publication Date: November 16, 2005
Citation: Logsdon, S.D. 2005. Time domain reflectometry for high surface area soils. Vadose Zone Journal. 4:1011-1019.
Interpretive Summary: Equipment is available that determines soil water content based on how the positive and negative ends of the water molecule move when an electrical field is applied. The information from this equipment depends strongly on other factors in the soil, such as the high amount of negative charges on the clays and the positively charged ions associated with the clays. These factors cause the calculated water content to increase when temperature is high and decrease when temperature is low. This research showed that including a temperature term in the calibration equation helps to correct the temperature effect; however, accurate water content determination was only possible for a few soils in the field. This information is important for scientists who use this equipment or related equipment to determine soil water content. The information is also vital for irrigation managers who use related equipment for irrigation timing.
Time domain reflectometry (TDR) is commonly used to determine water content. Many laboratory studies have shown accurate water determination across a wide range of soils, often with a unified calibration equation. Recent data has shown a strong diurnal temperature influence on TDR data. The purpose of this study was to calibrate TDR for water content under both laboratory and field conditions. TDR waveguides were installed horizontally into the sides of pits at four sites and nine or ten depth-positions on a 5% slope. Thermocouples were installed at the same depths. Neutron access tubes were installed within 3.3 m of the TDR sites. After the field study, soil was collected around the waveguides and repacked into columns for laboratory calibration. The columns, which were then progressively wetted and then dried over a few months. At each water content and at two or three temperatures, the waveforms, bulk electrical conductivity, and square root of apparent dielectric were saved for the laboratory data, but memory limitations prevented saving waveforms from the field data. Much of the field calibration data were shifted to higher values for dielectric than for the laboratory data, apparently due to problems with internal waveform analysis. High bulk electrical conductivities were apparent for some of the sites and depths even though these are not saline soils. For the laboratory data, adding a temperature term to the calibration equation reduced the root mean square error (378 data points) from 0.54 to 0.34 m/m, and for the field data (939 points) reduced it from 0.78 to 0.54 m/m. Saving the waveforms for post-processing should improve use of TDR for determining water content in the field; however, some sites might still not have useable data.