Explicit Effects of Water Content and Bulk Density on TDR Response.
Ben Goff, Louis M. McDonald, and Eugenia Pena-Yewtukhiw. Division of Plant & Soil Sciences, Evansdale Dr., West Virginia University, Morgantown, WV 26506-6108
Although the standard procedure to determine bulk density (BD) is straightforward, it requires destructive sampling and is labor-intensive, especially at depth. In addition, because the range of soil BD is small (0.9 to 1.4g cm-3) measurement errors often lead to a situation where BD is not a significant variable even when other soil properties suggest that it should be. Time-Domain Reflectometry (TDR) is a nondestructive method for determining soil volumetric water content (VWC). Because BD is part of the equation to convert gravimetric water content (GWC) to VWC, we attempted to use this relationship to determine the effects of soil settling on BD and soil water storage. For this relationship to work, TDR-VWC must be insensitive to changes in BD. Previous work suggests that TDR response is not affected by BD, but these studies used a range of soils with varying texture and BD. Our objective was to determine the effect of BD on TDR response on a single soil. Moist soils were packed into PVC columns to achieve a range of BD (0.9 to 1.35 g cm-3). Apparent dielectric content (K_A) of the media was determined by TDR using a three-prong 20 cm waveguide. VWC and GWC were determined independently. Apparent K_A was converted to VWC using equations available in the literature. Results indicated that the TDR determined VWC was not proportional to the actual VWC. Differences were larger as BD decreased. This suggests that BD needs to be accounted for when measuring VWC by TDR on settling soils.