proudly sponsored by
|
archiving of SOWACS proudly sponsored by |
|
Dr. Ioan C. Paltineanu writes: Dear All, On September 24, 1998, I sent to the owner-sowacs@agua.ccwr.ac.za an ample response (including recent presentations at the 16th World Soil Science Congress, held at Montpellier, France in August 1998) to the Lindsay's question about diurnal changes observed with the real-time soil water dynamics monitoring systems using TDR or capacitance sensors. Since I have not received any comments so far, [this post has only now been sent - apologies - Bruce] I would like to draw your attention to another problem, calibration of TDR and capacitance sensors in 2:1 clay type soils, as a special case of the generalized formulas obtained so far. Dr. J. L. Starr and I concluded in our paper "Real-time soil water dynamics using multisensor capacitance probes: laboratory calibration", published in the Soil Sci. Soc. Am. J. 61: 1576-1585 (1997), that more calibration research with these capacitance sensors is needed (as has been the case with all methods that use Ka to calculate soil water content) for special soils (e.g.., swelling 2:1 clays or high organic matter content) etc.. We cited as reference, among others, the paper of Bridge, et al. (1996) on "The dielectric behavior of clay soils and its application to time domain reflectometry", published in the Aust. J. Soil Res., 1996, 34, 825-35. The authors concluded, by calibrating TDR on a self-mulching swelling clay (Black Vertisol), that the apparent dielectric constant (Ka) versus soil water content curve was bellow the Topp curve (at least a 0.06 m3/m3 underestimation of soil water content by using Topp curve on similar vertisols). I will appreciate very much having some information on the laboratory calibration of the multisensor capacitance probes in 2:1 expanding clay type soils (montmorillonite especially) in order to compare to the scaled frequency (SF) versus soil water content formulas obtained by us and others experimenting on different soils. Best regards, Ioan. Dr. Ioan C. Paltineanu visiting scientist USDA, ARS, BARC, NRI, ECL Bldg. 007, Room 224, BARC-West 10300 Baltimore Ave. Beltsville, MD 20705-2350 U. S. A. Phone: 301-504-5032 Fax: 301-504-5048 email: ipaltin@asrr.arsusda.gov http://hydrolab.arsusda.gov/ecl