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Dear Sowacers, Very interesting TDR discussion. I'm learning lots. Just like to agree with Sam Hokett re working with 'difficult' soils and TDR. I worked with TDR in rainforest soils to 2 m depth, with a high clay content (> 50 %) and very high woody root density and I felt the error in obtaining accurate bulk density estimates for converting gravimetric to volumetric samples was large under these conditions. As I was more interested in relative changes I relied on the Topp equation. Comparisons between TDR volumetric and converted gravimetric were reasonable (r2=0.8) anyway. > > well i have been a tdr user and fan for quite some time and > am glad to hear all this good sense talk about keeping things simple > and using the good old topp equation. After all, not having to > calibrate for specific soil type is what makes tdr such a nifty > technique. In my experience, working with difficult desert soils, there > is generally more error in the bulk density measurement needed to convert > gravimetric water content to volumetric water content, than there was in > the tdr measurement to begin with! Steve e. also brings up > a very good point about the interpretation of the tdr trace which > is usually not mentioned in great detail and can make quite a significant > difference in the measured water content. The only soils where i have found > specific calibration necessary has been in conductive soils where tdr > over predicts water content. In such soils the problem is futher complicated > because the tdr traces become more difficult to interpret due to signal > attenuation. > > Sam hokett > desert research inst. > las vegas, nv > Lindsay Hutley CRC for the Sustainable Development of Tropical Savannas School of Biological Sciences Northern Territory University Darwin NT 0909 AUSTRALIA Ph (+61) 08 8946 7103 Fax (+61) 08 8946 7107