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Grant, I think Marty and Trevor did a good job answering your questions. My experience is that repeated measurements at the same location are much more precise than accurate for several methods of soil water content measurement, including some methods that are not at all accurate. (I'm thinking of some of the frequency domain methods). We recently did a study (submitted to Computers and Electronics in Agriculture) where we took 80 repeated measurements through 1 channel of 1 multiplexer of the water content of a saturated sand. The TDR probe was 30 cm long, trifilar, and remained undisturbed in the sand during the entire experiment. Standard deviation (SD) of the water contents was 0.0004 m^3/m^3. (For those who saw my earlier message, I did mean m^3/m^3 not m^2/m^2). We then took 80 more readings of the same probe but used a different input channel each time, and used 5 multiplexers to get the 80 readings (16 channel multiplexers). The SD of water content the second way was 0.0006 m^3/m^3, indicating very little additional error introduced by using the multiplexers. Either way, it is obvious that the precision of TDR can be very good, as you mentioned. In the field, for slowly changing water contents, we routinely observe precision of better than 0.001 m^3/m^3. As far as calibration goes, as Grant mentioned, the precision of gravimetric soil water contents can be much better than +/- 0.02 m^3/m^3. I routinely use a Madera probe bit to take soil samples. This bit has a 60 cm^3 volume so with careful sampling I get both volume and gravimetric water content for every sample. Calibrating neutron probes this way, our root mean square errors are routinely less than 0.01 m^3/m^3 for the calibration equation. The bottom line for a calibration is the error term for the calibration equation regression. Why not use Topp's equation? That's exactly what I do most of the time, and it's what I recommend to others, but with the caveat that for real accuracy (not precision) you have to do your own calibration. I say this partly because of the wave form interpretation dilemma that I mentioned earlier. Best regards and thanks for the good questions, Steve Steve Evett srevett@ag.gov http://www.net.usda.gov/cprl/ USDA-ARS, P.O. Drawer 10, Bushland, Texas 79012 U.S.A. (1/2 mile W., Interstate-40 S. access road) Tel. 806-356-5775, FAX: 806-356-5750