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Dear Josep Maria, My understanding of the literature is that Topp's equation hasn't proven to fit any particular soil 'exactly' but that it fits the ensemble of soil water content and K data very well. Some persons with whom I've discussed this have shown me results that indicated that Topp's equation was biased by 0.02 to 0.05 m^3/m^3 but their wave form interpretation methods were different from what Topp used. When they re-interpreted their data using methods more similar to Topp's they found that Topp's equation was not significantly different from theirs. This issue of wave form interpretation is critical for calibration efforts. Most papers in the literature do not describe the interpretation methods used in enough detail to allow reproduction of the method, or understanding of what was done. Different interpretation methods can easily produce water content differences of 0.03 m^3/m^3 or more. And, simply citing one of Topp's paper is hardly adequate - no single paper does this topic justice. The take home message is that if you want water contents accurate to better than +/- 0.02 m^2/m^2 then do your own calibration. That calibration will include effects both of your particular soil type, and of your wave form interpretation method. If your wave form interpretations are reproducible then your calibration should work. In reference to fitting the data both ways; K=f(theta) and theta = f(K): my experience is that there is little difference if the data exhibit small scatter. If there are important discrepancies, I would re-do the calibration and try to get data with less scatter. Yes, I know this isn't always possible. Best wishes, Steve Steve Evett srevett@ag.gov 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