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NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za with the body of the message containing the line: unsubscribe sowacs Dear Wenceslau, Steve and Cliff It is unfortunate that the hysteresis in soil water characterisitics is neglected in so many soil water modelling exercises. For this reason, the so-called physically based models (Richards equation) fail to give any better predictions than empirically based models. I measured "field" values of soil water characteristic using lysimeters (Water Resources Reseach (1980) 16:749-754). I believe that the difference between the laboratory characteristics and the field characteristics is most likely to be a reflection of the difference in "representative elemental volume" being measured by the difference measurement techniques. This also remains a neglected area of study in soil physics. Martin Parkes Wuhan University (WUHEE) 8 Donghu Nanlu, Wuchang 430072 WUHAN P R CHINA > I endorse Steve Evetts comments on the difficulties of getting truely > undisturbed > field samples. > The rule we use in Australia is that (ideally) the sampler must have the > following characteristic > > (area of the circle formed by the external diameter of the sampler minus > the area > of the sample) > divided by the area of the sample should not exceed 5%. > > This means the sampler must be a relatively thin metal ring with no reinforced > tips or outside sleeves - *(I think this is what Steve calls a double ring > sampler). It also means that each sample has to be quite short - > generally, the > length should not exceed the diameter. > > An illustration of a suitable device (Tanner sampler) is on my web site > www.soilwater.com.au - follow the links to 'equipment' > Cliff.hignett@soilwater.com.au > > owner-sowacs@aqua.ccwr.ac.za wrote: > > > Dear Wenceslau, > > > > I can think of three things that could easily bias the field vs. laboratory > > determinations. You have already mention hysteretic concerns and they are > > really troublesome in any such comparison. > > > > First is the question of how undisturbed the cores were. Many of the coring > > sets I have used tend to compact the sample (e.g. double ring samplers). > > The degree of compaction may be slight but still have a measurable effect > > on retention characteristics. The best cores I have obtained have been with > > thin-walled sample rings that had their own cutting edges, and that were > > driven into the soil with a driving head - not the usual practice of > > placing cylinders inside a sampling tube with a separate cutting bit. > > > > Second is the question of how quickly the tensiometers equilibrated. We > > know that the rapidity of equilibration will decrease as the soil dries and > > conductivities in the soil and the porous cup of the tensiometer decrease. > > Particularly in an instantaneous profile method, this lack of rapid > > equilibration can cause the tension read by the tensiometer to be behind > > what is happening in the field. In contrast, with laboratory methods we > > usually ensure equilibration by dewatering the core in steps, with long > > enough waits between steps to achieve equilibration or a close enough > > approximation of it. > > > > Third is the question of representativity of the cores vs. the field site. > > Cores usually cut across macropores and change the nature of the dewatering > > process. They really cannot be representative of a field situation. > > > > I am not surprised to see different results from these very different > > methods. Rather, I would be surprised if they gave identical results. Hope > > this helps. > > > > Steve > > At 07:06 PM 2/28/2001 +0000, you wrote: > > >NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za > > >with the body of the message containing the line: > > >unsubscribe sowacs > > > > > >Dear list members. > > > > > >I am working with experimental water retention data to fit parametric > > >equations, which I'd like to use in numerical simulations of water fluxes. > > > > > >I have determined in-situ volumetric soil water content by TDR readings and > > >soil pressure head by tensiometers. However, measured field data disagree > > >with the determined on undisturbed soil cores samples collected at the same > > >site. > > > > > >The problem of hysteresis was partially circumventing collecting the field > > >data in the drying brunch of the hysteretic curve (it was carried out on an > > >instantaneous profile method experiment). > > > > > >The volumes sampled by TDR and the cups of our tensiometers are not the > > >same and, not only the volumes were different but they are separated > > >because of installation procedures. Moreover, the soil samples for the > > >laboratory measurement were collected about 2 meter apart.These factors are > > >probably some explanation for the differences, but our data set is large > > >with many replications and, I believe that some other factors besides soil > > >variability are contributing to that difference ... > > >I have found a limited number of field studies (where in situ measurements > > >were carried out) in which undisturbed soil cores were taken. > > > > > >Someone knows studies concerning the problem of field x laboratory > > >determination of the soil water retention curves ? or have some > suggestions ? > > > > > >Any help on this matter will be greatly appreciated > > > > > >Best regards. > > > > > >Wenceslau Teixeira > > > > > >_____________________________________________________________ > > >Wenceslau Geraldes Teixeira > > >Embrapa - AmazŁnia Ocidental - Brasil > > > > > >Universitt Bayreuth > > >Bodenkunde - Abt. Bodenphysik > > >D - 95440 - Bayreuth - Germany > > >Fax: +49 (0)921 552246 > > > > ___________________ > > Steve Evett, Soil Scientist > > USDA-ARS, 2300 Experiment Station Rd., Bushland, TX 79012 USA > > 806-356-5775, FAX: 806-356-5750 > > srevett@cprl.ars.usda.gov, http://www.cprl.ars.usda.gov/programs/ > > -- > Cliff Hignett > Soil Water Solutions > 45a Ormond Ave > Daw Park > South Australia 5041 > pH 61 (08) 8276 7706 > WWW.SOILWATER.COM.AU