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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 Cliff Hignett's reply was correct for the kind of sand he described. Uniform sands such as he describes drain only a very small amount until the air entry potential is reached, for his sand at a suction of 49.5 cm of water. This is consistent with the physics of capillary rise. Soil physics texts such as those by Hillel; Iwata, Tabuchi, and Warkentin; Baver, Gardner and Gardner; Rose; and others all describe this phenomenon. Of course, many of us have done similar measurements on aeolian sands that we have in our own laboratories. Such sands are very useful as interface materials between tension infiltrometers and soil surfaces because they remain practically saturated at the tensions used when operating the tension infiltrometer and thus readily transmit water to the soil surface. Well graded sands (those with a mix of particle sizes, that is, not uniform) will behave differently because they have a wider range of pore sizes. They will drain over a range of suctions, rather than almost all at once when a particular suction is reached. Alluvial sands are sometimes well graded. Len's answer is also at least partially correct. For any sand (not containing clay or silt sized particles), even well graded ones, the pore sizes will be large enough that the capillary rise is not large; and the (nearly) saturated depth of the sand will descend as the water table descends. Part of the confusion here stems from a lack of defining what is meant by "water table". If we define the water table as being at the depth at which soil matric potential is zero then we see that there is often a zone of soil above the water table that is nearly saturated. If the soil is a sand, then that nearly saturated zone will descend practically as fast as the water table descends because the hydraulic conductivity of the sand is so great. I think that is what Len was trying to say. I would suggest to Jiny that she read some of the soil physics texts mentioned above. She will see there is no single relationship between matric potential and capillary rise because sands differ in particle size and in the grading of particle sizes. She will also see, as Cliff said, that the relationship differs depending on whether the sand is being wetted or drained. The best way to find out the answer for one's own sand is to make the necessary measurements. For sands they can practically all be done with hanging water columns and are easy to do. Best regards, Steve Evett At 08:19 AM 6/4/2001 +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 > >At 7:01 AM +0000 6/3/01, owner-sowacs@aqua.ccwr.ac.za wrote: > >Sorry to say, but Cliff Hignett's reply below is in error > > > > Reply to Ginny Carrera > >> > >> A sand, particularly a pure sand of dune origons will have a very > >> uniform grain size. That means it also has a uniform pore size which > >> in turn means that if a column of saturated sand is subjected to > >> higher and higher suctions (raised further above the water table) , it > >> will not change water content very much at all until a particular > > > suction is reached, then nearly all the water drains at once. > >This is physically impossible! > >If such a column of sand is TRULY saturated (all its pores are full >of water) most of that water will IMMEDIATELY begin to drain from the >pores as the water table drops. However the residual capillary water, >wetting the surfaces along contacts of sand grains, will drain at an >increasingly slower rate. > >Len Ornstein > > > > I have a > >> sand which drains at 49.5cm suction precisely leaving the sand water > >> content virtually zero.. As the silt and clay content increase, the > >> material will drain slowly over a wider range of suctions and will not > >> drain completely. > >> > >> The suction at which it wets will also be very precise - but it is > >> unlikely to be the same suction as that at which it drains. > >> cliff.hignett@soilwater.com.au > >> > >> owner-sowacs@aqua.ccwr.ac.za 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 > >> > > >> > Jiny Carrera asks > >> > > >> > In a sand (free of osmotic suction) what is the > >> > relationship between matric suction and the height of > >> > the soil with respect to the water table? > >> > > >> > Is this relationship the same for both, wetting and > >> > drying process? > >> > > >> > __________________________________________________ > >> > >> -- > > > Cliff Hignett > >> Soil Water Solutions > >> 45a Ormond Ave > >> Daw Park > >> South Australia 5041 > >> pH 61 (08) 8276 7706 > >> WWW.SOILWATER.COM.AU > >> > >> > >> > >> > >> ___________________ 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/