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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 Steve: Yes. I define the water table as being at 0 soil matric potential, and a "TRULY" saturated sand would also be at 0 potential, i.e, the water table would rise to the top of the sand. Len At 7:08 AM +0000 6/5/01, owner-sowacs@aqua.ccwr.ac.za wrote: > >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: > > > >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/