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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 I think Len Ornstein and I have a communication problem - what we are saying is not substantially different. Most basic soil physics texts describe the process quite well. I will try for a 'potted version' in different words.- All saturated porous materials have a 'bubbling pressure'. This is the air pressure needed to force water out of the largest pores (or the suction in the soil water to drain the largest pores - same thing) In terms of the relationship between water content and suction, it means that an initially saturated material when subjected to a steadily increasing suction (starting at zero) will hold all its water until the bubbling pressure is exceeded. In materials with a wide range of pore sizes - like loams, there are usually enough very big pores that this point is close to zero suction - say 0.01kPa. In a sorted, uniform pore size sand, such as exist in beach or dune sand deposits, the bubbling pressure is often as high as 1 to 5 kPa (corresponding to hydraulic heads of 10 to 50 cm suction) Until this pressure is reached, there is no drainage at all. When this pressure is exceeded, nearly all the water drains. This produces a water retention curve , and a hydraulic conductivity curve which approximate step functions. I agree with Len that some water remains on the grain surfaces and grain contacts which drains slowly but this usually represents only 1-2 % of the saturated water content. A typical sand might go from (say) 30% water content at saturation to 2% when its bubbling pressure is exceeded. An interesting side effect of all this came to light on a survey of the water use characteristics of wheat plants across southern Australia. The survey used the varieties selected by farmers for optimal production in their soil and climate conditions We found that the varieties bred on sandy soils grew much greater root lengths than those bred on clay soils - even when rainfall was the same. This makes genetic sense. If a plant is to survive in a sand where the only water is held tightly to grain surfaces (most of the time) with poor hydraulic conductivity (most of the time) then it will need to have a roots separated by as small a distance as possible. In a clay or loam soil, the hydraulic conductivity is much higher even at quite high suctions so roots can be further apart and retain the same water gathering capability. 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 > > 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 > >> > >> > >> > >> > >> -- Cliff Hignett Soil Water Solutions 45a Ormond Ave Daw Park South Australia 5041 pH 61 (08) 8276 7706 WWW.SOILWATER.COM.AU