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"Richard G. Allen" writes to the sowacs list: ===8<============== Dear Mike: Some experiences in Idaho find that on occasion, granular matrix sensors have problems rewetting after they have been dried to very dry levels. This seems to occur even when the sensors have good contact with soil. Some of this phenomenon is attributed to the ability of water films to reenter the coarse medium of the GMS from a fine soil. It is similar to the build up of water in a fine soil layer over a coarse layer, during infiltration, until the soil water potential is high enough at the interface to create sufficient hydraulic conductivity in the underlying coarse medium. Rehydration of dry GMS sensors does not seem to be as much of a problem with surface irrigation, where the soil water potential during infiltration becomes high enough surrounding a GMS to rehydrate it. The problem seems to be greatest with center pivot irrigation (as has been noted by Dr. Bradley King, Univ. Idaho), when the soil layer surrounding a GMS is allowed to become very dry due to progressive underirrigation. Subsequent increased irrigation and soil rehydration by the center pivot system may not show coincident response from the GMS if the surrounding flux of water in the fine soil is occurring at too low a water potential to provide for entry into the more coarse GMS. A paper on this topic was published by McCain, Kincaid and Wang in an ASAE publication in 1992. Their work was with the older plastic jacketed WaterMark sensors, that have since been replaced by stainless steel jacketed sensors. However, the phenomenon is similar. McCann, I. R., D. C. Kincaid and D. Wang. Operational characteristics of the Watermark Model 200 soil water potential sensor for irrigation management. Applied Engineering in Agriculture, 8(5), 603-609, 1992. As one other contributer noted, vapor diffusion into a GMS may help to override the hydraulic conductivity problem described above and eventually cause rehydration of the sensor, but this may occur at too slow a rate to be very useful for irrigation monitoring work. Best regards, Rick Allen Univ. Idaho. > > Mike Schulz writes to the sowacs list: > ===8<============== > greetings, > As pore size has a major impact on soil moisture movement i have a query > regarding whether granular matrix resistance blocks could potentially have > problems rewetting in clay soils. > Its apparent that gypsum blocks have very fine pores as they loose little > water before ~60kPa, ie. they don't work in this range. Granular matrix > sensors are therefore made of coarser material in order to allow them to > function in the >10kPa range. That is they have larger pores. > In a clay soil that has been rewet after drying to say 250kPa, can water > move from the wet but not saturated clay into the dry but larger pored > matrix of the granular matrix block. > If the soil around the block is saturated at each irrigation or rain > rewetting should proceed rapidly. My query is what happens when the block > is positioned on the edge of say a dripper wetted ball and the soil texture > is finer than that in the block. > Comments on your experiences would be appreciated. > > Regards > > Mike Schulz > Serve-Ag, Victoria. > > PO Box 615, Mooroopna, Vic, 3629. > Tel: (03) 5825 4783 > Mobile: 0408 395 076 > > Serve-Ag provides a range of services to the agricultural and environmental > communities. > For additional information consult our website (www.serve-ag.com.au), call > me or, call our head office (03 6427 0800) > > ===8<============== > NOTE: To get off this list, send an email to list@sowacs.com > with the subject line: > unsubscribe_sowacs > > For full instructions see this page: > <http://www.sowacs.com/subscribe/index.html> >