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SOWACS Thanks to all the replies re soil water potetial versus content. I have commented on three posts together to save on the number of mailings. ============== Comments on >>Trevor Finch's postMetelerkamp. >> Coefficients of permeability change by orders of magnitude (factors of 10), and in any case flow in real soils can be totally dominate d by flowin cracks, etc. Water can 'appear' to flow vertically through essentially impermeable clay only by travelling down cracks - once the cracks are full, almost zero penetration. Right on © and am I correct to say that this preferential crack©flow is practically impossible to predict or quantify? If not, please fill us in! (I have only ever seen two references to this, and both were not conclusive). >>I was recently shown the results of a simple trial in sugar cane of buried drip, on the four major soil types used in the area. They had used about 6 buried TDR sensors at different radial distances from the dripper. It took only a few weeks of measurements to develop a simple chart for growers in the area showing maximum pumping time for each different soil type before through drainage occured. From memory, one soil type had vertical upwards flow from the dripper !!! They didn't need to calculate coefficients of permeability - they knew what actually occured This is fine, but to now interpolate or extrapolate to soil types other than the four used, some predictive tool is needed to calculate pumping time. (Taken to the extreme, if one wanted to calculate pumping times or other related hydrological soil indices, for an entire region for example, it would be impossible to place TDR sensors in reprentative areas for the whole lot!) >>What is depressing is the approach of (some) researchers and advisors - that a mathematical model can describe the way real plants grow. Soils and soil profiles really are not made up of 'sand', 'loam' or 'clay'. A recent paper presented at the IAA conference in Adelaide on irrigation scheduling said that the grower will be given a schedule of 'based on soil type and crop', but this 'may need to be adjusted depending on weather'. Last week a colleague was show a case in Spain of a field of tomatoes irrigated automatically, with schedules calculated using a (very expensive) weather station. The final yield was about half that of the next field that was irrigated the old way. It appeared the problem was that a clay layer meant that the tomatoes were completely water logged at depth, but because the drip tape was buried too deeply the surface roots were dry. However, it appeared that the model (based on the weather station) said all ok. (Just a spade would have shown the problem) Technology is only as good as the gut feel (from experience?) of the operator of that technology? Point taken, though. >>Cotton is a relatively new crop in Australia - it may be significant that almost the entire irrigated crop is scheduled based on direct soil moisture measurements. Now cotton grows, I understand, under fairly dry conditions, and can handle the black swelling clays that a lot of other crops won't. How do you determine the refill and full points from first principles for a new area of irrigated cotton (without knowing the matric potential range that cotton prefers © and the water retention characteristic?) >>On the question of automating the points of inflection - The inflection points dicussed were well described and illustrated, made sense, and I learnt a lot here, thank you. (My education in this area must have somehow slipped through a crack!?!) But what I was talking about was a slightly different (and less useful) topic altogether. I was talking about the response of the neutron probe over the full range of water content. Above about 3/4, the slope of the calibration line changes (to a flatter slope), and if this is not realised, water content is underestimated at high water contents. (Malcolm Hensley and Joop Botha © here is your cue....!) >>I gather that possibly the most important soil moisture measurement is *below* root extraction depth - changes at this point give a more reliabe indicator of full (and refill as well) This is an interesting concept, particularly in trees where the depth of root extraction is far from clear © and usually far below that which is easily monitored. In addition, trees growing in deep soil in more arid areas will no doubt take up water from the topsoil, and then from deep soil reserves, if not a water table! Even in irrigated trees © such as in one of the very rare trials on irrigated eucalypts in the world, the trees in the 100% supply plots still sent down deep roots presumably to tap deeper water in time of drought. ==================== Comments on >>Bill Pogue's email to Metelerkamp Dear Bruce: >>The range of 10-1500 kPa can be very misleading, and if you consult an agronomist you would find out that only about 10% of this total range is of interest if the goal is to achieve a well watered crop. Also, The well "worn" 50% depletion "rule of thumb" applies only to the AVAILABLE portion, not the TOTAL water content. Water that is NOT AVAILABLE, practically in this discussion, is of no interest to a farmer who is trying to irrigate for production purposes. OK, but is it 50% depletion of 10 to 100kPA, (ie 55kPa) or 50% of the equivalent depths of water at these two values? (eg 300mm/m and 100mm/m) ie 200mm/m? Because these two are not the same, or are they? >>Your discussion on the "turning point of the curve" ("inflection") is also a very important issue. I refer to this as the "rate of change"---and if a farmer plots the readings on an X-Y graph (suction vs. time) then he can see this "inflection" very dramatically. You can also demonstrate the effect of soil type in this depiction--where the "inflection" point (or shape of the curve) between a coarse, sandy soil (with limited water holding capacity) is much more dramatic (fast rate of change) than a silty, clay type soil (greater water holding capacity where rate of change is ever so slow). The value of this in a production mode, to me, is all too simple and logical. Further, once the "pattern" is established for a given field or site, it becomes a rather boring routine because it repeats itself over and over again over time. Maybe that is why someone would report that farmers routinely abandon tensiometers in the orchards. A pity--for whatever reason. And you will not get me to admit that the answer to all of this is to "stick to a set irrigation cycle"--this is simply the WRONG solution for the WRONG reason. Fair enough, but I had to play Devil's advocate to ensure some discussion! <;=> Regards, Bill Pogue, Irrometer Company =========== Comments on >>Jean Piaget's post by Metelerkamp >>In the early seventies on my reccommendation we in the Western Cape discarded the -15000kPa point. The reasons being that I found the past guesstimated 50% ( which had worked quite well actually ) to roughly equal 100kPa reading of tensiometers----under our soil conditions. Hooray - a figure with which we can work! 100kPa sounds good to me. Now my point is that this [order of magnitude of] soil water content value is more translateable across soil types and conditions than any other. It seems to me that it is a lot more descriptive than x00 mm/m soil water content. >>To the relief of the lab staff who never had much liking for the high pressure equipment we changed the plate pressures to 10 and 100kPa to obtain what we called easily available water. The chaps in the field then decided to use 50% of that! (Oops, there goes the 100kPa figure!) Which means that the figure that was arrived at for a "refill-point" had very little to do with the 50% rule, and was in fact found by experience (or trial and error?) with a particular set of soil conditions. Jean/ ============= Regards -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-. Bruce Metelerkamp SOIL WATER RESEARCH OFFICER Institute for Commercial Forestry Research, University of Natal, PO Box 100281 Scottsville, ZA3209 Rep. of South Africa Voice:27 331 62314 E-mail: bruce@icfr.unp.ac.za FAX:27 331 68905 URL http://www.icfrnet.unp.ac.za/~metele /SoWaCS.html /RR.html Host of SoWaCS (Soil Water Content Sensor) Discussion List. SEND info sowacs OR subscribe sowacs TO majordomo@aqua.ccwr.ac.za