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Sorry for this late response. Moshe wrote: We may trust the sensor manufacturers .... I believe that "field calibration" of soil water sensors depends on: (a) what volume of soil is sampled in the calibration exercise - to be processed by oven drying. (b) how the volume sampled in the calibration relates to the volume of soil sensed by the instrument - do they match in both size and location? (c) how do both volumes relate to the so-called "representative elementary volume". This last concept is important but no-one appears to have put numbers on it, for soil water measurements. The idea is that small samples do not contain the full range of variability of the property being measured. So with increasing sample size, a point is reached where the variability, found from taking several samples, becomes constant. ie the larger sample contains no further components of variability, which were not all always present in each of the very small samples. Moshe wrote: Water moves and equilibrates within the soil by potential gradients, not by water content. ....... I agree except that: The amount of water held in soil at a particular tension depends on the recent history of wetting and drying, in technical terms, which "scanning curve" is being followed in the hysteretic relationship between soil water and soil water tension. This hysteretic behaviour means that, for example, at a tension of 5kPa (0.05 bar) the water content of a field fine sand sample could be anything between 0.08 vwc to 0.25 vwc ( This is a worst scenario but has been recorded). The typical envelope of water contents at this tension appears to be closer to 3-4% vwc. This band of water content, corresponding to a given tension, is subject to both internal drainage and potential crop water uptake. Hysteresis arises from differences in the wetting behaviour causing dry soil to re-wet. Under normal conditions, this depends on the intensity of rainfall/irrigation at the soil surface, or flux at the emitter. The higher the flux the greater the likelihood that air will be entrapped and full rewetting of the pore space is prevented. The consequence for supplemental irrigation is that the amount of water required to bring the soil back to a "full" field capacity condition ( no entrapped air) will depend on the degree of air entrapment created, during rainfall re-wetting events. I believe that a tensiometer cannot identify this, while the soil water sensor can suggest what change of water content has taken place - relative to a "full" field capacity condition. Of course this has to have been identified earlier. A colleague in Botswana reported difficulty in developing a soil water characteristic to interpret tensiometer readings. This is the only practical reference to difficulties arising from hysteresis that I have heard, in the context of scheduling. In arid conditions, and with drip irrigation promoting a complete recharging of the pore space during irrigation, then hysteresis is probably irrelevant. Can anyone help get the IAEA report posted on our discussion network along with Martin Shmitz's results? regards Martin Parkes WasteTrim at Technology Transfer Centre, Alrick Building, Mayfield Road, Edinburgh EH9 3JL, SCOTLAND tel (44) 131 472 4708 fax (44) 131 662 4678