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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 Dear James, Very interested in your Kazakh experiences and would like more details as I hope to be going there in May to install EnviroSCANs and Equitensiometers, Watermarks and Gypsum blocks. Not sure whether it is the same site as you. I don't quite understand your email: have you diagrams of the set-up? You don't give your personal email, but you can reply to me at r.knowles@aspects.net. I am living and based in China at the moment. Regards, Rupert Knowles Olivers Orchard Ltd (in association with Peter White Water Management) Olivers Lane Colchester CO2 0HH UK tel: +44 (0)1206 330208 mob: +44 (0)7860 687760 email: r.knowles@aspects.net www.irrigationworld2000.com -----Original Message----- From: owner-sowacs@aqua.ccwr.ac.za [mailto:owner-sowacs@aqua.ccwr.ac.za] Sent: 02 March 2001 04:44 Subject: potentials 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 Could anyone explain something to me? I have some suction measurements from Kazakhstan last summer, using laboratory constructed mercury tensiometers. When the data is plotted as potential it indicates that moisture was flowing upwards from the water table. Indeed, the capillary fringe was about 1.30m, and the actual moisture from the capillary fringe was 'pulled' much further upwards, possibly another 60cm. However, the shallowest tensiometer at 30cm shows that moisture was slowly draining from the top 30cm of the soil profile, or being drawn downwards by the plants. TDR measurements at the same depths show moisture contents close to field capacity, and at maximum 1 bar suction. What I can conclude from this, as has been found in previous studies in the area, is that a plough 'pan' has formed, and limits the irrigation waters downward, or drainage flow. TDR results hardly indicate an irrigation took place, yet over 100mm was applied at this time. Could the plough 'pan' be restricting drainage flow (hence the equipment did not register the irrigation), so that a very slow drainage downwards took place, at the same rate of wetting, as that of drying, therefore actual volumetric moisture content didn't change? If this is the case, the pan could almost prevent soil evaporation, as it is restricted in downward movement by the physical barrier of the 'pan', but actually slowly pulled down by the plant roots (at a higher rate or force than bare soil evaporation)? If this is the case, am I really only seeing transpiration, and not ETo? If so, how can I calculate transpiration alone, or at least compare an empirical method to my soil moisture measurements (i.e. page 135-136, FAO 56)? A reply to Wenceslau, I have also experienced the same problem, TDR and field samples giving different results. Soil variability will of course play a part, but in the end we used very thin walled sample rings, originally tubing from a hang glider, so it was of aircraft quality. These were very slowly inserted into the soil in a trench, using a modified car jack to slowly push the ring in. Yet differences were still apparent. In the end we concluded it was due to soil density, and possible TDR insertion errors, or calibration errors. Thanks for any help James Dalton James A. Dalton Research Assistant Institute of Irrigation and Development Studies Department of Civil and Environmental Engineering University of Southampton Highfield Southampton SO17 1BJ Hampshire United Kingdom Tel: +44(0)23 8059 2746 Fax: +44(0)23 8067 7519 _________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com.