From: Evan Christen (evan) To: SOWACS@aqua.ccwr.ac.za Date: Tuesday, May 28, 1996 1:52 am Subject: To TDR or not to TDR? I think Bruce has started the great debate, something along the lines of: "Best practice for TDR". My two bobs worth (not very technical!): We have used TDR on heavy grey self mulching grey clay soil(70-80% clay, ECe 2-3dS/m) and on red clays (similar properties but not self mulching). We found that using 15cm long probes installed vertically at the surface worked well for investigating vegetable crops. We also tried using 30cm long probes but they rarely gave us sensible results. The reasons were unclear, possibly that they were not absolutely parallel?, the high clay content over that length of probe resulting in a poor returned signal? The 15cm probes worked well until the soil started to crack, even minute cracks aff ected the signal. Using buried TDR. Work on crop water use in rainfed conditions used 3 methods of soil moisture estimation NMM, gypsum block, buried TDR (horizontal). Now although the soil was put back in the hole as carefully as possible it soon became obvious that the site had been changed. The wheat above and around the hole ended up taller and much more vigourous with higher DM than the surrounding crop. This obviously made the readings unrepresentative. Other practical points: all connections etc must be checked minutely on a regular basis. We were suprised to find that in the normal course of events play in wires, corrosion in connectors etc created errors. I now know this happen with all types of logging!! Regards Evan Dr Evan Christen Irrigation and Drainage Management for Horticulture CSIRO Division of Water Resources Griffith Laboratory Griffith NSW 2680 Australia Tel # 61 69 601586 Fax # 61 69 601600