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I would agree with Grant Upchurch - the resolution of these instruments is much better than their absolute accuracy. Soil moisture really does vary spatially to such an extent that 'calibrating for each site' can give, unless done very carefully, even more errors than just using a 'standard' equation. The *resolution* is giving what is usually wanted - has the water content changed, and by how much. In south australia, tree belts are being planted to help with salt groundwater problems. One research programme is monitoring soil moisture down (I think) to 12 m, at different distances from the trees. I assume they are trying to find out the radial distances that trees absorb soil moisture. The academic steering committee recently made the decision that each sensor must be individually calibrated... It seems to me that errors in deriving the calibration equations are likely to be much greater than errors from just using a 'standard' equation. (And in practice - like so many PhD's - they are going to use up the entire research programme on calibrating, and not find out how much water trees do use) It also seems to me that sensors must be left in place, and not moved. Bedding errors and changes in water content in even slightly different locations will completely hide changes in soil moisture. Banana growers in Central America were using TDR by carrying the instrument and probe from site to site. Changes they were recording could have been due either to real soil changes; instrumentation bedding errors; or spatial variation. I think in the future they are going to leave waveguides buried at each site, and either connect each sensor in turn to the TDR instrument, or use a multiplexor. If you can bear with the digression, the 'permanent sensor and trust the resolution' approach is similar to social scientists using small focus groups instead of mass surveys to identify changes in public opinion. Very large samples are needed to measure significant changes in opinion, whereas if they track in detail a small group of people (the same people) they can identify changes in opinions and attitudes. ---- Trevor Finch Research Services New England 8/16 Nicholson St, Balmain NSW 2041 Australia email: rsne@mpx.com.au tel: +61 (2) 9810 3563 fax: +61 (2) 9810 3323 ----