proudly sponsored by
|
archiving of SOWACS proudly sponsored by |
|
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 > I think Tom Reynolds is (to use an Australian expression) 'having a bit of a > stir'. But I am a 'bit of a stirrer' also, and I think he is quite right, > there is far too little work done in comparing sensor methods. For obvious reasons, the manaufacturers will not finance that sort of work. Governments won't finance it - probably because there is no big 'pay off' at the end of it. Consultants can't afford to do it from their own resources. So it does not get done. After 35 years studying soils, water and crop water use, I am convinced that there is NO device on the market that allows trouble free , always accurate, measures of soil water. In my experience, ALL indirect measures (ie electronic, neutron, microwave.....) have problems, and the user who is not aware of those problems is taking a serious risk. Unfortunately, journals do not readily accept papers about experiments which have failed due to 'equipment error' and informal discussion between those with similar interests is the only way to transmit this vital information. Even on this discussion group, some caution is needed in case a manufacturer decides to take legal action against a person criticising his device. ANYONE who claims to have a foolproof device is doing so from a position of ignorance. The complexities of soil types are massive and very few devices have even been tested on more than a few % of the possible variations. As an example (and to take up one of Tom's points) , YES there is a different set of problems when dealing with man made soils. The problems of measuring 'soil water' in a small mountain of waste material from a mine are quite different from the range of problems you might experience in an agricultural situation. Generally, such waste comprises pockets of fine material which we may loosly call 'soil' (but may contain all sorts of exotic minerals which alter an electronic sensors calibration) in a matrix of 'pores' which may be several cm across and rocks which may be metres across. If you think that you can avoid problems by using the most basic direct water measures (ie a volumetric sample and an oven) - think again. Consider the case (which I once encountered) where a B horizon soil swelled but (possibly due to a small organic matter difference - or past history), some areas had fine cracking (on a scale of 1mm or so) , some areas cracked on a scale of 20cm or so. The first soil is easy - just take a volumetric sample with a sample ring about 5 to 7 cm diameter and handle in the usual way. The second soil was much harder. Unless the sample includes a fair average number of the cracks, the answer you get will be quite wrong - and who deliberately samples cracks? This is VERY relevant to sensor technology. Most modern sensors sample a VERY small volume. A TDR (for example) set in the centre of a 'clod' of perfectly swelling clay which is drying should (and probably will) show the same water content until the clod gets to near air dry and starts to crack internally. This is not a problem with the TDR. If you could repeatedly take a volumetric sample from the centre of such a clod, you would get the same answer. This is because the volume of such a clod shrinks EXACTLY in proportion to the water loss over most of the drying range. (Arie Nadler's peat probably behaves in much this manner) Most sensors work very well in some circumstances. ALL sensors fail in some circumstances. Scientific detachment is very important - keep an open mind. Gather all the information you can and TALK to your collegues. Thats enough to go on with Cliff Hignett > > Bruce and Fellow Sowac'ers: > > I for one would like to see these two continue the discussion. The = > apparent next "play" is "Since when does soil water content measurement = > need to be on a "disturbed" or "man-made" basis?"=20 > > If I understand this new condition, then we might ought to have a SOWACS = > (SOWACSAG) for those who till, cultivate, and trample the soil and a = > separate SOWACS (SOWACSENV) for those who monitor natural, = > non-agricultural soils. Obviously we need to separate the two so that = > those who water (irrigation) managers) can quit being distracted by = > those who monitor the ebb and flow of bogs, marshes, and mine tailings. > > Two years ago I challenged this group to issue multi-method comparisons = > of soil moisture content. (Given the above, I must now preface this to = > mean, Soil Moisture Content Under Economic Crop Cover and Soil Moisture = > Content Under Ornamental Landscape Cover). Has anyone yet considered = > MONITORING AND RECORDING of soil moisture regimes under equal water = > deliveries, equal crop requirements, equal soil types, equal = > replications, equal statistical scutiny, to deduce the correlations, = > over time, between the various SOIL MOISTURE MONITORING TECHNIQUES? Yes = > Mabel, the relationship between volumetric content and soil water = > potential needs to be converted into like terms for any meaningful = > correlation analysis, and the investigator must provide all = > assumptions, experimental design, statistical software, climatological > conditions, background "noise/traffic", irrigation methods, irrigation = > design, irrigation control protocol/technologies, all quality controls, = > etc. > > Tom Reynolds > Water Balance > Mesa, AZ ( tomrinaz@earthlink.net ) -- Cliff Hignett Soil Water Solutions 45a Ormond Ave Daw Park South Australia 5041 pH 61 (08) 8276 7706 WWW.SOILWATER.COM.AU