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Cliff Hignett writes Reply to Marcus - re 'universal neutron meter calibration' Clinton Shock is dead right - anyone who uses a NMM without a local calibration is asking for trouble. HOWEVER, one trick we learned was that the NMM is VERY good at measuring water content change - and not so good at measuring water content. This may seem a silly comment - but after 20 years, I assure you it is not. You can ALMOST get away with a 'universal ' calibration for change in water content. If we are talking about homogenised soil in a calibration vessel then water content and water content change are indeed, the same. As soon as you go into the field, water content and water content change are quite different because (in my experience) all fields have soil variability. The effect is particularly pronounced in duplex soils or in soils with a varying amount of clay at a particular depth. This means that the H present in clay which is NOT H2O is causing the count at a particular depth to vary independently of water content. (Where I come from, soils are notoriously low in organic matter, so H in organic matter is not a problem) We looked at 50 Australian clays and measured the (non H2O) hydrogen by first heating them to 105 degree C (to drive off 'water') then burnt them in a tube furnace at 800 degrees C in dry oxygen and used a steam absorbant material to collect steam given off We called the quantity of steam the 'equivalent water'. the relationship we found was We = 0.124 (+-0.012) C + 0.015 where C is clay content in g/g and We is equivalent water also in g/g.- measured relative to 105 degree soil weight. This is published in 'Soil Water Assessment by the Neutron method' ed E L Greacen publisher CSIRO Australia. 1981. We also published relevant work in Australian Journal of soil Science : Greacen E L and Hignett C T (1979) 'Sources of Bias in the field calibration of a NMM' AJSR 17 405-15. Knowing why we have variability is all very well but how do you USE this information? It is my experience that most users of the NMM ultimately want water content CHANGE from the device as their objective. ie they calibrate for water content (against count or count fraction), they measure neutron count then use the calibration to get water content then CALCULATE water content change between reading times. We found that not only was this counter productive in error terms (subtracting one value subject to field and calibration curve error from another point similarly affected by error doubles the calibration curve error) but as the above reference showed, it actually produced the WRONG answer BECAUSE of the field error. The result was biased - ie even with reduced error, the mean was tending to the wrong answer. What do you do about it? a) if you have already calibrated against a field using randomly selected sites then you must tolerate the bias. However, you can still decrease the error in estimation of water use down to that inherent in your calibration by a simple modification of the arithmetic procedure which can be done retrospectively if required. Instead of calculating water content at each depth and replicate in the field for each time and then subtracting water content at one time from another, calculate the CHANGE in counts at each access tube and depth between two reading times. THEN use the slope of your calibration only, to calculate the water used. If you have a field where the clay content varies with depth or position, then you will see a spectacular reduction in field error when you analyse the error in change in water content compared to water content itself. A full explanation of this phemomena requires too much space to give it here, but you can get some idea of what is happening if you consider an extreme case of a field with loam at all depths in all but one replicate access tube which has clay at all depths. The field error associated with (any) water content measure in this field will always be large because of the higher intrinsic water content of the clay - most of which is below wilting point and will not be used. However, the change in water content in the loam over a time interval is not that different to change in the clay over the same interval therefore the error in water content change across the field is smaller. This effect is further enhanced by the NMM characteristics which is VERY sensitive to clay content due to its H content. b) If you have not yet calibrated (or can face that horrible job again) i) make the decision as to how many sacrificial calibration access tubes will be used as you would have before, based on the field situation. ii) Then group the tubes into pairs - and situate the tubes comprising the pair as close to each other as possible one to be sacrificed in the dry condition and one in the wet. The assumption will be made that the field clay content is the SAME for the two tubes in the pair and that field error will be manifested BETWEEN pairs. (In my experience, this assuption is reasonable if the tubes are within 2m of each other) iii) treat each soil horizon in each pair separately when doing the calibration. - you should end up with a series of parallel lines with the position of the line depending on the clay content at the site. (You may like to use the soil clay content and the above equation to see if the displacement along the water axis is equal to the equivalent water content) iv) to calculate water content change use the average SLOPE of these lines and the CHANGE in counts or count fraction at each reading point. You will find a much reduced error term in any field where clay content varies v) if you need to know water content then use the same data to plot a conventional calbration - use it once only (at the driest or wettest time of the year) then use (iv) to calculate change in water content from this time. If you really want to be horrified, try using the same field calibration data to prepare a conventional and a difference calibration curve and then do the corresponding arithmeticfor each method - you may find up to 30% difference in water use - this is the bias referred to in the reference above. The bias results because the slope of the calibration line through all the points will NOT be the same as the average of the slopes of the calibrations for each soil location. (If you doubt this then try drawing a series of parrallel lines on a page , mark a point at the ends of each line and do a regression on those points. ) For those that are still with me, you might like consider that the basic regression formulae assume that there is NO error in the independent variable. Under some calibration procedures, the smallest error may be in the water content . Specifically those procedures were the calibration measures ALL the soil counted by the NMM. Therefore regressions should be done with count vs water content not water content vs counts. (NO the answer is NOT the same, if there is error then the the two lines are QUITE different). Cliff Hignett Cliff Hignett CPSS CPAg CSIRO Land and Water PMB 2 Glen Osmond South Australia 5064 ph (08)8303 8459 fx (08) 8303 8551 ah(08) 8276 7706 email cliff.hignett@adl.clw.csiro.au