Answer to Trevor Finch's question

[Cliff Hignett <cliff.hignett@adl.clw.csiro.au>]

Answer to Trevor Finch's  question about differences between neutron
moisture menter calibration for water content change and water content -

The concept that I was explaining is not an easy one to grasp - even if you
were in my office and I could sketch the effect we have found.    However, I
will try to do it in words.

First SOME DISCLAIMERS

1) If your field is uniform in clay content (ie a pure sand or a clay which
is uniform in type and % at all depths) then my comments DO NOT APPLY.
2) If you are using the NMM to measure water in a field and are not
concerned about errors up to +-20% in water content or +_10% in water
content change then my comments DO NOT APPLY    (for many commercial
applications this error level is quite acceptable)
3) if the scatter of the points in a conventional  calibration is trivially
small (ie <+-5%)then my comments DO NOT APPLY

WHERE DOES IT MATTER?

My comments only apply where the NMM is being used as a precision instrument
in a research application where it is worth spending effort and money to get
the error down to 5% or better.   In these applications, there is usually
some form of replication of access tubes and an 'experimental design' which
will be analysed in a statistical sense - eg a replicated block design
comparing water use of different crops.     

In such work, field error is often a major consideration especially in
geographic areas (like mine) where soil variability is a major problem.   In
my location we have duplex soils which vary in clay content from  2% in the
surface to 30-50% at 300mm depth then reduce to 20% or so at 800mm depth.
Not only is there this vertical variation in soil type but variation in
depth to the clay horizon can occur across any experimental site along with
variation of clay content at a given depth.

 If the variation is associated ONLY with depth (as in a duplex soil) then
it is simple enough to do a separate calibration for each soil horizon.
BUT  clay content also varies at a single depth across a field.   The worst
case is near the interface between the clay and surface loam horizons when
the depth to clay varies in a random fashion across the field which is seen
at the NMM measuring depth as field variability.

Needless to say, we have been strongly motivated to seek a 'universal
calibration' for the NMM to reduce the effort going into calibration.  We
also have been motivated to look closely at the effect of this field
variabilty on the calibrations and the results which we have obtained.

SOME HISTORY AND BACKGROUND (I don't know what your prior knowledge is)

I should point out that this work is not new, it was done by Dr E L Greacen
and myself in the 1970's and (mostly) published in Australian Journal of
Soil Research. 

 NMM calibrations done in sands and clays have a different slope.  In
searching for a universal calibration, we sought a reason for this
difference - preferably something we could measure in the laboratory on a
gravimetric soil sample.    

At first we thought it was soil density, and NMM theory supports this -
neutrons are slowed and scattered (ie reflected back to the counter tube) by
H , but they are also scattered (without slowing) by any atom including Si,
Al and oxygen - ie other soil components.  So it was conceivable that the
size of the slow neutron 'swarm' around the counter tube was being changed
by density - hence affecting the calibration.    We tested this with both a
simulation model of neutron scattering and field calibrations.  (AJSR 1979,
17:405-15).   We were only partly right.- density has an effect, but that
effect is caused mainly by the non water  H content in the clay.   (This H
can be associated with organic matter, but we have very little of that
here).   As the soil density decreased going from sand to clay, so did the
concentration of this non water derived H, which we were not including in
the calibration.  At the time the book 'Soil water assessment by the neutron
method' was published, we  were measuring this non water H  and multiplying
its mass concentration by 9 and calling it 'equivalent water' ( in H2O,  H:0
as 1:9) and adding it into the calibration as if it was water which did not
'move'   

This answered a lot of questions, but it still missed the most critical
effect of variation in  clay, equivalent water, and density.    That was,
the effect when a field calibration includes variation in the clay content
of the soil being calibrated. Normal calibration procedures assume that the
only variables are counts and water (as released from soil at 105 degrees
C).   If there are other variables affecting the relation between water and
counts then they show up as error either in the slope or the  intercept.
By measuring equivalent water and adding it to the ordinary water we
accounted for a lot of the error.    However, to use this information in the
field, we would have to know the density and equivalent water for every site
and depth in the field.  (At this stage gravimetric sampling started to look
more attractive than the NMM) .  However, we found a more general way to
solve the problem. 

FIRST, WHAT IS THE EFFECT?
Consider the following calibration data 

count           water
fraction        cc/cc
 , pair 1
0.2	0.2
0.37	0.32
, pair 2
0.3	0.3  
0.4	0.37
, pair 3
0.3	0.35
0.4	0.42

 The regression of water on count fraction is Vw = 0.044(+-0.03) +
0.85(+-0.21)*Cf
ie  a change in counts of 0.1 indicates a water change of 0.85

While I have stretched these points a little for effect, I think you will
agree that the error associated with this calibration is not unduly large
for a field calibration in a highly variable field.   The calibration slope
is also typical of many  in loam type soils.

Now, consider the additional information that each pair of points came from
a pair of access tubes situated close to each other (one coordinate sampled
wet and one sampled dry)  at each of  three locations spread across the
field.    Under these conditions it is reasonable to assume that the field
variability between the tubes of each pair is negligable (because they are
close together) compared to the error between different pairs (say 100m
apart) .  If you plot the points and connect the wet and dry point for each
pair you will see that the slopes for all pairs are identical (no error here
at all -  yes,  I did cheat - I have never got data this good, but I need to
make a point)

 You will also note that the slopes are 0.70,  NOT the average slope of 0.85
from the regression done on the SAME set of points.   Clearly, if three
individual sites showed that a count change of 0.1 is equivalent to a
moisture change of 0.07  and the combined calibration shows a change of 0.1
is equivalent to a moisture change of 0.085 then something is wrong.   The
discrepancy is an unacceptable 0.015/0.07 = 21%

I have emphasised the effect here for demonstration purposes but it is real!
It does happen!

Our explanation of this phemomena is that the three  sites had different
amounts of clay H     eg pair 1 had 1% volumetric equivalent water, pair 2
had 1.3%, pair 3 had 1.8%.  If you plot total water (ordinary water plus
equivalent water) against count, you will see the slope is 0.7 and the error
is near zero.  

HOW TO INTERPRET THIS

We interpret this sort of data as meaning that
(a)  in a field with high clay variability the statistical uncertainty
associated with measuring, or calibrating for,  water content is very high
and unavoidable.  Because of displacement of points along the water axis of
the calibration by an unknown amount.
(b)   this uncertainty primarily applies to the water content, it does not
affect the change in water content because the clay at a single measurement
point does not vary.
(c) the problem can be avoided by pairing the wet and dry  calibration tubes
to get an accurate measure of the calibration slope without the effect of
clay variation.  

NOTE 
1) that the best measure of field water content still is the conventional
calibration analysis (ie regression of all points in together)  You must
live with the statistical error caused by clay H. 
2) if this conventional calibration is used to calculate water content
change the result is not only subject to the above  statistical error but is
biased. ie even  if the error is zero, the answer will be wrong!
3) If water content CHANGE is needed then the best measure comes from the
average of the slopes of the individual pairs of tubes. (in this case you
don't know the actual water content)

WE RECOMMEND
1) pairing calibration tubes (ALWAYS ! - it doesn't cost any more) 
2) do a conventional calibration data analysis for use in measuring water
content.
3) calculate the average slope of the pairs of tubes (we call this a
diference calibration) and use it to measure water content change.
4) DO NOT use a conventional calibration to calculate the water content at
two points in time and subtract them to get water use.


Hope this helps describe our findings

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