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Soil water measurements for Grain & Graze Murrumbidgee
Interpreting the measurements
It is important to be aware that these are measurements of soil water
potential, not soil water content. The two are related, but not uniquely;
the relationship is far from linear and varies with soil type and from
place to place within a given soil type as individual soil properties
vary.
Unlike soil water content measurement, water potential measurements cannot,
by themselves, indicate how much water is stored in the soil and available
for crops. Soil water potential measurements are, however, much easier
and cheaper to make that soil water content measurements and still provide
much useful information.
Soil water potential provides an absolute measure of how wet or dry the
soil is:
- a value of zero indicates that the soil is saturated - ie. water
will ooze from the soil;
- a value of ~10 kPa indicates that the soil is at field capacity or
drained upper limit - this is the practical upper limit of wetness;
a well-drained soil, would rarely spend more than a day or two wetter
than this
- a value of 1,500 kPa indicates that the soil is at wilting point
- ie. so dry that plants cannot extract more water from it.
In contrast, a measurement of soil water content can only be interpreted
in this way if the values of water content at these three limits (saturation,
drained upper limit, and wilting point) are known in advance, which
is usually not the case.
Temperature effects
A complication in interpreting the output from gypsum blocks, and most
other soil water sensors (including the very expensive ones) is that they
are temperature dependent. Their output is affected not only by water
content, but also by the soil temperature at the time of the measurement. Soil
temperature changes in response to air temperature, although the magnitude
of the change in soil temperature decreases with depth.
At shallow depths (less than about 200 mm), soil temperature reaches
a peak in early afternoon and a low around dawn. Because of the time
it takes for the soil to heat up and cool down, these daily variations
are not seen below 200 mm, and so aren't a concern for the measurements
at this site. However, the seasonal oscillation in air temperature
shows up at much greater depths, because in 6 months heat can travel down
as far as 2 meters into the soil and cause a temperature rise. The seasonal
oscillation of gypsum block output caused by temperature can be seen by
following the link to an example from a paddock
at Charles Sturt University.
The consequence of the temperature effect on gypsum block measurements
is that some extra care is required when interpreting their output:
- in spring and summer, a gradual wetting in gypsum block output is
more likely to be a result of increasing soil temperature than an increase
in soil wetness
- in autumn and winter, a gradual drying in the gypsum block output
is more likely to be a result of decreasing soil temperature than a
decrease in soil wetness
Detailed Interpretation
For a detailed interpretation of each data set, see "Commentary
on the data" sections associated with each individual project.
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