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SOWACS - Please excuse me for getting a bit carried away...
>>Excuse my ignorance, but could you explain matric potential to
me?
The matric potential is the "suction" with which water is held in
the soil by virtue of the physical structure of the soil
particles. Smaller particles (clays etc) will hold onto the water
much tighter, so creating a higher (more negative) matric
potential. Any plant trying to get at this water will have to
pull harder to get at it.
Soil water is also influenced by the amount of salts in solution
(thus it has an osmotic potential as well). The saltier it is,
the more "thirsty" the plant must be to be able to (and to want
to) take it up.
Lastly, the vertical position relative to a fixed point
determines the gravitational potential of water. Anything above a
point is a positive potential as it now has the "potential" to
fall to the lower level. Anything below the point has a negative
pressure (like the water pressure in a dam increases with depth ¬
only this is the "negative" of that positive pressure).
The latter two pressures (osmotic and matric) don't vary much,
and so the most variable pressure is the matric component,
which is what needs to be measured in order to relate the
amount of water in the soil to how accessable it is to the plant.
Sandy and clayey soils can have very different soil water
contents at the same matric potential. Thus is it far better to
use the measurement unit of kiloPascals (kPa) or other pressure
unit to describe how wet a soil is. Quoting the soil water
content as a percentage by mass or by volume has to be qualified
by knowing what the soil type is (clayey or sandy).
Working in pressure units normalises all situations because it
transforms
soil water content to the amount of energy required to move the water.
Irrespective of what the soil water content is, it is traditional
to use certain matric potential values as threshholds for Field
Capacity (full point) about -10kPa, wilting point (WP -1500kPa)
and then irrigators use 50% between these as a "time-to-refill" point.
I often have wondered when I see this 50% figure quoted, whether
this refers to 50% by volume, as it is usual - and much easier
(and more logical) to work in mm of water applied.
If we know (or can calculate) that crops use say 5mm per day, then,
we know that after 4 days we need to put on 20mm of water to
replenish this.
This sytem is effectively, the same as working in percentage
volume water per volume soil. Divide a volume by an area (a unit
area of 1 square metre), and you are working in depth - or in mm.
So that is fine if two irrigators are working on the same soil
type.
But by virtue of having different matric potentials at the same
water content, a clay and a sand will release (dry out) at very
different rates. Sands have a lot of water very easily available,
but it tends to run out quickly, whereas clays hold onto a lot
more water more tightly. They can therefore supply water to a
plant for longer, but at a higher tension.
> Because of the hills created in potato production I placed
> tensiometers directly under the hill and the furrow positions.
> With the matric potential I can find the hydraulic gradient and
> multiply by the hydraulic conductivity to get the flux. I
> determined the hydraulic conductivity from the average matric
> potential.
More explanations please.
Water (and energy) flow from areas of low potential to areas of
high, and the flow is proportional to two things. The potential
difference (like more electrical current flow with a larger voltage),
and more flow with less resistance (or greater conductivity).
In soil, the wetter the soil, the faster the conductivity, and
under completely wet conditions, the conductivity suddenly
increases by many orders of magnitude.
Thus to work out how much water is moving between wet and dry
points, the conductivity has to found by measuring the matric
potentials at the two points, and then calculating flow rates.
>> As far as economics, tensiometers are cheap compared to the
>> pressure transducers and a data logger.
>The farmers I have contact with will not agree. Lots of
tensiometers are abandoned in orchards.
Tensiometers, like neutron probes and datalogging systems are OK
for researchers, consultants and DEDICATED irrigators, but
without an understanding of the system and how they work, I, too
would stick to a set irrigation cycle...
>> My collegue used TDR to measure water distribution in a fine
grid > under the potato hill. This method has high initial cost
but is > relatively cost free after purchased.
>Disregarding the time and transport cost to collect the data
then?
Exactly as I see it. As long as the data collection, processing
and USE of teh information is seamless and easy.
>> We also used neutron probe tubes placed in the hill. These
>have> a sphere of influence about 20 cm or so. Once tubes are
>in place> you can read them as often as you want but is not
>practical for> hourly measurements. We used them for weekly
>measurements.
>The neutron probe still has the time and transport disadvantage.
>If data is collected hourly, two points on the graph can be
obtained:
>1. The maximum percentage moisture reading, at the end of an
> irrigation period or rainfall. The soil around the sensor then
>is saturated.
Only if you go out during a rainfall event large enough to get
saturation (extemely uncommon?!?) assuming an unirrigated area.
>2. The field capacity point. The turning point of the curve.The
>point where the tangent to the curve approximates to 45
degrees.
> (The point of inflection is it called?)
>Given those two readings, why should I not have a
>self-calibrating program?
This point of inflection needs to be found, as without this,
readings above a certain wetness (I think around 75%), will show
artificially high values. It has something to do with the neutron
method itself. There are therefore two calibration lines required
for each soil, and this is often not done.
I will attempt to illustrate this to see if we are talking about the
same thing (set your mail reader to a monospaced font):
*
*
*
* inflection point (perhaps exaggerated)
* above a certain wetness.
*
*
*
*
*
*
*
soil water content -->
^counts
This is especially so under irrigated conditions where such wet
values occur often. In my dryland situation, I rarely get above 50%,
so have not come across it often.
But how does this two-point process do the self-calibrating?
ANY help here would be appreciated.
It seems the time has come to discuss Neutron probes!
Regards
-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.
Bruce Metelerkamp SOIL WATER RESEARCH OFFICER
Institute for Commercial Forestry Research,
University of Natal, PO Box 100281
Scottsville, ZA3209
Rep. of South Africa Voice:27 331 62314
E-mail: bruce@icfr.unp.ac.za FAX:27 331 68905
URL http://www.icfrnet.unp.ac.za/~metele
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