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The problems of using
ET calculations in drip, potatoes grown in ridges etc
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ET
calculations are used in many parts of the world to schedule
water, but is it the way to go?
In
our view it is not good enough for modern high value cropping.
Why?
- see spread sheet below and compare actual water use with
ET data.
1.
It is only ever as good as the information entered into the
computer.
2.
Most computer modellers do not know when the soil is really
full.
3.
Limiting deficits are only theoretical, not actual.
4.
They have no way of showing the grower the correct amount
to apply.
5.
They have to assume an irrigation\rain efficiency that will
vary considerably from crop to crop, year to year, soil type
to soil type.
6.
Irrigation equipment application rates are often not known.
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So how do we explain this
and show better ways forward?
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| 1.
For example: rainfall varies over, at best, a farm, but most
growers do not enter field specific data. |
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2.
It is easy and often erroneous to make assumptions about soils
being full. This EnviroSCAN graph above, shows the
water content of the top 20cm of soil in potatoes after planting
and through to the middle of summer. The drip irrigation (Pathfinder
(from Wroot Irrigation)
in the ridge) was started to get the soil to field capacity
before tuber initiation. As can be seen the water content
increased from 25mm to 50mm (explaining a 25mm deficit). However,
after a lot of rain in July it can be seen that the soil continued
to wet up to a full figure of 67mm. This shows that in fact
the soil needed 42mm to fully wet up the soil.
Why?
The
water from the drip was not fully wetting the ridge and the
EnviroSCAN (which measures up to 10cm around the 10cm tube)
was picking up wet and dry soil and averaging it. However,
many people around the world use a Neutron Probe for water
monitoring, which measures a far greater volume of soil and
averages it making it far more difficult to find full. Then
the rain wetted the whole ridge and we reached a true full
figure for the ridge.
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3.
Limiting deficits are scientific figures based on an ideal
world of 'normal' rooting and spread of roots. This rarely
happens! - see photo (below)of Eric Anderson from SAC at the
recent
Potatoes in Practice near Dundee (double click to see
fuller report). Email if you want examples to:
peterwhitewater@irrigationworld2000com
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Where
are most of the roots??
In
the ridge - thats why you need to monitor closely what happens
in the ridge not what is in the valley.
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| 4.
If the true deficit is 50mm, then applying 25mm as most growers
do, then this will only wet up the top 15cm, leaving the next
15mm dry. If the plant needs water from these roots it will
be stressed even though the top of the soil appears to be fully
wet. Compare water use at 30cm on the 8th (large steps\water
use)and the 18th July (small steps). |
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| 5.
A grower may have 25mm of rain in 15 minutes, most of which
will run off the field as the soil cannot absorb it at that
rate - it can be that only a small percentage is used efficiently
- it is no good putting those rainfall figures in the computer
- it will be wrong! |
| 6.
Water being applied by various irrigation equipment needs to
be calibrated so that application and infiltration rates are
equal - otherwise water is wasted. You can only quantify this
by measuring in the soil with the right
equipment. |
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An
example of why using Computer models in drip or any other
crop where the water does not wet the whole rooting area,
does not work
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Click graph to see full size
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Date
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Rainfall
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ET
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Actual water use measured
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13-7-01
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0.4mm
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3.0mm
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18.0mm
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14-7-01
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0.0mm
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3.9mm
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15.7mm
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15-7-01
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0.4mm
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1.6mm
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watered
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16-7-01
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0.0mm
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3.3mm
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20.3mm
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17-7-01
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8.2mm
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2.4mm
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rain
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18-7-01
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10.0mm
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0.0mm?
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drizzle
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19-7-01
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19.4mm
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1.1mm
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rain
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20-7-01
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0.2mm
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2.3mm
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10.5mm
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21-7-01
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0.2mm
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1.9mm
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7.7mm
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22-7-01
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0.0mm
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2.9mm
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8.5mm
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23-7-01
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0.0mm
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3.3mm
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6.3mm
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The
water use figures on the 13th (18.1mm), 14th 15,6mm and 16th
(21.8mm) are all after drip irrigating when the whole of the
ridge is not wet and the water use is concentrated into a
small volume of soil. This shows that with drip a large number
of roots develop in the wet area allowing this to happen.
Between
the 18th and 20th there was a lot of rain (40mm+) and the
whole ridge became wet and therefore more roots were then
able to operate, which means that the volume of water needed
by the plant came from a larger area and hence the water use
figures from the EnviroSCAN are lower, but still greater than
ET - 2 to 3 times.
If
you base you irrigation scheduling on the wrong information
you will get the wrong results!
If
you would like to know more then email:info@irrigationworld2000.com
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