Miscellaneous thoughts
concerning all methods of
irrigation scheduling
Several different techniques are available
which can be used to effectively monitor or
directly measure soil water content. Some are
extremely simple and are well worth the
investment of time and labor. Many irrigation
scheduling consultants are using these
different methods
Table 5. (Thomas W. Ley, Robert G. Stevens, Richard R.
Topielec and W. Howard Neibling)
Qualitative evaluation of soil water monitoring devices. 1 is
Least favorable while a score of 10 is most favorable.
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NP Neutron Probe;
TDR Time Domain Reflectometry;
GS Gravimetric Sampling;
AP Troxler Sentry 200-AP;
AQ Aquaterr Probe;
TM Tensiometer;
GB Gypsum Block;
WB Watermark Block;
Disadvantages of Fixed Point Soil
Moisture Monitoring:
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Several locations and depths must be monitored to
make sure your measurements are representative of
the entire field. Irrigation systems often are
extremely variable
Movement of water on the surface of the soil leaves
wet and dry spots, often within a few feet of each
other. Moving a water mark sensor or tensiometers
a few meters away might give very different
readings. Moving to a different location in the field
will likely encounter a completely different soil.
Limitations of Hand Augering
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Using a hand auger or tile spade can be much
more representative of the field as a whole;
The actual estimate of percent soil moisture
is less accurate.
Hand augers are not as convenient to quickly
measuring soil moisture in deeper depths.
Tile spades are limited to 24 inches or so;
augers are limited by gravel or small rocks in
the soil
Using the Checkbook Method of irrigation
scheduling, ”When should you begin the next
irrigation following an extended rainy spell when the soil
profile is completely full across the entire field.??”
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Be careful! This is often the time a
farmer will get into trouble and “burn
up” a crop.
Here’s how I suggest proceeding:
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1. Wait one day on sandier soils, two days on
heavier soils to allow the excess soil moisture to
drain from the soil. Do not consider ET during those
days.
2. Begin the next irrigation when the accumulated
ET has reached around one half of the planned
irrigation amount. Actually you would begin irrigating
when depletion is only 25% of the Available Water in
the root zone
Slow the pivot down to additionally apply the each
successive day’s ET. Continue to slow down only
until you reach your Planned Application Amount
Irrigation Scheduling Falls into Three
Categories for Center Pivots that have
adequate water
available on demand
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1. Time Based: eg. Irrigate every 3 days
applying ET for those days.
2, Amount Based: eg Irrigate when
accumulated ET equals a pre planned
amount.
3. Fixed Time – Fixed Amount : Irrigate
every two days with 0.5 inches. Actually this
is NOT IRR SCHEDULING
Time Based Irrigation
Schedule
1.
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When water supply is adequate, this
almost always results in too much
water applied.
Lack of soil oxygen often results in
poor root growth, soil pathogen
problems, etc.
Amount Based Irrigation
Schedule
If water supply is overestimated, the
system fails unless:
1.
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% Timer Charts are updated as needed
And Soil moisture is regularly checked
before and after irrigations.