After a rain event and drainage, the Average daily use can be high.
1. Drainage
– After a rain or irrigation event, drainage occurs as excess water moves
through the soil profile. During the next 24 hours, and sometimes up to 48
hours, the daily water change may be higher than expected crop water use due to
this drainage.
2. Air Pocket or
Crack Around the Sensor – If you see a large spike or an increase in
moisture at a specific sensor depth, there may be an air pocket or a rock near
the sensor. This can cause the sensor to read drier than normal. For reference,
a probe placed in pure water would read 100%, while one exposed to dry air
would read 0%.
3. Interpolation – The calculations used to
estimate moisture between deeper sensors may sometimes overestimate the
decrease in moisture at those depths, leading to higher calculated water use.
This issue most often occurs in heavier soils and is usually not a major concern. The key is to focus on rooting depth, infiltration depth, and the estimated moisture at each depth. Watch for the “stepping” pattern in the data. You can continue to push the refill line deeper until you see crop water use begin to decrease.
When the Daily Water Change starts to decline and the sum graph begins to flatten, the crop is starting to experience stress. This is the point where irrigation is needed. Anticipating this trend allows you to be ready to start the pivot on time.
Also consider the probe’s location in relation to the pivot. If the probe shows dry conditions and the pivot has already passed over it before being shut off, the rest of the field is likely even drier. System capacity is another important factor. Think about how long it takes to apply 1 inch of water across the entire field. If the power unit is on interruptible service, irrigation time may be significantly longer, potentially doubling the time needed to apply water.
The probe shows an increase in moisture that is more than the irrigation or rain. Why?
1. Air Pocket – A
small pocket of air around the probe can fill with water during a rain or
irrigation event and show a large increase in moisture. Even if this occurs at
only one sensor depth, it can significantly affect the Sum Graph. Watch how
each sensor responds during the event. If one sensor increases much more than
the others and then drops quickly as the water drains, this may indicate an air
pocket.
2. Tillage – Deep tillage can sometimes leave open
channels below the soil surface. These channels can move water directly to the
probe, causing higher moisture readings than expected.
3. Preferential Pathway – Preferential pathways occur
when water has an easier path to the probe than to the surrounding soil. Rodent holes, hard pans, or other soil irregularities can create these pathways and
lead to higher moisture readings at the probe.
4. Runoff – If the probe is installed in a low area or at
the bottom of a slope, it may receive more water from runoff than the rest of
the field, resulting in higher moisture readings.
5. Sub-Irrigation – Subsurface water movement can
increase moisture around the probe. This is more common in lighter soils, where underground water flowing downhill can raise measured moisture levels.
Depending on the cause, you may need to visit the field to correct issues such as an air pocket or an unintended water pathway around the probe. To reduce the impact of runoff or sub-irrigation, carefully consider field slope, soil type, and water movement when selecting the probe location. Proper placement helps ensure the data accurately represents field conditions.