Why is the Average Daily Water Use on my probe so high?

Why is the Average Daily Water Use on my probe so high?

Why is the Average Daily Water Use on my probe so high?
After a rain event and drainage, the Average daily use can be high.

  1. Drainage- After a rain or irrigation event drainage occurs. During the next 24 and sometimes 48 hours the Daily Water Change will be higher than expected crop water use because of that drainage.
  2. Air Pocket or Crack Around The Sensor- If you are seeing a large spike or increase in moisture at a specific sensor, then there might be an air pocket or even a rock near the sensor causing it to read drier than normal. Remember sticking the probe in pure water would read 100% and out in open dry air would be 0%.
  3. Interpolation- The equations estimating the moisture in between the deeper sensors could be overestimating the drop in moisture at those depths.

Generally, this problem exists in heavier soils and is not a big problem. It’s important that we focus on rooting depth, infiltration depth, and estimated moisture at each depth. Watch the stepping. We can continue to push the refill line down until we see decreasing crop water use. When Daily Water Change is decreasing and the sum graph starts to flatten out the crop is starting to stress. At that point we need water. It’s important to anticipate this so we can be ready with the pivot. You should also consider such things as where the probe is in relation to the pivot. If it shows dry and the pivot ran over it before it was stopped, the rest of the field is even drier. Also, what is the system capacity? How fast can you get a 1″ application on the whole field? Is the power unit on interruptable service? If the grower is on any time interruptable your application time could be doubled.

The probe shows an increase in moisture that is more than the irrigation or rain. Why?
Again a few things can be happening.

  1.  Air pocket- A small pocket of air around the probe can fill with water during a water event and indicate a huge increase in moisture. Even though it may only be happening at 1 sensor the Sum Graph can be skewed quite a lot. The main thing is to watch each sensor increase during the event and see if a lower sensor increase much more than another sensor and then decreases dramatically again as the water leaves the pocket.
  2. Tillage-Deep tillage sometime leaves a open channel below ground that moves water directly to the probe.
  3. Preferential Pathway- This occurs when path to the probe from out around the probe leads more water to the probe. Rodents and possibly even hard pans or other anomalies can cause this.
  4. Runoff- If the probe is in a low spot or at the bottom of a hillside, then the probe will receive more water than the rest of the field.
  5. Sub Irrigation- This may occur more on lighter soils, but underground water flowing down hill can cause higher moisture contents around the probe

Depending on what problem is causing this you may need to go to the field to fix the pathway or air pocket. To prevent Runoff or Sub Irrigation from causing this problem be careful in considering the factors that lead to this when placing the probe.