Know When to Stop Irrigating

Irrigation is one of the most expensive inputs in crop production—but when used efficiently, it can deliver a very high return. As the growing season wraps up, it’s important to ensure your crop has enough water to reach physiological maturity, but not more than necessary.

The goal is to leave the field at 60–70% depletion of available water content by harvest. This level of depletion:

• Reduces the risk of soil compaction during harvest,

• Enhances soil structure over winter through freezing and thawing,

• And provides a buffer if fall precipitation delays harvest.

With precision irrigation tools, growers can strike this delicate balance with confidence.

The Three Essentials for Ending the Season Right

Regardless of the crop, three data points are essential to build your end-of-season irrigation strategy:

1. An accurate crop model that reflects the actual growth stage of your field.

2. Soil volumetric water content data, typically from in-field sensors.

3. A reliable weather forecast to estimate upcoming evapotranspiration (ET).

If all three are available, you can time the final irrigation accurately and efficiently.

Using the Crop Model

Crop models help estimate when physiological maturity will be reached. Most models use historical averages, but actual crop stage can vary significantly by:

• Growing Degree Unit (GDU) accumulations

• Planting and emergence dates

• Hybrid or variety differences

Once you determine how many days remain until physiological maturity, you can estimate water needs using forecasted ET.
Example: If there are 14 days left and the daily ET is 0.15”, you’ll need 2.1” of water to finish.

Crop models also support irrigation scheduling and water requirement projections. For example, research shows that if corn has a milk line halfway down the kernel, it typically needs about 2” of water to reach maturity.

Measuring Water in the Soil Profile

Next, assess how much usable water is already in the soil. Precision sensors can measure the volumetric water content across the root zone.

• In this example, the Profile Sum shows 12.4” of water in the rooting depth.

• The refill point is 10.2”, meaning you currently have 2.2” of usable water before needing to irrigate.

However, at the end of the season, you can draw down the soil profile deeper than normal, often up to 70% depletion (instead of the usual 35–40%)—unless you're planning a cover crop.

Let’s adjust accordingly:

• Full point = 13.6”, Refill point = 10.2”

• Usable water at 35% depletion: 13.6 - 10.2 = 3.4”

• Adjusted for 70% depletion: 3.4” × 1.3 = 4.4” usable

• To reflect this, drop the refill point by 1” to 9.2”, resulting in 3.2” of usable water.

If the crop only needs 2”, no further irrigation is needed—even if there’s no rain.

What if You’re Short on Water?

If your soil doesn’t have enough stored moisture, look to your last variable: historical rainfall. For example, if the 10-year average shows 0.75” of precipitation expected between now and maturity, include that in your calculations before deciding to irrigate again.

Finish Strong with Precision Tools

With the right combination of crop modeling, soil moisture data, and weather forecasting, you can plan the exact timing and amount of your final irrigation. This allows you to:

• Maximize yield

• Avoid unnecessary irrigation costs

• Minimize harvest delays and complications