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When to irrigate

Thresholds are reference points that you determine for your own site and application, identifying the upper and lower boundaries for managing allowable depletion. This range depends on soil type, crop, plant development, and cultural practices for managing the field, but a typical starting point is to manage between 10% on the wet end and 40-50% on the dry end of available water depletion.

Soil types vary in their ability to hold water, so the available water corresponds to a different range of tension for each soil type. In other words, 50% depletion in a pure loam would be 84 centibars of soil water tension, while in a sandy loam it would be 40 centibars. The following chart visually displays these relationships and provides a reference guide to assist in selecting appropriate threshold levels.

How much to irrigate

By using sensors at two or more depths in the root system, you can get a more accurate picture of the whole root zone and also determine how much water to apply. If the shallow sensor shows a rapidly increasing reading but the deep sensor shows adequate moisture, you can run a short irrigation cycle as you only need to replenish the shallow root profile. If the deep sensor also shows a dry condition, then a longer irrigation cycle is needed to fully re-wet the entire root zone.

Having a third sensor at the bottom of the root zone can quickly indicate over-irrigation.

Evaluating the results and adjusting subsequent irrigations will allow you to quickly refine the amount of water required.

With multiple sensor depths being used, a weighted average of the sensors can be used to determine the threshold point more accurately.


The on-line calculator allows for entering values from multiple depths and calculates the weighted average automatically.

Trend Analysis

While manual readings can be logged and charted, trend analysis is made easier by data logging devices. By recording frequently and allowing for easy visualization via software, loggers can make longer term trends easy to understand. This is particularly true when the device is also recording irrigation run-times, making the relationship between water applied and the response at different depths clearly visible.

Choosing the correct product 





Lighter to Medium

Sensitive to Medium Sensitivity

Drip, Sprinkler, Micro Spray, Center Pivot, Furrow, Flood


Non-Soil Media, Amended Soils, Coarse or Sandy

Very Sensitive

Drip, Trickle, Micro Spray, Sprinkler, Capillary


Medium to Heavy

Very Sensitive

Drip, Sprinkler, Micro Spray, Center Pivot, Furrow, Flood


15 to 75 Centibars

5 to 20 Centibars

30 to 200 Centibars

Using the Information

When using either IRROMETER instruments or WATERMARK sensors, use the following readings as general guidelines:

  • 0-10 Centibars = Saturated soil

  • 10-30 Centibars = Soil is adequately wet (except coarse sands, which are drying)

  • 30-60 Centibars = Usual range for irrigation (most soils)

  • 60-100 Centibars = Usual range for irrigation in heavy clay

  • 100-200 Centibars = Soil is becoming dangerously dry- proceed with caution!

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