May 24, 2013

Watermark sensors monitor soil water status while atmometers estimate crop water use. Both can be used in conjunction to provide efficient, season-long irrigation water use. (See related CW article.)

To monitor soil water for the Nebraska Ag Water Management Network, we use Watermark sensors installed at 1-, 2-, 3-, and even 4-foot depths in representative areas of the fields. That is a key which we found out last year when we observed pivot sprinkler patterns last August (Figure 1).

The sensors are placed in the row between plants after corn or soybeans emerge. These sensors are glued onto PVC pipe so they can be installed, used, removed at the end of the season, and reused in following years. Watermark sensors should be installed when conditions permit before the crop gets too large and can be used throughout the growing season to manage irrigations (soil moisture-based irrigation management), using suggested UNL trigger points.

Preparing and Installing Sensors

Before heading to the field to install Watermark sensors, you’ll need to do a little preparation. Soak the sensors in water (Figure 2) and then allow them to dry out, preferably three times prior to installing them. To speed up the process, place the sensors in front of fans to dry them out quickly. When installing sensors, be sure that they are reading near zero. Also, be sure that no extra water has soaked up into the PVC tube by tipping the sensor and making sure no water sloshes around

Using Sensors to Manage Irrigations

Initiate irrigation events when trigger points indicated by the Watermark sensors are reached. Trigger points listed in the online table, Irrigation Trigger Levels by Soil Types, are when 35% of the available soil water in the soil profile has been depleted. It’s important that growers vary these triggers points depending on system capacity, crop stage, predicted crop ET, and point of the growing season.

We don’t recommend waiting until the traditional 50% depletion is reached because it may take several days to make a full circle application with the center pivot system. Thus, if the irrigation is initiated at the 50% depletion level, by the time the pivot completes the circle, the available soil water in some parts of the field may be significantly less than 50%, causing plant stress.

Learn More

For more information on how to use ETgages and Watermark sensors for irrigation management, see these UNL resources.

• Cost Effective Tools to Improve Water Use Efficiency  (May 24, 2013 CropWatch)
  
• Using the Atmometer or ETgage (May 24, 2013 CropWatch)
• Agricultural Irrigation section of the UNL Water website. This site also includes videos and publications on water management and a table of irrigation triggers by soil type.
• Watermark Granular Matrix Sensor to Measure Soil Matric Potential for Irrigation Management, Extension Circular EC783
• Using Modified Atmometers (ETgage) for Irrigation Management, UNL Extension NebGuide G1579

Chuck Burr, Aaron Nygren, and Gary Zoubek
Nebraska Extension Educators