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ETgage and Watermark help watering

01/11/2011 @ 1:28pm

It’s the best of both worlds, says Gary Zoubek. In this case, the University of Nebraska specialist is referring to using two devices to determine water needs: ETgages and Watermark sensors.

ETgage (also called an atmometer) readings are employed along with a crop’s growth stage to estimate water use or evapotranspiration.  

Watermark soil matric potential sensors can be used to measure available water in the soil profile. These sensors are glued into PVC pipe and, when the crop is small, placed in the row at depths of 1, 2, and 3 feet. Handheld meters as well as dataloggers can be used to obtain readings.

An ETgage costs approximately $205, says Zoubek. Watermark soil moisture sensors and a handheld meter or data logger cost $300 to $500.

Zoubek says farmers have reported that using ETgages and the Watermark sensors together can save 2 inches of applied water per season. That could equate to a savings of $2,000 to $3,000 per pivot revolution. “These tools can more than pay for themselves in the first year,” Zoubek says.

ETgages can be used alone to manage irrigation. But this requires knowledge of when to initiate the first irrigation. If you don’t have Watermark sensors (or another type of soil moisture instrument) installed, you can determine the first irrigation trigger point using the traditional checkbook method. This determines the soil water depletion from the estimated available water at the beginning of the growing season and adjusts it according to rainfall and ETgage information.

It is important to note that if the ETgage is used alone, this assumes that the first irrigation is triggered at the proper soil water depletion level (of 35% to 38%), says Suat Irmak, an irrigation specialist with the University of Nebraska.

After determining the first irrigation, the ETgage can be used to calculate actual evapotranspiration on a weekly basis as well as irrigation for the rest of the growing season.

Clear Picture Of Actual Needs

Watermark sensors provide actual soil moisture conditions, so they can be used to determine when to initiate the first irrigation; whereas the ETgage can be used to estimate actual water use since the last irrigation. A combination of ETgages and Watermark sensors provides a very accurate picture of soil moisture conditions while accounting for the impact from irrigation or rainfall.

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