Machinery Chatter
Russ Quinn DTN Staff Reporter

Wednesday 08/20/14

ET Gauges Help Schedule Irrigation; Nebraska is No. 1!!

No, this is not the University of Nebraska's spot in college football rankings. Although, that would certainly be okay with me and my fellow Husker fans. I am referring to the state's ranking in number of irrigated acres. According to the 2007 Census of Agriculture, the Cornhusker State ranks first nationally with about 8.5 million acres irrigated.

Gary Zoubek, University of Nebraska-Lincoln Extension educator for York County, explains an ET gauge to those attending a meeting at the UNL Soybean Management Field Days held at four locations across the state Aug. 12-15. This field day was located near Snyder in northeastern Nebraska on Friday Aug. 15. (DTN photo by Russ Quinn)

When irrigating, it is important to estimate evapotranspiration (ET) for specific crops, which is also important for irrigation scheduling and water management. ET is expressed in units of water depth per unit of time, such as inches per day, inches per week or inches per month.

Estimating ET is not a simple equation so there are tools available that do not require irrigators to have to figure out complex calculations or buy expensive data recording equipment. A simple tool that can measure ET is an atmometer, which is designed to simulate ET from a plant canopy.

An atmometer looks like an overgrown rain gauge and consists of a canvas-covered evaporation plate mounted on a water reservoir, which is 11.8 inches deep. The green canvas material covers the evaporation plate and mimics the absorption of solar radiation characteristics of a plant leaf and controls the rate of evaporation.

Gary Zoubek, UNL Extension educator for York County, said people who buy and use an atmometer, better known as ET gauges, need to make sure birds don't roost on the plate. In addition, other creatures can also mess with the gauges.

"I had a guy tell me once that raccoons were taking the green canvas off his ET gauge," Zoubek said during a presentation at the UNL Soybean Management Field Day near Snyder, Neb., on Aug. 15. "I told the guy he should plant sweet corn nearby; that should take care of that problem."

Suction through a glass or plastic tube provides water to the plate and a rubber stopper secures a length of the glass tubing from the bottom of the reservoir to the plate, according a UNL NebGuide detailing atmometers. A check valve allows water to flow upwards to the plate but not backwards and the check valve keeps the plate charged but prevents absorption of rainwater through the plate.

The ET gauge also is ventilated by two small holes drilled in the upper end of the pipe. The depth of the water in the meter is read through the graduated sight tube.

The atmometer usually is mounted on wooden post about 40 inches above the ground and it is usually mounted somewhere for easy access like at the edge of irrigated field or service road. The device should be set in a place so the plate is never in the shade and away from trees, buildings or even tall crops which may prevent full exposure of the gauge to all environmental factors affecting evapotranspiration.

The gauge should only be filled with distilled water. This will prevent accumulation of salts in the plate which could affect its accuracy. The gauge also should be stored inside during winter to prevent freezing and breaking.

Now that we know what an atmometer is, how are they used to schedule irrigation? As water evaporates from the gauge, the water level in the reservoir and sight tube decreases.

ET can be recorded by the decrease in water level over a certain period of days. The period can be three to five days or even longer, depending on the capacity of the irrigation system and stage of plant growth.

The UNL NebGuide on ET gauges provided specific examples of use in different crop situations. One example is a corn crop at the 12-leaf growth stage and the water level in the ET gauge sight tube decreased 1.5 inches during a 7-day period since the last irrigation. No rain fell during this time.

To determine the actual crop ET, you would multiple 1.5 inches by 0.88, which is the crop coefficient (Kc) established at different intervals in various plant growth. This calculation comes to 1.32 inches.

To figure net irrigation requirements (NIR), take 1.32 inches minus rainfall during the time frame. In this example there was no rainfall so NIR would be 1.32 inches.

Gross irrigation requirements (GIR) is calculated by the NIR divided by the efficiency of the irrigation system (IE) expressed as a ratio ranging from 0-1.0. The NIR in this example was 1.32 inches and the IE is 0.60 so the GIR would be 2.20 inches.

Most of the state of Nebraska is blessed to be right over the deepest part of the expansive Ogallala Aquifer, the endangered water table located beneath much of the Great Plains. While Southern Plains states sitting over the Ogallala are struggling to maintain water levels for irrigation needs, Nebraska has largely avoided these issues due to the state closely regulating groundwater as well as irrigators utilizing water management tools like atmometers.

Now if ET gauges could get the Husker football team to No. 1, we'd be set.

The UNL Water website can be found at…


Posted at 10:16AM CDT 08/20/14 by Russ Quinn
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