Gary Zoubek
Extension Educator
Aaron Nygren
Extension Educator
Chuck Burr
Extension Educator
As growers complete corn and soybean planting, it's a good time to install Atmometers or ETgages as well as Watermark Sensors. We hope you won't need to irrigate for several weeks, but now is the time to begin making plans.
Crop consultants and producers in the Nebraska Ag Water Management Network (NAWMN) have been using ETgages and soil water sensors to aid their irrigation planning and management for nearly 10 years. These two tools are economical and can help take the guesswork out of your irrigation management decisions.
ETgages
ETgages measure evapotranspiration — water loss from the soil through evaporation and from the plant through transpiration.
ETgages should be placed in open areas like fence lines, grass, alfalfa, soybean or small grains fields at least one foot above the canopy. ETgages take into account the humidity, temperature, solar radiation, and air movement of the area to provide an accurate estimate of potential crop evapotranspiration or ET.
Think of the ETgage (Figure 1) as being the opposite of a rain gauge. Instead of measuring rainfall, it measures how much the water level has dropped. This occurs as the water evaporates from a ceramic plate covered with green canvas to simulate a crop leaf. The ETgage is read at least once a week, preferably at the same time and day of the week each time.
Figure 1. ETgage installed near soybean field
Typically we use an ET gage with an alfalfa canvas (No. 54) to estimate the evapotranspiration for a fully irrigated alfalfa field, which is the basis for crop water use in Nebraska or the reference ET. To convert this reference ET to our actual crop ET, we use an adjustment factor called the crop coefficient (Kc) as shown in Table 1. If you know the ETgage drop for the week (reference ET) and your crop coefficient for your crop growth stage, you can multiply the two to calculate your crop's ET for the last week.
For example, let's calculate evapotranspiration water loss if your ETgage dropped 1.50 inches for the week and your corn was at V4. The crop coefficient for that stage is 0.18. Multiply 1.5 x 0.18 = 0.27 inch evapotranspiration for the week. If your neighbor planted corn earlier than you and that crop was at V8, the crop coefficient would be 0.51, so the crop ET would be 1.5 x 0.51 = 0.77 inch.
One of the advantages of using an ETgage is that it is located near your field and does a great job of estimating ET based on local conditions and your crop's growth stage. Once corn gets to the 16-leaf stage through beginning dent and soybeans are between beginning pod and full seed they both have a crop coefficient of 1.1, so you multiply the ETgage change for the week times 1.1 to estimate your crop ET.
For more information about using ETgages go to UNL's water.unl.edu/NAWMN ETgage link. There you can view information from producer's that are posting their ETgage readings as well as automatic weather station data. At the NAWMN website, check out the NebGuide "Using Modified Atmometers (ETgage®) for Irrigation Management" for more information as well as the crop coefficients for various crop growth stages.
The automatic weather stations, which show up as blue balloons on the map provide daily reference ETs as well as readings for the past three weeks.
Do you have an ETgage? If you do, we hope you'll consider posting your information to the website so that other producers can access the info.