With the majority of Nebraska’s crops planted, now is a good time to be installing atmometers or ETgages. As with soil water sensors, a few questions may need to be addressed before using an ETgage

ETgages®, marketed by the ETgage Company, Loveland, Colo., are one tool promoted by the Nebraska Ag Water Management Network (NAWMN) to help irrigators better manage their irrigation. For those unfamiliar with the ETgage (Figure 1), they are a simple tool that takes into account humidity, temperature, solar radiation, and wind to provide an accurate estimate of potential crop evapotranspiration or ET. Potential crop ET estimates the amount of water use from evaporation from the soil surface and plant canopy plus the amount of water transpired through the crop. One advantage of using an ETgage is that it is located near your field and does a great job of estimating ET based on local conditions along with your crop’s stage of growth.

2)   What canvas cover do I need?

To simulate crop ET, water evaporates from a ceramic plate covered with green canvas to simulate a crop leaf. Typically in Nebraska an alfalfa canvas (No. 54) is used. This cover estimates the evapotranspiration for a fully irrigated alfalfa field, which is the basis for crop water use in Nebraska or the reference ET.

Users need to inspect their canvas cover at the beginning of the season and replace covers that are worn, bug eaten, or stained.

In addition, underneath the canvas cover there is a wafer which needs to be replaced yearly. This wafer prevents rainwater from entering the ceramic while at the same time allowing water vapor from the ceramic to pass through. Over time, mineral deposits may build up on this wafer, resulting in errors in ET estimation

3)   How often do I need to read my ETgage?

The ETgage is read at least once a week, preferably on the same day of the week each time. Once you have read your ETgage, we would strongly encourage you to go to the Nebraska Agricultural Water Management Network (NAWMN) website. We hope you’ll consider posting your information to the website so other producers can access view it too. You'll also be able to view information from other producers posting ETgage readings as well as automatic weather station data. The automatic weather stations, which appear as blue balloons on the map, provide daily reference ETs as well as readings for the past three weeks.

4)   Where do I install my ETgage?

ETgages should be placed in open areas like fencelines, grass, alfalfa, soybean or small grains fields at least one foot above the canopy. This location needs to be outside the throw of any irrigation nozzles if installed close to a center pivot.

5)   I forgot how to prime my ETgage so now what?

When using an ETgage it is important to make sure that the ceramic top is primed properly so that as water evaporates, additional water will be pulled up the plastic supply tube. The easiest way to do this is to remove the ceramic top by pulling on the clips. Then remove the rubber stopper and supply tube and fill the ceramic top with distilled water. You will likely need to top it off once more as water soaks into the ceramic. Next, fill the reservoir to the top with distilled water. At this point there are several different techniques that can be used. One method is to hold the rubber stopper and supply tube underneath the water and then use your finger to trap water in the tube and then quickly insert the rubber stopper into the ceramic top. You can also insert the tube into the water and then hold it lower than the water level to start a siphon and then insert into the ceramic top.

For a visual reminder of this process, view the Assembly and Placement of an Evapotranspiration Gauge video (right) and at UNL CropWatch on YouTube.

6)   What kind of water do I use to fill my ETgage?

It is important to only fill the ETgage with distilled water, otherwise minerals may quickly clog the ceramic top.

7)   How do I calculate my actual crop water use?

The water level drop in an ETgage gives us a reference ET. To convert this 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, by multiplying the two together you can calculate your crop ET for the last week.

For example, your ETgage dropped 1.50 inches for the week and your corn crop was at V2. The crop coefficient for that stage of corn is .10. Multiply 1.5 x .10 to get a crop ET of .15 inch. If your neighbor planted corn earlier than you and their crop is at V4, the crop coefficient would be .18, so the crop ET would be 1.5 x .18 = .27 inch. Remember that this is the crop water use for the last week, so you would need to divide by 7 to get a daily ET value.

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.

View the NebGuide “Using Modified Atmometers (ETgage®) for Irrigation Management” for more information on using ETgages as well as the crop coefficients for various crop growth stages.

Suat Irmak is the project coordinator for the Nebraska Agricultural Water Management Network.