Using an Atmometer or ETgage
May 24, 2013
Figure 1. Key components of an atmometer installed approximately 40 inches above ground in a pasture.
The Nebraska Agriculture Water Management Network (NAWMN) has been using atmometers or ETgages since it began in 2005. The ETgage (Figure 1) does a great job of estimating crop water use or evapotranspiration (ET). It's basically a PVC tube filled with water and acts as a mini weather station to provide reference (ET) information for nearby fields.
Information is displayed on a sight tube mounted in front of a ruler on the instrument. Reading the sight tube (Figure 2) is like reading a rain gauge in reverse as you watch to see how much the water level drops each week. Growers or crop consultants can use an atmometer to estimate how much water various crops are using based on stage of grow and changes in the sight tube from week to week and season to season with changing weather conditions. Using an ETgage is much more accurate than using average numbers for a given season or growth stage. Last year at this time much of the corn was at V4 or V5 compared to V1 or V2 this year. The same is true for soybeans this year, which shows how much this varies from season to season.
Atmometers consist of a wet, porous ceramic cup (Bellani plate) mounted on top of a cylindrical water reservoir. The ceramic cup is covered with a green canvas (Gore-Tex®) that simulates the canopy of a crop. The reservoir is filled with distilled water that evaporates out of the ceramic cup and is pulled through a suction tube that extends to the bottom of the reservoir. Underneath the fabric, there is a special membrane that keeps rain water from seeping into the cup. A rigid wire extending from the top keeps birds from perching on top of the gauge.
Figure 2. View of the sight tube water level. An atmometer costs approximately $250 and can be used alone to manage irrigations. Typically, Nebraska fields begin the season with a full soil moisture profile. We then use a check book method and subtract ET as indicated by ETgage and add effective rainfall amounts. Growers should leave a little room for rainfall.
Atmometers are typically mounted on posts near irrigated fields of low-lying crops such as alfalfa or soybean. They also can be located along a road if it's surrounded by low-growing crops. The site should represent average field conditions. Do not install near farm buildings, trees, or tall crops that may block the wind. The top of the ceramic cup should be at least 39 inches above ground and at least one foot above the crop canopy. Don't install an atmometer under the throw of an irrigation system, as evaporated irrigation water will leave minerals on the green cover that can inhibit water flow. University of Nebraska-Lincoln research conducted by Suat Irmak, Extension soil and water resources engineer, showed that an atmometer's ET reference values closely match the values calculated from weather station data. More importantly, the atmometers are located in growers' fields, making them especially useful for areas without nearby weather stations or for people who do not have ready access to this information. A grower or crop consultant can install an atmometer to help schedule irrigations for any field within a radius of several miles.
Crop water use can be estimated by recording the weekly drop in water level. It is important to try and read the ET gage on the same day of the week and around the same time to get a good weekly reference ET value. Multiply this reference ET value by a crop coefficient based on the crop's stage of growth. Nebraska coefficients are listed in the publication Using Modified Atmometers (ETgage) for Irrigation Management (UNL Extension NebGuide G1579) and in the Growth Stage Charts on the Water.unl.edu website.
The larger the crop, the larger the crop coefficient. For corn from V16 through dent, the coefficient is 1.1; however, for corn at V4 the coefficient is 0.35. So, for example, if the ETgage dropped 1.5 inches for the week and the crop was in the V6 stage, the crop coefficient would be 0.35. Actual crop ET would be 1.5 inches multiplied by 0.35 for a total of 0.525 inches of crop water use for the week. However, if the crop stage was tassel, we would multiply 1.5 inches by 1.1 for a total of 1.65 inches for the week.
Creating an ET Information Network
Figure 3. Network of ETgages in Nebraska.
If you install an ETgage in your field, we encourage you to share your findings through the NAWMN. Participating producers, consultants, NRD personnel, and Extension educators across Nebraska are posting their weekly ETgage information to provide more resources for managing irrigation. If you'd like to see the data from your area, simply go to the View Weekly ETgage data page and click on your county. You will find a GoogleTM Map noting the location of ETgages (Figure 3) and can access more detail for each site. The website also shows data from automatic weather stations in blue so you can obtain weekly and daily crop water use information.
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 Watermark Sensor (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