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Irrigation water management – ET gauges
Posted June 28, 2013
By Gary Stone, UNL Extension Educator, and
Dr. Gary Hergert, UNL Soil and Nutrient Management Specialist
An evapotranspiration (ET) gauge, or atmometer, is an accurate way for a farmer to determine how much water a crop will use each week of the growing season.
ET gauges simulate evapotranspiration from either an alfalfa crop or an actively growing grass crop. They are mounted near a field on a post about 3 feet above ground, in an open area away from shadows of trees or buildings.
The ET gauge consists of a plastic PVC cylinder / reservoir filled with distilled water, with a site gauge tube attached to the outside. Fixed to the top of the cylinder is a special porous ceramic disk and canvas crop cover.
Attached to the underside of the ceramic disk is a plastic feed tube that goes inside the plastic PVC cylinder. The ET gauge is filled with distilled water and “zeroed out.” As the water evaporates from the ceramic disk, the plastic tube draws water from the reservoir and the water level in the cylinder / reservoir goes down.
Any producer can access this web site to view weekly data and access any of the nearly 800 registered ET gauge sites registered across the state. Here’s how to use the site:
- Clicking the link titled “View weekly ETgage® data and local reference ET weather station data” near the top of the page opens a new page with a map of Nebraska and its counties. Clicking on a county takes the user to a page for that county showing all of the ET gauge site and weather station markers in that county.
- In the Panhandle and eastern Wyoming there are 48 crop (alfalfa) ET gauge sites marked in red and 9 actively growing grass ET gauge sites marked in green. A gray marker indicates that a producer has not posted readings within the last week. Blue markers are weather stations.
- Selecting the site (red) closest to his location takes a producer to a page that will help determine crop water use for the previous week. Across the top of the page are the crops that can be selected. Corn is always the default choice. Most of the crops grown in the Panhandle and eastern Wyoming are there.
- Using corn for this example, the producer determines what stage of growth his crop is in, as shown in the left column. The producer then selects the most recent ET gauge reading which is found under the different crop selections. Where the crop growth stage and the most recent ET gage reading come together on the chart will be the estimated crop water use for the previous week or time period.
- Taking this reading, and subtracting any precipitation received, leaves an accurate estimate of how much water the crop has used and how much water to apply to replace that use. In applying water, producers must allow for some loss in application efficiency. Most producers can figure their center pivot application is 90 percent efficient.
Homeowners interested in the water use of their lawns can follow the same instructions, but select one of the green markers closest to where they live. When they do this, there will be only one choice which is actively growing grass and the readings posted will be the estimated water use by grass for that time period.
For producers who need more information on how to determine the growth stage of their crops, a link on the main NAWMN page titled “Crop water use by growth stage for various crops” provides charts that can be downloaded, printed and taken to the field. Select a crop, then select “Download Chart (pdf)” and print it.
More information and cost-share
Several other links on the main NAWMN web page also have useful information for producers:
- Publications: ETgage® & Watermark™ Sensors
- CropWatch ET Resources: Maps of potential evapotranspiration rates for the High Plains Region
- Videos on evapotranspiration gauges and sensors (video topics include How to Stage Crop Growth for Managing Crop Water Use, and Assembly and Placement of an Evaportranspiration Gage)
Cost share might be available for ET gauges from the USDA Natural Resources Conservation Service, local natural resources districts (NRDs), electrical power supply companies or area conservation districts.
ET gauge sites in the Panhandle are part of a project funded by the North Platte NRD, South Platte NRD, Upper Niobrara White NRD, Nebraska Department of Natural Resources and the UNL Panhandle Research and Extension Center.
For more information, contact Gary Stone, Extension Educator, 308-632-1230.