ET Gauges Help Producers Determine When To Irrigate Crops

Take weekly readings and post those readings to UNL website.

Published on: Jul 11, 2013

It's easier than ever to determine how much water a crop needs each week during the growing season. The reason is tools that are readily available to irrigators.

One is the evapotranspiration (ET) gauge, or atmometer, which simulates evapotranspiration from either an alfalfa crop or an actively growing grass crop, says Gary Stone and Gary Hergert, UNL Extension specialists in Scotts Bluff.

An ET gauge is 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.

An atmometer simulates crop water use for irrigators.
An atmometer simulates crop water use for irrigators.

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. 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.

You can then takes weekly readings of how far the water level in the cylinder / reservoir drops and posts those readings and any precipitation received to the UNL Nebraska Agricultural Water Management Network web site, the specialist point out.

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 ET gauge sites marked in red and nine 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% efficient.

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.

For more information, contact Gary Stone, Extension Educator at 308-632-1230.