How to Measure the Evapotranspiration of Corn in a Glass Tube

Mount an atmometer on a wooden post in the middle of one of your fields, approximately 40 inches above the ground along a service road where it will be easy to access. Select a site that is representative of the general condition of your fields. Install the atmometer in your field when your corn is 12 to 15 inches tall and plan to remove it at the end of irrigation season, or when your corn is completely mature.

Use this equation to determine the evapotranspiration rate of your corn:

ETc = ETr x Kc

Understand that ETc = the evapotranspiration of your corn; ETr = the water depth reading from your atmometer at the time you check it (such as after a recent irrigation or rainfall); and Kc = a crop coefficient adjustment factor. Kc varies depending on the growing stage of your corn. Plug in the appropriate Kc value from the referenced chart from the University of Nebraska. For example, when your corn has two leaves, the Kc = 0.10 whereas when the corn is silking, the Kc = 1.10.

Use the formula and readings from your atmometer to manage your water usage. For example, if you notice the water level in your atmometer drops 1.50 inches in a six-day span since the last time you irrigated, and you know that during that time, .75 inch of water fell, you can easily determine the net and gross irrigation requirements to 95 percent efficiency. If your corn is at silking stage, the formula is as follows:

Evapotranspiration (ETc) = 1.50 x 1.10, or 1.65 inches. Subtract the rainfall amount from that figure (1.65 - .75) to determine the net irrigation requirement (.90 inches).