By Troy Ingram

Do you know how much water your crop is using on a daily basis? When I ask this question, most growers tell me somewhere between 0.20 and 0.40 inch per day. Sometimes they are close, but wouldn't it be nice to know for sure? It is rather simple to figure out if you have the right tools.

An atmometer, such as the ETgage, is what you need to calculate reference ET, or evapotranspiration. This is the amount of water evaporated from the soil and plant surface, and transpired through the plant.

An ETgage costs about $250 and can be placed on the outer edge of a field, attached to a post. It is filled with distilled water and should be read once a week. It has a ceramic plate on it that allows water to evaporate from the unit. In addition, there is a canvas cover simulating crop characteristics, typically for alfalfa. It has a sight gauge on the side that allows you to read how much water evaporated during the week.

Next, you need to consider the crop being grown and the growth stage it is currently in to determine crop coefficient (Kc). Then, simply take the weekly reference ET reading and multiply it by the Kc value to get the water use per week. The Kc values are available for most of the common crops grown in Nebraska.

Here is an example of how to calculate ET. I have an ETgage in a field in Valley County, Neb. The reading over the past week was 1.8 inches. This is a cornfield at the V6 growth stage. Referring to the chart, the Kc value is 0.35. Then, multiply 1.8 inches by 0.35 and get 0.63 inch. Divide this figure by seven days in the week, and it’s 0.09 inch of water used per day this past week. So, as you can see, when the crop is early in its development, it is not using much water each day.

To learn more about the ETgage, see Publication G1579, Using Modified Atmometers (ETgage) for Irrigation Management. 

Ingram is an irrigated cropping systems Extension educator in Howard County, Neb. This report comes from the Nebraska Water Center at UNL.