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Atmometers--Another Tool for Midsouth Irrigation

Irrigating crops is or should be a scientific endeavor. Gone are the days when producers can afford to just “water” crops without regard for how much water they are using or how much they may be wasting by unnecessary irrigation or overwatering without regard to the plant’s need.

Without a doubt, Midsouth soybean producers are doing a much better job of irrigating their crop according to its needs, and are cognizant of conserving their major source of irrigation water, the alluvial aquifer. Producers are increasingly using soil moisture sensors to schedule irrigations and Pipe Planner to properly meter water onto each field according to its shape. Some are even using surge valves in an attempt to apply water more efficiently. Using these tools is a necessary and vital first step to irrigating more efficiently.

We must not stop now–the job is not done. There are still lessons to be learned and tools to be used to become even more efficient irrigators.

One such tool is the atmometer (called an ETgage or Evapotranspiration Simulator by the Nebraska Agriculture Water Management Network), which is an instrument that simulates evapotranspiration (ET) from a growing crop. Atmometers consist of a wet, porous ceramic cup mounted on top of a cylindrical water reservoir. The ceramic cup is covered with a green canvas that simulates the crop canopy. The reservoir is filled with distilled water that evaporates from 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 rainwater from seeping into the cup.

Atmometers are typically mounted on posts near irrigated fields so that they can be easily read. Information is displayed on a sight tube mounted in front of a ruler on the instrument. Reading the sight tube is akin to reading a rain gauge in reverse–i.e., the water level drops over time vs. increases with rainfall.

Crop water use can be estimated by recording the drop in water level in the atmometer over the desired timeframe–e.g. daily or weekly. This number is then multiplied by a crop coefficient based on the crop’s growth stage. Regrettably, we don’t have crop coefficients for Midsouth soybeans at this time, but hopefully that will be rectified in coming years.

Fortunately, soybean water use or ET during the normal irrigation period of R1-R6 in the Midsouth when the crop is or should be at full canopy will roughly equal the water loss measured by the atmometer. In fact, this is probably the case regardless of the geographic location of the field–e.g. see the Crop Coefficients for Nebraska soybeans here.

Arkansas and Mississippi together have roughly 3.5-4 million acres of irrigated soybeans. This, in my opinion, justifies a commitment of resources to support the most efficient use of irrigation water on this sizeable acreage. Hopefully, and with time, these resources can be allocated to support a network of atmometers throughout the portions of these two states that have the largest concentration of these irrigated soybean acres. Once such a network and a central data collection point can be established and maintained, producers will have another tool they can access to determine how much water the crop has used in a given time period so that irrigation can be scheduled to replenish this water use or a fraction thereof.

Data from atmometers is most useful when used in coordination with crop coefficients; however, those have not been developed for Midsouth crops. Using atmometer data with crop coefficients is addressed here.

Caveat: Data from atmometers, like that from any of the other tools that are used to more efficiently irrigate, will only be accurate and useful if the instrument is properly installed and maintained. Thus, this and all instruments must be applied and/or used correctly so that data that are generated from their application/use will accurately estimate what has happened or what needs to be done. That is why producers and their crop consultants/advisors must be certain that they understand the correct use and function of all of the instruments that are used to schedule/apply irrigation more efficiently so that they can recognize their proper/improper functioning and make adjustments when necessary.

Composed by Larry G. Heatherly, June 2018,