With fuel prices climbing and spring just around the corn, this would be a good time to consider adding economical irrigation tools that can help you better manage your irrigation application timing and rates. Such improvements can save energy and water without sacrificing yield.
The first tool to consider is an atmometer or ETgage, according to University of Nebraska irrigation specialists. It acts as a mini weather station to provide reference evapotranpiration (ET) information for nearby fields. Information is displayed on a site tube mounted in front of a ruler on the instrument. Reading the site tube is like reading a rain gauge in reverse as you watch to see how much the water level drops each week. A grower or crop consultant can use an atmometer to quantitatively gauge how crop water use varies from week to week and season to season with changing weather conditions. This is more accurate than using an average number for a given season or growth stage
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, Gor-Tex, that simulates the canopy of a crop. The reservoir is filled with distilled water that evaporates out of the ceramic cup and is pulled through a suction tube that extends to the bottom of the reservoir. Underneath the fabric, the ceramic cup is covered by a special membrane that keeps rain water from seeping into the ceramic cup. A rigid wire extending from the top keeps birds from perching on top of the gauge.
Atmometers are typically mounted on posts near irrigated fields, for example in an alfalfa field. It can also be located alongside a road if it's surrounded by low growing crops. The site should represent average field conditions. Do not install near farm buildings, trees, or tall crops that may block the wind. The top of the ceramic cup should be at least 39 inches above ground and at least one foot above the crop canopy. Don't install an atmometer under the throw of an irrigation system, as evaporated irrigation water will leave minerals on the green cover that can inhibit water flow.
Research by Suat Irmak, University of Nebraska-Lincoln water resources engineer, shows that an atmometer's ET reference values closely match the values calculated from weather station data. More importantly, the atmometers are located in growers' fields, making them especially useful for areas without nearby weather stations or for people who do not have ready access to this information. A grower or crop consultant can install an atmometer to help schedule irrigations for any field within a radius of several miles.
Crop water use can be estimated by recording the weekly drop in water level.
An atmometer costs approximately $250 and can be used alone to manage irrigations. Typically in Nebraska we begin the season with a full soil profile. We then use a check book method and subtract ET information provided by the ET and add effective rainfall events. We'll want to always leave a little room for rainfall, but it can be difficult to accurately estimate the soil water content to decide when to make that first irrigation.
Atmometers do a great job of estimating crop water use, but it's important to also monitor soil water status. To monitor soil moisture, UNL specialists and farmers in a Nebraska network have been using Watermark sensors that are installed at 1-, 2-, 3-, and even 4-foot depths in representative areas of the fields. The sensors are usually placed in the row between plants just after corn or soybeans emerge. These sensors are glued onto PVC pipe so they can be installed, used, removed at the end of the season, and reused next year. Watermark sensors should be installed when conditions permit before the crop gets too large and can be used throughout the growing season to manage irrigations ,using suggested UNL trigger points.
Irrigations events can be initiated when trigger points indicated by the Watermark sensors are reached. Trigger points listed in the online table are when 35% of the available soil water in the soil profile has been depleted. Growers will need to vary these triggers depending on system capacity, crop development stage, predicted crop ET, and point of the growing season. "We don't recommend waiting until the traditional 50% depletion because it may take several days to make a full circle application with the center pivot system," Irmak says. "Thus, if the irrigation is initiated at the 50% depletion level, by the time the pivot completes the circle, the available soil water in some parts of the field may be significantly less than 50%, causing plant stress. The 35% depletion
For more information on ordering ETgages and Watermark sensors, check with your local NRD, since many NRDs across the state offer cost-share programs for these tools.