Knowing when to irrigate can be a challenge. When crops show signs of water stress, it means irrigation was applied too late. Irrigating on a set schedule, for example once a week, can result in too little or too much water, depending on weather conditions. Crops use water from the soil through evapotranspiration (ET). Evapotranspiration is the combination of evaporation of water from the soil surface and transpiration of water from the plant. The amount of water crops use changes, for example due to the maturity of the crop and the weather (e.g., humidity level or temperature).
The soil has a certain capacity for holding water, known as the field capacity. Immediately following an irrigation or heavy rain the soil is saturated. Excess water drains, and the soil then reaches “field capacity”. Water is depleted from the soil through evapotranspiration. Plants can extract water until the wilting point. At the wilting point, the crop can no longer extract water from the soil and will experience stress. The soil should be kept wet enough so the plant never approaches the wilting point. Therefore, it is important to track crop water use to know how much water is depleted from the soil and when the soil needs to be replenished or irrigated.
Atmometers measure evapotranspiration. They track the crop water use in the field and allow users to know when irrigation should be initiated. The atmometer user selects the maximum amount of water that a crop can use before it needs to be irrigation (called the “deficit”). This number is set conservatively, using a safety factor, to ensure the crop never approaches the wilting point or becomes stressed. The deficit is the difference between the field capacity and the refill point (a set level above the wilting point). The deficit changes depending on the crop growth stage, and is also dependent on the soil texture and the irrigation system used (furrow, flood, or sprinklers).
University of Arkansas is doing on-farm research to validate and develop this method of irrigation scheduling for Arkansas growers. A draft chart for soybeans has been developed and is being tested through the Arkansas Discovery Farms and Soybean Research Verification Program. This chart indicates appropriate water deficits for various soybean stages, soil textures, and irrigation system types. (Note that deficit charts are specific to a region and crop.) Preliminary results indicate that irrigation recommendations based on the atmometer agree with irrigation recommendations based on the Arkansas Irrigation Scheduler.
Atmometers are made up of a small reservoir of water, covered by a ceramic cup. The ceramic cup is covered by a thin paper wafer and canvas. The water from the reservoir evaporates through the ceramic cup. The paper wafer covering the cup prevents rain water from entering the reservoir but does not hinder evaporation, and the canvas simulates the leaf surface. The amount of evapotranspiration can be viewed in the sight tube. The water level will fall one inch in the sight tube for each inch of water the crop uses. The sight tube contains two adjustable red rings. The top ring is set to the water level in the sight tube each time the soil water profile is full, while the bottom ring is slid down to mark the deficit when irrigation will be necessary (e.g., 1.8 inches below the top red ring). When the water level in the sight tube reaches the lower red mark, the user will know that it is time to irrigate. The atmometer does not account for rain water. Therefore the user should measure rainfall separately, and after a rainfall event, move down the lower red ring the amount of infiltrated rain, to account for water being added to the soil water balance.
Atmometers are simple to set-up and require minimal upkeep. They should be placed adjacent to crop fields, and mounted at least 39 inches high and above the crop canopy, to provide accurate on-site evapotranspiration information. At the beginning of the growing season, the atmometer reservoir should be filled with distilled water, and will likely need to be refilled once during the season. The paper wafer, which prevents rainwater from entering the atmometer, should be replaced annually. To learn more about scheduling irrigation and atmometers, contact your local County Extension office.
– Sarah Hirsh