This NebGuide describes the atmometer (evapo- transpiration gauge) and explains how it can be used for irrigation scheduling. Examples are provided to show how information collected with an atmometer can be used to estimate crop water use from corn and soybean. Estimation of evapotranspiration for speciic crops (ETc) is important for irrigation scheduling and agricultural water management. ETc for crops such as corn and soybean can be estimated using the following equation: ETc = ETo x Kc (Equation 1) where ETo is the evapotranspiration (ET) of a reference crop (usually grass or alfalfa), which is commonly called reference ET. Both ETc and ETo can be expressed in units of water depth per unit of time, such as inches per day, inches per week, or inches per month. ETo is usually esti- mated using equations that use weather variables as inputs. These variables include solar radiation, air temperature, wind speed, and relative humidity. Kc is an adjustment fac- tor called the “crop coeficient,” which mainly depends on the type of crop and its growth stage. In some instances Kc also can be adjusted to account for a reduction of ETc when the crop is under water stress or an increase in evaporation G1579 Using Modiied Atmometers (ET gage ® ) for Irrigation Management Suat Irmak, José O. Payero, and Derrel L. Martin Extension Water Resources /Irrigation Engineers when the soil surface is wet due to rain or irrigation. Estimat- ing ETo from climate variables, however, can be a dificult task for growers, consultants, and extension educators who may not be familiar with the use of complex energy balance equations to estimate ETo. Additionally, in some situations, climate variables may not be readily available for a given location to make ETo computations. Alternative methods of estimating ETo do not require complex calculations or sophisticated data recording equip- ment. Devices that measure the evaporation of water such as atmometers or evaporation pans may be useful. An evapora- tion pan is a large (4 ft in diameter) open container illed with water and used to measure how much water evaporates during a given period. The water surface in the evaporation pan is open to atmospheric conditions and offers little resis- tance to evaporation; however, when water evaporates from plants, some water has to travel through the plant before it is transpired as water vapor into the atmosphere. The plant shows some resistance to evaporation that limits ET in a way that is not represented by the open water surface of the evaporation pan. Another alternative for estimating ETo is an atmometer which is designed to simulate ET from a plant canopy in a way that agrees closely with a plant’s resistance to ET. Atmometers have been gaining acceptance among irrigators. The atmometer described here is a modiied Bellani Plate atmometer marketed under the brand name ET gage ®1 (or ETG). The simplicity of use and ease of interpretation of the data from the ETG make it a practical tool that farmers and consultants can use to estimate the crop water use of speciic crops. They can then use this information to improve their irrigation manage- ment practices. Figure 1A-B shows an atmometer, its water reservoir and the evaporation plate mounted on top of the reservoir. 1 Commercial names are provided only for the convenience of the reader and do not imply endorse- ment by the authors or the Univer- sity of Nebraska–Lincoln. Figure 1. Model A ETG at 40 inches above ground in a pasture (A) and basic components of the ETG (B). #30 green canvas cover Bird spike Sight tube and scale 300 mm (11.8 in) capacity water reservoir Bellani plate (ceramic evaporation surface) Rubber stopper Suction tube KLTELTKLTKLT KFSBOPFQVLCB?O>PH>ｨFK@LIKUQBKPFLKKPQFQRQBLCDOF@RIQROB>KA>QRO>IBPLRO@BP