Atmometers: A Simple, Site Specific Tool for Irrigation Scheduling
There are a myriad of irrigation scheduling tools out there. And whether they're as simple as evaporating pans or as high tech as weather stations connected to computers via satellite, the goal is to get accurate crop evapotranspiration (plant water use) numbers for more efficient irrigation scheduling.
Yet another tool designed for this purpose is the atmometer, a device consisting of a ceramic cup connected to a water reservoir by a tube. The porous ceramic cup is wetted and the cylindrical reservoir is filled with distilled water. As the water in the cup evaporates, more is pulled up from the reservoir. A canvas or Gore-Tex cover specific to the crop being grown covers the cup. This cover simulates canopy cover. A membrane below the canvas cover prevents rain water from getting into the gauge yet allows water vapor to escape.
A gauge running up the side of the atmometer measures the level of the water in the reservoir in inches. As the water evaporates, the reading increases. The difference between readings at two different times is the amount of water used by the crop during that time. Early in the season, these evapotranspiration (ET) numbers must be multiplied by a coefficient to adjust for the lack of full canopy cover.
To utilize this tool to the fullest, one must know the initial water content of the soil and how much of it can be depleted before the next irrigation. The "Feel and Appearance" method of determining percent soil moisture accompanied by tables of available water capacity for different soil textures provide this information. After that, it's just a matter of keeping track of the deductions the crop makes from the water supply and the additions irrigation makes.
If placed away from buildings or other obstructions in a representative location of the field, atmometers provide highly accurate, site specific ET values at a relatively low cost, giving producers another tool for their irrigation toolbox.