EVAPOTRANSPIRATION


Several recent tech


 make it easier to determine and use 


I By


Christopher Lund


irrigationtoday.org


rrigating crops efficiently is a goal of most growers as they look for ways to save on costs while being good stewards of natural resources. Water-use efficiency can be achieved using a


variety of approaches, from the choice of irrigation system type to irrigation management decisions based on soil moisture monitoring and/or evapotranspiration estimates.


ET is the combined flux of water from the soil surface (evaporation) and from plants (transpira- tion) to the atmosphere. It is controlled by both meteorological conditions and plant physiology. Whether you are providing the full crop water requirement or deficit irrigating, knowing the ET for your crop provides a powerful irrigation management tool.


Unfortunately, direct measurements of ET are costly and complex, and for the most part, they are limited to research settings. In operational


agriculture, most growers making ET-based irrigation decisions still use reference ET and crop coefficients (see FAO Irrigation and Drainage Paper No. 56 at www.fao.org).


Reference ET (ETO) is defined as the ET for a short, green, well-watered grass surface. It is typically calculated with the Penman-Monteith equation using solar radiation, temperature, wind speed and humidity measurements from weather stations located on reference grass surfaces. Many agencies in the United States maintain weather stations on reference grass surfaces and provide ETO data to growers free of charge (e.g., CIMIS, AgWeatherNet, FAWN).


Assuming equivalent meteorological conditions, ET for any other well-watered crop (known as ETC) can then be calculated by multiplying ETO by a crop-specific coefficient, KC. Historically, crop coefficients have been obtained from tables


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