The crop water requirement determines the irrigation system flow rate. Evapotranspiration (ET) is the combined process of evaporation from the soil and transpiration from the plant (Fig. 5.1). Transpiration is the process by which water passes from the soil and into the atmosphere through the tissues of the plant (Fig. 5.1). Without quantifying evapotranspiration, design and management of irrigation systems is guesswork. Quantification of evapotranspiration through crop experiments and theoretical physics has led to increased irrigation efficiency. In addition, governments have invested heavily in the establishment and maintenance of agricultural weather station networks. Most weather station networks use the Penman-Montieth model to calculate reference evapotranspiration. The model is a surface energy balance with sensible heat flux, latent heat of vaporization, net radiation, and soil heat flux. In addition to theoretical development and demonstration of the calculation procedure, the chapter includes Excel worksheets that calculate the Penman-Montieth hourly and daily ET. The chapter ends with a program that automatically acquires and downloads weather station data for use in evapotranspiration and crop models.
KeywordsEvapotranspiration Water requirement Radiation Reference evapotranspiration Evapotranspiration models
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