AbstractA laboratory evaluation was directed to provide a means for assessing the need for irrigation based on the simultaneous appraisal of the evaporative demands of the atmosphere and the free energy of water at the soil surface. The method essentially consists in recording the difference in evaporative cooling between a non-ventilated wet bulb thermojunction at the height of a crop canopy and less intensive evaporative cooling at the soil surface, as evaporation proceeds and the free energy of soil water decreases. Utilizing the principle of the thermocouple, a curve is obtained in a microvolt recorder which gets steeper as the evaporative demands increase and the plant gets farther away from the zero time of a given irrigation. Statistical data are presented which indicates that the continuously recorded potential (∆t) is a major function of evaporative demands of the air (ed), of the moisture content of the matrix material (Ø), and of its evaporation rate (em).
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