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Computational Approach to Assess Actual Transpiration from Aerodynamic and Canopy Resistance

The Jacob Blaustein Inst. for Desert Res., Ben-Gurion Univ. of the Negev, Sede Boqer Campus, 84493 Israel

D. W. Meek, R. B. Hutmacher and C. J. Phene

USDA-ARS, Water Management Res. Lab., 2021 S. Peach Ave., Fresno, CA 93727

^* Corresponding author.

A simple method for estimating actual transpiration may help to control precise irrigation. The Penman-Monteith equation can be a useful method, provided data on canopy resistance (r_c) and aerodynamic resistance (r_av are available. These parameters are complex and hard to obtain. The objective of this study was to estimate r_c and r_av with infrared thermometers (IRT). Tomato (Lycopersicon esculentum Mill. cv. UC-82B) grown under various irrigation regimes on a Panoche clay loam soil (fine-loamy, mixed, thermic Typic Torriorthents) were used to obtain differential values of canopy and air temperatures. With standard climatological data, they were used for the determination of r_c and r_av. Calculated values of r_c and r_av were used to estimate actual transpiration rates. A basic assumption is that both r_c and r_av are crop-specific coefficients of proportionalities rather than physically measurable parameters. While r_c varies according to water availability, r_av is not directly affected by soil water status, but is a factor of crop and climatic conditions. Thus, r_av determined under conditions approximating potential transpiration (7_p) was also used to evaluate r_c under water deficit conditions. Calculated values of r_c and transpiration agreed with values measured with a steady state diffusion porometer. A computational link exists between measurements of canopy, air temperature, r_c, and r_av. Infrared thermometers can be effectively used in irrigation management.

Received for publication February 16, 1987.