Computational Approach to Assess Actual Transpiration from Aerodynamic and Canopy Resistance

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Abstract
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 (rc) and aerodynamic resistance (rav are available. These parameters are complex and hard to obtain. The objective of this study was to estimate rc and rav 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 rc and rav. Calculated values of rc and rav were used to estimate actual transpiration rates. A basic assumption is that both rc and rav are crop‐specific coefficients of proportionalities rather than physically measurable parameters. While rc varies according to water availability, rav is not directly affected by soil water status, but is a factor of crop and climatic conditions. Thus, rav determined under conditions approximating potential transpiration (Tp) was also used to evaluate rc under water deficit conditions. Calculated values of rc and transpiration agreed with values measured with a steady state diffusion porometer. A computational link exists between measurements of canopy, air temperature, rc, and rav. Infrared thermometers can be effectively used in irrigation management.

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