Abstract
The objectives of this study are to propose a remotely measurable plant transpiration transfer coefficient (hat) and to verify its characteristics under various environmental conditions. The hat is defined as (Tc - Ta) / (Tp - Ta), where Tc, Tp, and Ta are the temperatures of the canopy, a non-transpiring canopy, and air, respectively. Theoretically, hat ≦ 1, and determines transpiration from its minimum value (zero) to its maximum value (potential transpiration rate). Five experiments were conducted between 1994 and 1999 to verify the characteristics of hat, using 3 plant species (sorghum, tomato and melon). Theoretical validation showed that the proposed model agreed well with conventional models. Experimental results showed that the hat value was approximately equal to the value of the ratio of sensible heat fluxes (H/Hp), and the slope of the regression line between them was close to 1, the intercept was close to 0, and the regression coefficient was r2 = 0.70. In addition, hat was not only an indicator of the water status in the plant root zone, but also an indicator of atmospheric variables. Under waterstressed conditions, hat was affected mainly by the water status in the plant root zone. Therefore, hat can be used as an indicator of plant water stress. The main advantages are that hat can be easily measured and applied under various conditions. Due to its simplicity, hat is a suitable coefficient for analyzing the process of transpiration and for determining the transpiration rate.
