Theoretical analysis and experimental verification of a remotely measurable plant transpiration transfer coefficient

The objectives of this study are to propose a remotely measurable plant transpiration transfer coefficient (h(at)) and to verify its characteristics under various environmental conditions. The h(at) is defined as (T-c-T-a) / (T-p-T-a), -here T-c, T-p, and T-a are the temperatures of the canopy, a non-transpiring canopy, and air, respectively. Theoretically, h(at) less than or equal to 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 h(at), 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 h(at) value was approximately equal to the value of the ratio of sensible heat fluxes (H/H-p), and the slope of the regression line between them was close to 1, the intercept was close to 0, and the regression coefficient was r(2) = 0.70. In addition, h(at) was not only an indicator of the water status in the plant root zone, but also an indicator of atmospheric variables. Under water-stressed conditions, h(at) was affected mainly by the water status in the plant root zone. Therefore, h(at) can be used as an indicator of plant water stress. The main advantages are that h(at) can be easily measured and applied under various conditions. Due to its simplicity, h(at) is a suitable coefficient for analyzing the process of transpiration and for determining the transpiration rate.