Stomatal control of transpiration: Examination of the Jarvis-type representation of canopy resistance in relation to humidity

The Jarvis-type parameterization of canopy resistance is commonly used to m odel the canopy energy balance in large-scale applications. In its most com plete form it is written as a minimal stomatal resistance multiplied by fiv e stress functions involving solar radiation F-1(S), air temperature F-2(T) , air saturation deficit F-3(D), soil or plant water status F-4(Psi), and C O2 concentration of the air F-5(C), respectively. One or several functions, however, can be missing according to the experimental conditions or the as sumptions made for the modeling process. This general scheme is examined in relation to environmental humidity to determine the exact correspondence b etween the mathematical representation and the physiological response in te rms of canopy resistance and actual transpiration. The strict feedback resp onse, i.e., the response to air or soil humidity through plant water status , is simulated by means of the sole function F-4 involving leaf water poten tial Psi (l) (without the need for another stress function related to envir onmental humidity). In this case, canopy resistance increases with increase d saturation deficit, and transpiration also increases. When soil water pot ential decreases, canopy resistance increases and transpiration falls. A fe edforward response to air humidity, i.e., a direct response of stomata inde pendent of plant water status, is modeled through the stress function F-3(D ), with or without the function F-4(Psi (l)), depending on whether the feed forward response is combined with or without a feedback effect. In this lat ter case, canopy resistance increases with an increased saturation, deficit , while the transpiration rate increases up to a threshold and then falls. A strict feedforward response to soil moisture is modeled through function F, in which soil water potential replaces leaf water potential. When the ca nopy resistance formulation includes a feedback response, i.e., when the fu nction F-4(Psi) is used with leaf water potential Psi (l), Jarvis' scheme i s equivalent to the parameterization developed by Monteith (as a function o f the rate of transpiration), and the transpiration rate can be rewritten a s a simple function; of the climatic demand, the soil water availability, a nd the decoupling factor Omega.