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


  • J.-P. Lhomme


The Jarvis-type parameterization of canopy resistance is commonly used to model the canopy energy balance in large-scale applications. In its most complete form it is written as a minimal stomatal resistance multiplied by five stress functions involving solar radiation F1(S), air temperature F2(T), air saturation deficit F3(D), soil or plant water status F4(Ψ), and CO2 concentration of the air F5(C), respectively. One or several functions, however, can be missing according to the experimental conditions or the assumptions made for the modeling process. This general scheme is examined in relation to environmental humidity to determine the exact correspondence between the mathematical representation and the physiological response in terms of canopy resistance and actual transpiration. The strict feedback response, i.e., the response to air or soil humidity through plant water status, is simulated by means of the sole function F4 involving leaf water potential Ψl (without the need for another stress function related to environmental humidity). In this case, canopy resistance increases with increased saturation deficit, and transpiration also increases. When soil water potential decreases, canopy resistance increases and transpiration falls. A feedforward response to air humidity, i.e., a direct response of stomata independent of plant water status, is modeled through the stress function F3(D), with or without the function F4l), depending on whether the feedforward response is combined with or without a feedback effect. In this latter 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 F4 in which soil water potential replaces leaf water potential. When the canopy resistance formulation includes a feedback response, i.e., when the function F4(Ψ) is used with leaf water potential Ψl, Jarvis' scheme is equivalent to the parameterization developed by Monteith (as a function of the rate of transpiration), and the transpiration rate can be rewritten as a simple function of the climatic demand, the soil water availability, and the decoupling factor Ω.