Modelling Leaf and Canopy Transpiration and the Soil-Plant-Atmosphere Continuum

Abstract

Introduction The concept of the soil-plant-atmosphere continuum (SPAC) was first proposed by Phillip in 1966. In this conceptualisation, water flows from soil to the atmosphere via a plant's hydraulic pathways (Chapter 3). The flow is driven by the gradient of water potential from high to low, arising principally from differences in solute and turgor potentials along the SPAC (Chapter 3). The biophysical and physiological processes occurring through the soil-plant-atmosphere continuum (SPAC) can be divided into three components: (1) radiation absorption and energy balance; (2) water transfer; and, (3) photosynthesis and physiological regulation of carbon and water fluxes. This chapter focuses on the second process, that is, water transfer, while Chapter 9 deals with the first and Chapter 10 deals with the third process. There are two significant differences between canopy-scale and leaf-scale transpiration which need to be considered explicitly in models. These are: (a) radiation interception; and, (b) turbulent transfer. It is the shading and multiple layering within canopies that increases the complexity of radiation interception and turbulent transfer. A brief consideration of these is given prior to a more complete description of leaf and canopy transpiration. Canopy Radiation Exchange Radiation Interception Solar radiation is the key input to leaf energy balances and this drives transpiration, in addition to it being the energy source for photosynthesis. The net radiation balance of a leaf is partitioned into sensible heat by heat conductance and turbulent transfer and into latent heat by transpiration. Because net radiation is a radiation balance between changes in sensible and latent heat exchanges, net radiation represents a steady-state, at any given moment. Leaf temperature, as determined by a leaf's energy balance, influences the biochemical reaction rates of all photosynthetic processes. Leaf temperature determines the leaf's emission of long wave radiation and also determines the saturated water vapour pressure in the sub-stomatal cavity (Chapter 2). Consequently leaf temperature influences stomatal conductance and transpiration. Turbulent Transfer of Energy and Mass Within and Above a Canopy CO 2 and water vapour pass into and out of stomatal pores, respectively (Chapter 2). The rate of CO 2 flux and water vapour transfer are determined by stomatal conductance, boundary layer conductance and differences in CO 2 concentration and vapour pressure between the sub-stomatal cavity and ambient air. In these processes there are complex interactions between photosynthesis, stomatal conductance and intercellular CO 2 concentration. These are discussed later in this chapter.