Chapter 13 – Steady-State Heat Balance: (i) Water Surfaces, Soil, and Vegetation

Building on the principles of radiation, momentum, heat, and mass transfer in previous chapters, we now address the steady-state heat balance of water bodies, soil, and vegetation by applying the First Law of Thermodynamics. We begin with the heat balance of dry-bulb and wet-bulb thermometers to establish basic principles and introduce the concept of resistances to heat and mass transfer. The heat balance of wet surfaces introduces adiabatic and diabatic processes. This leads to the Penman Equation, discussion of its application to estimating evaporation from natural surfaces, and analysis of the dependence of evaporation rate on the weather. Considering the heat balance of leaves, the Penman-Monteith (PM) Equation is developed, identifying the distinction between boundary layer resistances to heat and mass transfer and the stomatal resistance. Differences between factors influencing evaporation from wet surfaces and those influencing transpiration from leaves are discussed. The PM Equation is used to explore how transpiration and leaf temperature depend on radiation, humidity, windspeed, and stomatal resistance; rates of dew deposition are also discussed. Developments from the Penman and PM Equations conclude the chapter, including discussion of the “big-leaf model” for vegetation canopies, the equilibrium evaporation rate, and Priestley-Taylor coefficient, and the concept of “coupling” between vegetation and the atmosphere.