Showing papers by "John L. Monteith published in 2014"

The Scope of Environmental Physics

Abstract: This chapter defines our interpretation of the scope of Environmental Physics and explains the framework for the further chapters. Environmental Physics, as we choose to define it, is the measurement and analysis of interactions between organisms and their environments. The physical environment has five components critical for the survival of plants and animals: radiant energy; water, minerals, and trace elements; temperature; stimuli in space and time; and transport mechanisms for pathogens and parasites. To understand the interactions between organisms and their environment, biologists and ecologists must seek links between the biological and physical sciences. Environmental Physics is one of these links, recognizing that the existence of an organism modifies the environment, just as the environment imposes change on the organism. In this book we explore these inter-relationships, focusing in chapters that follow on the exchange of heat, mass, and momentum between organisms and their environment. The chapter also introduces the use of simple electrical analogs to describe rates of transfer of heat, mass, etc. and the resistances that control those rates.

Microclimatology of Radiation: (i) Radiative Properties of Natural Materials

Abstract: This chapter describes how radiant energy reaching a surface is absorbed, reflected (scattered), and transmitted. These processes depend on wavelength and on the angle of incidence of direct radiation. Radiative properties of water, soil, leaves, vegetation canopies, and animal coats are reviewed. Still water is an effective reflector of the solar beam at low angles of elevation, but only reflects about 5% of solar radiation when the sun is high. Water absorbs least in the blue-green portion of the spectrum. In the infra-red region water absorbs strongly in several wavebands. Reflection of solar radiation by soil depends on soil structure and water content. Leaves and canopies absorb least (reflect most) around green wavelengths and transmit near-infra-red radiation very effectively.

Micrometeorology: (i) Turbulent Transfer, Profiles, and Fluxes

Abstract: This chapter reviews some aspects of micrometeorology, the study of atmospheric physics on the scale of vegetation canopies. In Environmental Physics micrometeorology has been very effectively used to study hydrology, environmental physiology, and ecology. The turbulent boundary layer that develops over extensive soil and vegetation surfaces grows at a rate depending on surface roughness and atmospheric stability. Properties of turbulence are discussed, leading to a review of the principles of the eddy covariance method of studying rates of turbulent transfer of heat mass and momentum (fluxes) in the surface boundary layer. Instrumentation requirements for, and corrections necessary to eddy covariance measurements, are summarized. The less direct method of studying turbulent fluxes by relating analyzing mean vertical gradients (profiles) of windspeed, temperature, and mass concentration is also reviewed. Roughness parameters depending on vegetation height and structure influence profile properties. Aerodynamic resistance is derived as a quantity relating momentum flux to windspeed gradients. The influence of atmospheric stability on profiles and therefore on fluxes is reviewed, including issues encountered when measuring close to vegetation elements. Principles of the aerodynamic and Bowen ratio methods for estimating fluxes from gradients are reviewed, and the strengths and weaknesses of eddy covariance and flux-gradient methods are summarized. Finally, aspects of turbulent transfer in vegetation canopies are discussed.

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

Abstract: 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.

Transport of Heat, Mass, and Momentum

Abstract: This chapter introduces some of the major concepts and principles on which environmental physics depends. It considers how the transport of entities such as heat, mass, and momentum is determined by the state of the atmosphere and the corresponding state of the surface involved in the exchange, whether soil, vegetation, the coat of an animal, or the integument of an insect or seed. A simple general equation can be derived for transport within a gas by “carriers”, which may be molecules, particles or eddies. A carrier can “unload” its excess of property P at a point where the local value is less than that at the starting point. This relation provides the “eddy covariance” method of measuring vertical fluxes. All three forms of transfer (heat, mass and momentum) are the direct consequence of molecular agitation, as they are described by similar relationships. Because the same process of molecular agitation is responsible for all the three types, the diffusion coefficients for momentum, heat, water vapor, and other gases are similar in size and in their dependence on temperature.

Microclimatology of Radiation: (iii) Interception by Plant Canopies and Animal Coats

Abstract: Building on the principles of Chapter 7 , this chapter is concerned with estimating the radiation interception of vegetation canopies and animal coats. The interaction of radiation with canopies is approached first by considering idealized canopies of leaves that absorb fully at all wavelengths (“black leaves”). Expressions for estimating interception are derived for canopies with assumed leaf angle distributions (horizontal, vertical, ellipsoidal, conical), and for direct and diffuse radiation separately. Then the principles are extended for canopies of leaves with spectral properties, using Kubelka-Munk equations. The chapter continues with discussion of the absorption of photosynthetically active radiation (PAR), known to correlate well with crop productivity. Principles behind remote sensing of vegetation from satellites and aircraft are discussed. Interception of radiation by animal coats depends on the radiative properties of hair and skin, and on the depth and density of the coat, analogous to the roles of leaf and soil properties and canopy density in determining radiation interception by canopies. The chapter concludes with discussion of modeling and measuring the net radiation absorbed by canopies, leaves, and animals.

Micrometeorology: (ii) Interpretation of Flux Measurements

Abstract: This chapter is concerned with describing the principles of methods used for interpreting micrometeorological fluxes, and illustrating these principles with examples from agricultural and forest science. Flux measurements are most useful to biologists, hydrologists, and environmental scientists if they are interpreted to reveal how the canopy or surface controlled or responded to the measured flux. Resistance analogs serve this purpose. Canopy (or surface) resistance to water vapor or carbon dioxide transfer is a parameter comparable to the stomatal resistance of single leaves, but with several important caveats that are explained. Methods of deriving canopy resistance and aerodynamic resistance for momentum are reviewed. An additional resistance is needed to parameterize aerodynamic heat and mass transfer, and measurements and models of this resistance are discussed. Fluxes of pollutant gases and particles to canopies can be interpreted similarly, and the relation between canopy resistance and surface depostion velocity for pollutants is explained. Examples are given of flux interpretation for water vapor and transpiration, carbon dioxide, and crop and forest growth, and pollutant gas uptake to crops and natural vegetation. Finally there is discussion of measurement and interpretation of fluxes within vegetation canopies.

Transient Heat Balance

Abstract: In previous chapters we treated the temperature of a system (e.g. leaf or animal) as constant by assuming that input terms in the heat balance exactly balanced output terms. This situation is rare in natural environments. This chapter considers the equations that describe how a system responds thermally to changes in external temperature. The time constant of the system describes the rate at which the system temperature adjusts to an external change. Equations are derived for general cases such as step changes of external temperature, ramp changes, and harmonic changes. Examples discussed include responses of leaves, animals, and streams. Heat flow in the soil is analyzed in terms of the thermal properties of soil components. The influence of soil moisture on soil thermal conductivity and diffusivity is explained. Formal analysis of heat flow in soil is developed for the case of uniform soil composition with depth, allowing discussion of diurnal and annual changes of soil temperature and soil heat flux with depth. The chapter concludes with discussion of methods of modifying the heat flow in soils.

Transport of Radiant Energy

Abstract: This chapter reviews properties of electromagnetic radiation and principles of radiation absorption and emission by surfaces. The concept of “black body” or “full radiation” is introduced, with discussion of the laws of Wien, Kirchhoff, Planck, and Stefan-Boltzmann. Radiative exchange between surfaces or atmospheric layers is discussed to derive the concept of “radiative resistance.” Spatial relations for emission from point sources and surface elements are explained, along with the Cosine Law. Spectral reflectivity and absorptivity, and relationships between irradiance and radiance are introduced. The chapter concludes with discussion of the attenuation of radiation passing through a medium, leading to Beer’s Law and the Kubelka-Munk equations.

Mass Transfer: (i) Gases and Water Vapor

Abstract: Transfer of water vapor, carbon dioxide, and other trace gases between plants, animals, and the atmosphere takes place by molecular and turbulent diffusion. This chapter uses principles from fluid dynamics to develop methods for relating mass and heat transfer, and applies the methods to analyze mass exchange between leaves and the atmosphere. Mass exchange between the air in greenhouses and similar structures and the atmosphere is also analyzed. Mass diffusion through stomatal pores on leaves is a process limiting photosynthesis, respiration, transpiration, and the uptake of pollutant gases. The concept of stomatal resistance is introduced and measured resistances are compared with theoretical predictions. Water vapor transfer through animal skin, coats, human clothing, and through pores in artificial “breathable” fabric is also analyzed by resistance analogs.

Mass Transfer: (ii) Particles

Abstract: Small particles such as pollen grains, spores, pathogens, and pollutant materials may be transported long distances by the wind and deposited at the surface or inhaled by animals. This chapter reviews the physics of particle transfer and deposition. In still air particles sediment under gravity at speeds depending on their mass and shape, and very small particles move with random Brownian motion. In moving air they may be impacted on or intercepted by objects in their path. Equations describing these mechanisms are developed, and measurements of particle depostion are interpreted to show the influence of particle size, windspeed, target size, and surface microstructure on deposition and retention. Particles with diameters in the range 0.1 – 2 μ m deposit much more rapidly on tall canopies than current theory predicts, but above and below this size range there is reasonable understanding of the physical principles governing deposition. Hygroscopic particles grow rapidly in humidity gradients, altering their depostion rates. The chapter concludes with a section on deposition mechanisms when small particles are inhaled, indicating the hazards posed by inhaled sub-micron-sized pollutant aerosols.

Microclimatology of Radiation: (ii) Radiation Interception by Solid Structures

Abstract: In microclimatology and micrometeorology, radiative fluxes are usually expressed per unit area of ground. In environmental physics, we are also concerned with radiation intercepted by leaves at angle in canopies, sloping land surfaces, and animal coats. This chapter describes methods for estimating the radiation intercepted by solid structures in terms that depend on the geometry of the surface (defining a “shape factor”) and the directional properties of incident radiation. Shape factors are derived for simple geometric shapes such as ellipsoids, cones, and cylinders, to which the shapes of animals, trees, and shrubs can often be approximated. Direct and diffuse radiation is treated.