LANDSCAPE ECOLOGY in AGROECOSYSTEMS MANAGEMENT - CHAPTER 3

CHAPTER 3 Mitigation of Radiation and Heat Balance Structure by Plant Cover Structure Janusz Olejnik, Andrzej Kedziora, and Frank Eulenstein CONTENTS Introduction Heat Balance Components Measurements and Modeling of Heat Balance Components Daily Value of Heat Balance Components for Selected Ecosystems Seasonal Value of Heat Balance Components for Selected Ecosystems and the Landscape Changes in Heat Balance Components under Changed Climatic Conditions References INTRODUCTION The Earth’s atmosphere is an extremely important medium that links the other elements of the environmental system. There are permanent fluxes of energy and matter exchange between the Earth’s surface and its atmosphere. All processes occurring on the Earth are possible because of spatial differentiation of potential energy. The realization of each irreversible process is connected with entropy production — in other words, with the increase of “disorder” in a thermodynamic system. With regard to the Earth–atmosphere system, it means that if no energy flux flows into the system from outside, the temperature within the system will equalize and all processes will be stopped.Yet constant flux of solar energy acts as a brush; it removes this disorder from the system by restoring the differentiation of the temper-ature field because it takes away more entropy than it brings into the system. Thanks to this perpetual solar energy flux (Figure 3.1), the realization of all thermodynamic © 2002 by CRC Press LLC Balance of: radiation heat water Rn = K - K + L - L Rn = G + S + LE E + H = P common element Figure 3.1 Schematic connection of radiation, heat, and water balances in agricultural landscape. processes, which are essential for life on Earth, is possible. The most important of these are the matter-cycling processes, especially water cycling. Due to these fluxes, one can consider the atmosphere a mirror, in which the present conditions of the Earth’s surface environment are “reflected.” On the other hand, the condition of the environment can be modified by the state of the atmo-sphere, both by short- and long-term interactions. Industrial and agricultural devel-opment in the 19th and 20th centuries led to unfavorable changes in the natural environment. At present, while these changes are commonly recognized on both the global and local scales, it is important to stop or at least minimize the effects of those processes that have negative effects on the environment. The crucial question is how to manage the landscape to create conditions that will enable us to return (if possible?) to ecological equilibrium (Stigliani et al. 1989, Freedmen 1992, Ke˛dziora 1999). To answer that question, we first must learn more about interactions between the atmosphere and the Earth’s surface. One of the ways to achieve a better understanding of all the processes that occur in the natural environment is to measure or model them. Results of measurement of energy fluxes create the possibility of parameterization of these processes, conse-quently leading to development of these models, which can be applied to different environmental conditions. Modeling of heat and water balance components very often has a practical application, e.g., for estimating evapotranspiration on a field scale (Olejnik 1988, Olejnik et al. 2001) and regional scale (Thom and Oliver 1977) by determining the water condition due to land-use condition (Kirchner 1984), or even by forecasting and estimating the yield (Tanner 1981, Gurney 1988). © 2002 by CRC Press LLC One way of describing environmental processes on different scales (from micro to global scale) is by the analysis of energy flux exchange between the Earth’s surface and the atmosphere. The knowledge about these fluxes and their interactions provides infor-mation not only about energy exchange in the environment but also about water, carbon, and nutrient balances and can be helpful for their estimation when direct measurements over large areas are difficult or impossible. Therefore, the analysis of heat balance structure is one of the best descriptions of environmental conditions on both local and global scales (Paszy´ski 1972, Rosenberg 1974, Kim and Verma 1990, Olejnik 1996). The driving force of energy in water, carbon, or nitrogen cycles and the interac-tions of these cycles provide a basic framework for understanding the potential feedback of the active surface and the atmosphere. Most of these relationships exert strong biological control on land-atmosphere interactions. Therefore, land-cover and land-use changes are driving variables for understanding energy exchange and, con-sequently, climate processes on both regional and global scales (Valentini et al. 1999). The landscape is a very complex, open, dynamic system of relationships in which nonlinear processes and feedback mechanisms play an important role. Many of these feedback mechanisms depend on the process of energy and mass exchange between the atmosphere and the Earth’s surface. The so-called surface processes are strongly connected with the transfer of water or water vapor from and into the atmosphere. Part of the water that reaches the Earth’s surface returns to the atmosphere via evapotranspiration and part goes to sea by runoff. The available energy is the most important physical factor that limits the amount of evapotranspiration. On the other hand, the kinds of plants that cover the Earth’s surface as well as the plant devel-opment stage are the most important biological factors. For model investigations on heat and water balance structure, both physical and biological factors have to be included in models for all scales — from field to global. HEAT BALANCE COMPONENTS The middle section of Figure 3.1 shows the heat balance of an active surface. In general, energy exchange processes are driven by solar radiation. The right part of Figure 3.1 shows the radiation balance. The short-wave radiation reaching the Earth’s surface is called global radiation (¯K). The short-wave radiation is partly absorbed by the active surface (soil, plant, water, etc.) but partly reflected (­K) from the active surface. The ratio of reflected to incoming global radiation is called albedo (a = ­K/bK) and is one of the most important parameters characterizing the input of energy into agroecosystems, forests, deserts, etc. For example, in natural conditions, high albedo (e.g., 0.3) means that a substantial part of the short-wave radiation energy reaching the ecosystem is reflected and only 70% is absorbed by the active surface. The absorbed short-wave radiation increases the temperature of the active surface, which in turn is reflected as increased long-wave radiation of the active surface (­L in Figure 3.1). This long-wave radiation is partly absorbed in the atmosphere by the greenhouse gases (water vapor, CO2, etc.). This absorption causes the increased tem-perature of the atmosphere, which in turn increases the long-wave radiation of the atmosphere. Some of the long-wave radiation of the atmosphere goes to the higher © 2002 by CRC Press LLC atmosphere and to space, but part returns to the Earth’s surface (¯L). If one adds all four fluxes — two incoming (short wave from the sun and long wave from the atmosphere back to the Earth’s surface) and two outgoing (short wave reflected and long wave emitted by the Earth’s surface) — one can construct the radiation balance of an active surface (Figure 3.1). The result of the balance of all these fluxes is the net radiation (Rn), and it denotes energy supply for all processes in the environment driven by natural energy, including photosynthetic processes, which utilize solar radi-ation for organic matter production. However, metabolic processes may be considered negligible relative to the magnitude of other energy fluxes, and, therefore, it is possible, following the energy conservation law, to write the energy (heat) balance equation as: Rn = Le+S+G (3.1) where Rn is net radiation, LE is latent heat flux (L is evaporative heat of water and E is evapotranspiration), S is sensible heat flux, and G is soil heat flux. Evapotranspiration of the plant community (or its energy equivalent, latent heat flux), warming of the soil surface, and consequently, also deeper soil layer (due to conductivity) or convection heating of the atmosphere are well-known processes occur-ring in the environment. Rn sets the energy utilized by these processes. The partition of net radiation for these processes is described by the heat balance equation and depends on both physical and biological factors that determine environmental condi-tions: plant type, land-use structure, plant development stage, weather conditions, etc. Through the latent heat flux (or evapotranspiration), there is a direct connection between heat and water balance of an investigated area. Figure 3.1 shows the connec-tions between radiation, heat, and water balances. The links between these balances play a very important role for the complex description of energy-water conditions of the environment and are often used by investigators to calibrate or validate different models for estimating mass and energy exchange processes.At present, many modeling activities use these principles to calculate the mass and energy exchange for several land-use/cover types, from the patch (small area having the same characteristics of habitat) to regional or global scale. Nevertheless, since the concept is relatively new, it has not been tested widely against field measurements of mass and energy exchange, nor has this concept been tested over an assortment of vegetation types. Experimental measurements of biosphere-atmosphere mass and energy exchange have the additional advantage of providing a new synthesis view of ecosystem processes, which allows us to improve our conventional classification of vegetation types and forms, achieving a deeper insight into functional biodiversity at the landscape level (Ke˛dziora and Olejnik 1996, Valentini et al. 1999, Ke˛dziora and Ryszkowski 1999). MEASUREMENTS AND MODELING OF HEAT BALANCE COMPONENTS In the 1990s, developments in micrometeorological technology and theory made it possible to study regularly the interactions between vegetation and atmosphere. © 2002 by CRC Press LLC Recent improvements in the eddy covariance method, which provides a direct measure of biosphere-atmosphere mass and energy fluxes at the ecosystem level, were partic-ularly important for studies (Ryszkowski and Ke˛dziora 1993b, Tenhunen and Kabat 1999, Ke˛dziora et al. 2000). In the past, most of our knowledge of carbon, water, or energy exchange in vegetation was based on measurements taken at the leaf level with enclosure chambers or portable cuvettes (Field et al. 1982). With the develop-ment of micrometeorological methods, a large change in scale of focus was achieved, moving from the leaf level, species/individual-dependent approach to an integration of overall ecosystem fluxes in a direct, nondestructive way. There are many microme-teorological methods based on physical principles, which allowed measurement of the turbulence components of heat balance (LE and S in Equation 3.1). According to the eddy covariance theory, the eddy flux of any scalar can be written as: F = rc ×w (3.2) where Fc is the flux density of scalar c, w is the vertical wind speed, and rc is the density (or concentration) of the transported species. The overbar represents the mean of product over the sampling interval. The eddy covariance method technique, developed during the 1980s and the 1990s, needs very technologically advanced sensors, and therefore it is not so commonly used. To measure eddy fluxes with this technique, rapid response sensors are necessary. These sensors and incorporated data acquisition systems are able to measure and collect data with very high frequencies (10 Hz and more). To measure heat or mass fluxes, sonic anemometer and open- or close-path gas analyzers are used (Arya 1988). Less technologically advanced sensors can be used for eddy flux measurements using gradient or heat balance methods. To determine the latent and sensible heat fluxes (LE and S in Equation 3.1) measurements of temperature and water vapor pressure in the air at several levels are necessary. For such measurements, adequate electrical psychrometers and cup anemometers can be used. It is known that latent (LE) and sensible (S) heat flux densities are proportional to water vapor and air temperature gradients, respectively (Monteith 1975), as follows: LE = Kvrcp g−1 ¶e (3.3) S = KH rcp ¶T (3.4) where g is the psychrometric constant equal to 0.65 hPa·K–1, e is the water vapor pressure [hPa], T is the air temperature [K], r is air density equal to 1.25 kg·m–3, cp is the specific heat of air equal to 1004 J·kg–1·K–1, and KV and KH are the eddy diffusivities for water vapor and heat, respectively. There are also methods of eddy flux determination that are based on the com-bination of two methods: gradient (with measurement at few levels) and heat balance © 2002 by CRC Press LLC ... - tailieumienphi.vn