Exergy of solar radiation: information approach

Abstract

A new thermodynamic approach for the analysis of ecosystems is presented. Vegetation is regarded as an active surface, which is interacting with solar radiation. Absorption, reflection and emission of radiation are consequences of this interaction, resulting in a new composition of the spectrum of outgoing radiation. From a macroscopic point of view this is equivalent to the change of energy and information. It is natural to assume that the difference between these macroscopic variables (incoming and outgoing radiation) is not only defined by the properties of these radiation fluxes but also the properties (state) of the active surface (vegetation). Measuring the change of energy by the radiation balance and the increment of information by the Kullback measure we define a special function (exergy) as a function of these variables. We postulate the following minimax principle, which is a generalisation of Jørgensen's maximal principle: during the process of self-organisation the vegetation tends to maximise its exergy in respect to the increment of information and to minimise it in respect to the radiation balance. From this point of view, the functioning of vegetation may be interpreted as a work of some ‘thermodynamic machine’, which can produce both mechanical work and information. This hypothesis was tested for seasonal dynamics of several ecosystems located at geographically different sites, where radiation data for two spectral intervals (long-wave and short-wave) was available. It was shown that the minimax of exergy is attained in June–July near the maximum of photosynthesis. The comparison of these values for different ecosystems shows that more self-organised ecosystems (like forests) are characterised by higher value of exergy at the minimax point.