# Gc Cc C GcDc cc cc

If we substitute into Eq. (3.3) the numerical values of qw/qc < 240 g H2O/g CO2 and qh/qc < 76 kJ/g CO2, which were calculated under the climatic condition of temperate latitudes, and set the value 10.7kJ/gCO2 as yc then hR = 1/(1 + 7.1 + 55) < 1.6% (compare with Section 5.3). If we now keep in mind that for most of Eastern Europe the summer radiation balance is equal to 55 -60% of the total solar radiation of the summer season (Budyko, 1963) then the efficiency coefficient, which was recalculated for this value of energy input, is equal to h < 0.9%.

We have assumed earlier that plants are normally using only a small part of the potential diffusion flux of carbon dioxide. If now we presuppose that all the potential flux is being used, i.e. Cc - Cf < Cs, then the efficiency coefficient h in this theoretical case will be equal to 7%.

10.4. Thermodynamic model of vegetation: internal entropy production

Let us represent the vegetation layer as a certain volume filled by leaves (not necessarily everywhere compactly). It has a unique basement (for instance, 1 cm2) and a thickness (or height) equal to l2: It is an open system, and the change of its entropy, caused by the production processes, is represented as dS=f - <->

where the term diS/di corresponds to dissipation processes (in particular, evapotranspiration and thermal diffusion), while the term de S/dt corresponds to an external flow of energy. It can be either the full radiation balance or its share. Then the term dS/dt will correspond to the total increment of biomass, i.e. the gross primary production (per some time unit).

The main assumption is: "all heat produced within the vegetation layer by all metabolic processes, like transpiration and respiration, is exported into the environment (atmosphere). The rate equals the rate of entropy production (or, simply, the entropy production) diS/dt."

We shall consider the following two processes: (1) transpiration of water by leaves and subsequent transportation of water into the atmosphere by turbulent diffusion, and (2) thermal diffusion of air between interior and exterior of leaves and its subsequent turbulent transportation. In this case the total function of dissipation (see Section 2.5) is

where Ch and (Cw)p T are the items describing the dissipation of energy as a result of the heat conduction and the diffusion of water vapour, correspondingly. The interaction of these two irreversible processes, namely heat conduction and diffusion, leads to the appearance of an additional source of energy dissipation corresponding to the item Chw. The first item of sum (4.2) is represented as