Under the same conditions as in the previous case we have Cc — Cf < 0.1C
Ti — T = 5 K (we assume that the temperature of the leaf interior at the surface of parenchyma is close to the optimal one for photosynthesis: 25°C), cp = 1J/gK. Then from Eq. (2.14) it follows that qh/qc < 76kJ/gC02. From this it follows that assimilation of 1 g of C02 is accompanied by direct dissipation of 76 kJ of heat. If we measure the assimilation in carbon units, then the previous statement could be reformulated as: in order to assimilate 1 g of carbon it is necessary to dissipate 278 kJ of heat.
10.3. Energy balance of a vegetation layer and the energy efficiency coefficient
We have assumed above that the vegetation cover was rather dense so that the transpiration prevails on the evaporation from a bare soil. In this case the equation of energy balance for the vegetation layer is written as
where R is the radiation balance, and qw, qh, qc are the fluxes of water vapour, heat and CO2, respectively, as already mentioned in Section 10.2. The coefficient yw is the specific enthalpy of water vapour — yw = 2450J/gH2O. The coefficient yc is the energy expense for the assimilation of one unit of CO2. The latter can be estimated from the thermal effect of photosynthetic reaction, 470 kJ/mol CO2. This value is the molar enthalpy of CO2 in the process of its assimilation. By recalculating it to 1 g of CO2 we get yc = 10.7 kJ/g CO2.
The flux of assimilated carbon dioxide qc determines the gross primary production of plants. Therefore, the ratio of two values can be considered as some efficiency coefficient of vegetation: the first is the solar energy expended for the maintenance of photosynthesis and the second is the part of incoming radiation absorbed by the underlying surface, i.e. the radiation balance:
By substituting Eq. (3.1) into Eq. (3.2) and after simple transformation we get 1
Gc qc Gc qc
In Section 10.2 we calculated the ratios qw/qc and qh/qc (see Eqs. (2.12) and (2.14)). By substituting these expressions into Eq. (3.3) we have
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