Info

The Kiel values were averaged over the years 1991-1996. The Hamburg grassland values were averaged over 1973-1993. The Central and South Germany values were averaged over 1974-1997 and 1998-1999. The Oklahoma values are from 1996.

The Kiel values were averaged over the years 1991-1996. The Hamburg grassland values were averaged over 1973-1993. The Central and South Germany values were averaged over 1974-1997 and 1998-1999. The Oklahoma values are from 1996.

Finally, the third hypothesis can be formulated as:

3. In the process of productivity maximisation, vegetation uses the minimax strategy, minimising the exergy efficiency coefficient with respect to the radiation efficiency coefficient and maximising it with respect to the increment of information.

We think that the statement can be considered as a generalisation of the J0rgensen maximum exergy principle (1997), although applied to one concrete case, i.e. to the interaction between solar radiation and vegetation. If the third hypothesis is a minimax exergy principle then we can get a relationship connecting such originally unconnected values as the Kullback measure K and the radiation coefficient Hr. It is natural to assume that these values are independent, if our "active surface" is passive in relation to the incoming radiation. However, if the "active surface" is really "active", i.e. it actively interacts with incoming radiation partly using it and transforming its spectrum (like a vegetation cover), then we have the right to expect that these values will be connected. If we postulate that the minimax exergy principle is true, then the productivity of vegetation is maximal when

Although "comparaison n'estpas raison" it would be interesting to compare the annual means of exergy, radiation balance, the corresponding efficiency coefficients and the increment of information (Kullback's measure) for the different sites. The results are shown in Table 10.2. Looking at these sites the differences between more natural and anthropogenic ecosystems are apparent. Besides, in the colder climate of North Germany the values are lower than in South Germany or the US.

To outline the principal difference between forest and agricultural ecosystems we present these values in Fig. 10.2 on the two superposed planes {K, 7fR} and {yEx, 7fR}. One can see that the forest ecosystems seem to have higher potential in using exergy regarding both absolute values and efficiency coefficients than crops and grassland. For example, the beech forest in Kiel has higher values for all parameters than the Kiel crop field and the Hamburg grassland. Moreover, it has higher exergy efficiency than the wheat field in

Was this article helpful?

0 0

Post a comment