On the role of entropy in water quality modelling

The use of the entropy principle in phenomenological water quality models is not only necessary, but also of great advantage. A deterministic ecosystem model must obey the 2nd law of thermodynamics. Gibb's equation is a constraint additional to the balances of mass, energy and momentum. The entropy principle supports the unified treatment of physical, chemical and biological processes in water bodies, offers stability criteria and controls the further development of the aquatic ecosystems. Thermodynamic criteria also allow the determination of the bifurcation points of the model equations. Especially near these points the state and structure of the ecosystem can be strongly changed by fluctuations of the variables and parameters of the ecosystem.Results of the thermodynamic theory of selforganizing systems (Glansdorff and Prigogine, 1971; Nicolis and Prigogine, 1977) are of very great importance for water quality modelling. Furthermore, the entropy principle bridges the phenomenological, stochastic and cybernetic approaches to water quality modelling.While the paper deals with general aspects of the role of entropy in water quality modelling, the basic system of equations, taking the entropy principle into account, can be found in a previous paper (Mauersberger, 1978).