This book represents the outcome of the Second IWA Leading-Edge Conference held in Sydney, Australia in November 2004. Sustainability is a paradoxical concept. We know we want to protect the environment from human-induced change, yet ecosystems are dynamic, constantly changing and adapting in response to a multitude of factors, the combined effect and subtleties of which are probably well beyond human calculation. Furthermore, our conscious desire to protect the environment - which forces us to think of humans as sitting outside ecosystems - conflicts with the unavoidable fact that we are an unconscious actor within those ecosystems. We must also recognise that the goal of 'protecting the environment' is not a clear-cut objective. Perhaps, because of its complexity and propensity to change, we cannot know what the fully protected environment would look like. Individual preferences too make the conceptualisation of an ideal state impossible; do we strive for an ecosystem in which we play a minor part - barely influencing natural outcomes - or one that is more actively managed and provides for our needs or wants? Neither this volume, nor the conference from which it draws, resolve the paradoxes described above. The papers presented here do, however, provide insight into the innovative thinking and practical projects undertaken across the globe that move us from patently unsustainable conditions to those in which our economic activity, impact on ecosystems and desire for positive social outcomes are in better balance.

Since the International Institute for Applied Systems Analysis began its study of water quality modeling and management in 1977, it has been interested in the relations between uncertainty and the problems of model calibration and prediction. The work has focused on the theme of modeling poorly defined environmental systems, a principal topic of the effort devoted to environmental quality control and management. Accounting for the effects of uncertainty was also of central concern to our two case studies of lake eutrophication management, one dealing with Lake Balaton in Hungary and the other with several Austrian lake systems. Thus, in November 1979 we held a meeting at Laxenburg to discuss recent method ological developments in addressing problems associated with uncertainty and forecasting of water quality. This book is based on the proceedings of that meeting. The last few years have seen an increase in awareness of the issue of uncertainty in water quality and ecological modeling. This book is relevant not only to contemporary issues but also to those of the future. A lack of field data will not always be the dominant problem for water quality modeling and management; more sophisticated measuring techniques and more comprehensive monitoring networks will come to be more widely applied. Rather, the important problems of the future are much more likely to emerge from the enhanced facility of data processing and to concern the meaningful interpretation, assimilation., and use of the information thus obtained."

During 1978-1982 the International Institute for Applied Systems Analysis (IIASA) was responsible for a research project on Environmental Quality Control and Management. The project was begun under the direction of Professor O. F. Vasiliev (from the Institute of Hydrodynamics of the Siberian Branch of the USSR Academy of Sciences) and was subsequently led by myself. This review is very much a re'fiection of that IIASA project. The major themes of the IIASA project were: (i) research into the methodological aspects of modeling river and lake sys tems [some of the principal results of this research appear in M. B. Beck and G. van Straten (eds. ) (1983), Uncertainty and Forecasting of Water Quality (Springer, Berlin (West)), and in K. Fedra (1983), Environmental Modeling Under Uncertainty: Monte Carlo Simulation (IIASA Research Report RR-83-28)]; (ii) case studies in the application of mathematical models to lake eutrophi cation control [results of which are summarized in L. Somlyody, S. Hero dek, and J. Fischer (eds. ) (1983), Eutrophication of Shallow Lakes: Model ing and Management (The Lake Balaton Case Study) (IIASA Collaborative Proceedings CP-83-S3), and in K. Fedra (1983), A Modular Approach to Comprehensive System Simulation: A Case Study of Lakes and Watersheds (in W. K. Lauenroth, G. V. Skogerboe, and M. Flug (eds. ), Analysis of Ecological Systems: State-of-the-Art in Ecological Modelling, pp. 195-204. Elsevier, Amsterdam)]; iv (iii) a policy study of operational water qua,lity management [M. B. Beck (1981), Operational Water Quality Management: Beyond Planning and Design (IIASA Executive Report ER-7)].