Hydrological analysis and prediction is usually carried out for a purpose, to inform decisions about future investments and requirements in the management of water resources. These decisions are made within the context of legal requirements that stem from high-level social, economic and environmental policies adopted by governments. In the European Union (EU), there are a variety of legal directives that influence the management of water. Of these, the most important recent directives are the Water Framework Directive1 and the Floods Directive,2 discussed in Chapters 16 and 17 and elsewhere in this book. Both of these directives set out a time table for improving the management of water quality and floods, although the details of implementation are decided individually by each of the nations in the European Union.