D4.5 describes the development of a reference ontology for the DWC project. Based on requirements elicited from cases in DWC, the report describes the conceptualisation, implementation, evaluation, and documentation of the reference ontology. The work performed has shown that most requirements can be satisfied by existing ontologies describing water-related concepts, notably ontologies in the ETSI SAREF suite. However, a few extensions in the form of new ontology classes, object properties and data properties are needed to fully address the DWC requirements. These extensions will be forwarded to the appropriate ontology maintenance body.