The hydraulic system surrounding the City of Murcia (South East Spain) shows a contradiction in terms of social justice. In spite of the existence of a number of elements in favour of the development of a well-balanced socio-economic structure, such as several irrigation principles, rules and institutions based on common sense and the practical use of irrigation, the system was distorted by the ambitions of a well-established oligarchy and the creation of a class of poor tenants and day labourers. The paradox of the hydraulic system of Murcia lies on the fact that once an acceptable democratisation and social balance level were reached, its economic capacity failed. As a matter of fact, social justice was not provided by the system itself, but by the general positive social evolution and economic growth since the 60s-70s. A number of socio-economic features are explored in addition to institutional parameters to measure the evolution of the equity of the system