The OECD defines a water supply system as such: A water supply system is a system for the collection, transmission, treatment, storage and distribution of water from source to consumers. All existing water supply systems are built on top of preexisting natural hydrologic cycles. Water supply systems are thus a form of socio-ecological system composed of both ecological components and engineered components, the two networks overlap, interact and are interdependent throughout the water cycle.
Water systems are invariably complex systems in that they consist of many interacting and interdependent variables on many different levels. From various ecological components, including climatic, geological and biological elements, to technology in the form of pumps, pipe networks, dams to socio-economic elements such as various businesses, manufacturers, markets, regulations and end-users.