That depend on which country ...
Published by Diego Fernandez, Economist; M.A. in public policy and regulation.
That depend on which country and maybe in which state you are. In almost each one of all US states, there is a regulatory utility commission which defines methodologies to set the water rates. In some US counties (as in Florida Dade) in fact are local regulatory bodies in charge to define water rates.
In others countries, as England; Chile, Perú or Colombia the are national regulators in charge to define methodologies to set water rates or directly in charge to define water rates.
In general, the water rates have to be defined to recover the cost of the services: operational cost (OPEX) and capital cost (CAPEX)