The proposed solution is a modern take on the ancient technology of cistern rain water storage and usage. Ancient cities, like Lecce in Puglia, Italy, used to have cisterns below the houses, with the rise of modern plumbing, these have been converted to storage cellers. Introducing local rain water reservoirs to houses would make a second water source available, which can be used to water gardens, wash windows etc. with additional plumbing, this rain water could also be used to flush toilets. An additional benefit would be, that another water source is available in case of fire. Water tanks could be underground, or on roofs or as separate water towers.
It will reduce the amount of drinking water that is used to water green areas, flowers and to flush the toilets.
As an additional benefit, it will reduce the amount of rain water that flows through the canalisation and which in case of heave rainfall floods the streets, because a large amount of the rain water from the roofs is collected in cisterns and not released to the streets.