Rain Water Harvesting
Rain water harvesting is essentially returning to "cistern technology", that used to be commonly practiced - and still is in arid parts of the world. In the US, runoff from roofs or other impervious surfaces is captured and treated to a minimal level and used for non-potable needs - for things such as watering gardens, flushing toilets, laundry, etc. (not for human consumption)
If a system is sized to handle runoff from a significant amount of rain (a 1.25 inch event is the size of rain that water quality practices are designed for since most rainfall events are less than this amount) then a significant reduction in runoff will be achieved. Reducing the volume of runoff reduces the delivery of pollutants to streams and reduces the flashiness of stream flows (which helps reduce stream corridor erosion). Rain water harvesting also reduces demands on municipal water supply systems and can save money for high water use customers.