NYC is unique in meeting its water needs. 8.5 million residents plus another million or so commuters and visitors consume 1.3 billion gallons every single day. Moreover, there's clean drinkable water right out of the tap. Much of American cannot say this and, sadly, most of the world may never have anything close to this luxury. The 19th century sewer plan certainly did not count on this extraordinary volume - yet, it's still functioning! But there are some significant environmental drawbacks.
The system was designed to overflow at peak water points - heavy rainfall. There are a whopping 496 outflow points around the five boroughs emptying into the East and Hudson rivers (attached map) to create the Combined Sewer Overflow system (CSO, attached graphic).
In dry weather, the system is functioning at near capacity and when NYC receives a mere 1/20 inch of rain, the CSO is gushing. If that sounds like a bunch of sh*t flowing directly into NYC waterways, you are very much correct. And that's just the New York side - New Jersey has it's own problems dumping into the Hudson.
So I've decided to focus on a rainwater harvesting system for my rooftop garden. The plan is to keep rainwater out of the CSO system and provide my plants with an extra dose of a natural gift. I'll have to be cautious as water is heavy and I'm working on a rooftop with weight restrictions but this is totally doable. I've attached also a guide to rainwater harvesting from GrowNYC.org, a site I use a lot to navigate growing issues in the city.