What a fantastic question! New York City uses over one billion gallons of water every day. That essential water comes to us via aqueducts that connect us to two different watersheds – the Croton watershed just up the Hudson and the Catskill / Delaware Watershed Area further upstate.
(A watershed is all the land whose water feeds through tributaries into a given larger body of water, like a lake, river, or bay. Therefore, the Croton watershed is all the area whose water, including rain water and snow melt, eventually flows into the Croton River.)
The New York City Department of Environmental Protection is responsible for overseeing the quality of our water, including working with upstate authorities to keep our aqueducts full of clean water (emphasis on both full and clean).
If you're explaining this to students, its a great idea to talk about what these upstate reservoirs are like. You most likely have students who have never been to another part of New York state. To help them envision the scene, read the first few pages of Water Dance by Thomas Locker. This beautifully illustrated book is set in the Catskill Mountains in upstate New York, the region where our water originates.
After reading Water Dance, have a conversation with students about what would happen if people took too much water from the lake for wasteful purposes – would rain replace every drop we took? Would it change the natural landscape shown in the book? Then, ask students what they might do to use only the water they need from the lakes and reservoirs upstate. Students who aware of the source of our water and understand that it is not, in fact, an unlimited resource are more likely to appreciate the need to conserve water.
After students are in the habit of keeping track of daily water use, extend the activity to include products that need water to grow or function properly. For example, rain water helped grow the banana you are eating and irrigation helped grow the cotton used to make your t-shirt. Water cools the engine of the bus or car you rode to school. 
Above, the finished gardens are laid out in Brooklyn Children's Museum's Greenhouse.
Another great example is the Peruvian garden!
