the integration and balance of these uses and users requires a watershed approach the city has the largest unfiltered water supply system in the world and its reservoir system extends nearly kilometres north of the city limits within the catskilldelaware and croton watersheds supplying water to the million residents of the city the city has a department of environmental protection which administers a comprehensive watershed programme including protective or proactive and corrective or reactive measures for example the city has invested more than usd million to buy upstate land around the reservoirs in order to improve source water quality and eliminate the need for building expensive water quality treatment plants