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 200 kilometres north of the city limits, within the Catskill/Delaware and Croton watersheds, supplying water to the 8 million residents of the city. The city has a Department of Environmental Protection, which administers a comprehensive watershed programme, including protective (or pro-active) and corrective (or reactive) measures. For example, the city has invested more than USD 300 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.