Water consumption and population in NYC during last 30 years (data on water consumption were obtained from (NYCDEP, 2007)). 

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... billion gallons (531.8 million cubic metres); and the Delaware system capacity is 320.4 billion gallons (1,212.8 million cubic metres). The Croton system, with a capacity of 30.6 billion gallons (115.8 million cubic metres), also includes the Kensico reservoir, which receives both the Catskill and Delaware water, passing it down to the NYC distribution system via the Croton aqueduct. Thus, altogether the Croton system has a capacity of 87.8 billion gallons (332.3 million cubic metres), approx. 15% of the total capacity. A separate aqueduct from Croton reservoir reaches NYC directly, and discharges water into the Jerome Reservoir in the Bronx. Besides having low water storage capacity, the Croton system also does not meet the special criteria assigned by the “Surface Water Treatment Rule” which was implemented in 1989 under the 1986 amendments to the Safe Drinking Water Act. This requires all surface water supplies to be filtered. Although Croton’s water continues to meet all federal and State health-related water quality standards, except for elevated coloration due to organic matter and minerals, its’ closeness to the highly populated NYC and suburban area raises concerns. A proposed filtration facility will be able to improve colouration as well as reduce microbiological contamination. Being in a very rural area, the Catskill/Delaware system was granted exemption by the Environmental Protection Agency (EPA) from this filtration rule. This led to implementation of the USEPA Filtration Avoidance Determination (FAD) in 1993, a management plan designed to implement a series of watershed protection measures, as well as monitoring, and modelling systems. Summarized in environmental impact statements (EIS), the new regulations were designed to improve water quality parameters and to comply with federal and state regulations without the necessity to filter its water supply. However, the implementation of the new EIS watershed anti-pollution regulations negatively influenced the economic development in the counties within the Catskill/Delaware system, creating unprecedented tensions between these counties and New York City. The Coalition of Watershed Towns within five counties was created to oppose watershed regulations and numerous meetings were held between them and NYC officials. These tensions were eased in 1997 when New York City signed a Memorandum of Agreement (MOA) between NYC and the Catskill/Delaware counties in the watershed system. The MOA was also facilitated by USEPA, New York State and various environmental groups. It helped not only implement necessary measures for improving and maintaining water quality within the watershed, but also provided upstate counties with job opportunities and involvement in watershed protection. According to NYCDEP (2008), “DEP’s operations and investments translate into 1833 jobs in the West of Hudson watershed in addition to the more than $100 million in taxes that DEP pays to the watershed communities each year.” In 2007, the NYC water supply was granted a 10-year FAD from USEPA. More than $1.5 billion was invested in the previous FAD, primarily in watershed protection programmes such as land acquisition, septic repairs, storm water controls for new development, wastewater treatment plant upgrades and economic development grants and loans through a local not-for-profit corporation. New York City currently owns approximately 125 000 acres (505.8 million square meters) of land surrounding its watershed and plans to invest an additional $300 million in land acquisition over the next decade” (NYCDEP, 2008). A new FAD will target similar goals, but will also expand on stream restoration projects, enhancement to the existing Watershed Agricultural Program, and invest in a new UV Disinfection facility to add an extra measure of protection to the Catskill/Delaware drinking water systems (NYCDEP, 2008). Water conservation measures implemented during the last 30 years decreased water consumption per capita despite noticeable population growth (Fig. 2). During the 1950s, 1960s and 1970s water consumption increased at about 1% a year. Conservation measures implemented between the 1980s and 2002 such as metering, education, leak detection, toilet replacement and water use regulations (EPA, 2002; Liebold, 2008) decreased water consumption despite the concurrent increase in population. Today, the management of one of the largest surface water supply systems in the US experiences new challenges associated with climate change, continuing water demand due to population growth, enhancement and maintenance of current conservation measures and continuing demands for economic development in upstate watershed communities. The combination of these factors poses unique social, economic, political, geological and hydrological challenges that help to identify future risks for water supply management. NYC watershed area covers more than 4970 km . Table 1 contains information on the characteristics of each of the water supply reservoirs. It should be noted that the Ashokan reservoir receives much of its water from the Schoharie reservoir via the 18.1 mile-long Shandaken tunnel that releases water into the Esopus Creek, a major tributary of the Ashokan reservoir. The Kensico reservoir, part of the Croton system, has a very small watershed area. However, as a terminal receiving reservoir for the Catskill/Delaware systems it routes approximately 85% of the annual drinking water supply for NYC. Kensico receives water from the Catskill/Delaware supply system by gravity through tunnels under the Hudson River. The Croton system consists of 12 reservoirs that supply water to New York via the Jerome Reservoir in the Bronx (i.e. northern NYC). It is connected with the Croton reservoir by an aqueduct. The safe yield is the amount of water that can be delivered to NYC considering the potential occurrence of the latest and most severe drought. This amount was determined using the worst droughts of the 1960s. Fig. 3 shows that the water consumption for the city on 3 June 2008 was 1.09 billion gallons (4.1 million cubic metres) which is lower that the safe yield currently estima- ted as 1.29 billion gallons (4.9 million cubic meters). Variations in yield depend on reservoir levels, associated with variations in weather patterns (droughts, floods, etc.) For example, on 2 August 2006, during extremely hot and humid weather, the daily flow was 1560 mgd (5.9 million cubic metres per day) and the peak flow reached 2020 mgd (7.6 million cubic metres per day) (NYCDEP, 2008). The size of the system, reservoir design and the inter-connectivity between reservoirs plays an important role in its management during the periods of droughts and floods. Water can be transferred and diverted. This gives operational flexibility to prevent local water shortages, reduce water excesses during floods, and also deal with water quality issues by closing the gates of the reservoir containing degraded water quality. Periods of severe droughts prompted NYC to develop a Drought Management Plan. The latest version includes three main drought phases outlined in NYCDEP (1998). Drought criteria are based on an analysis of historical data together with the current situation for the period of the “water year” that spans from 1 June to 31 May. If a comparison reveals a hazardous situation, there are contingency and emergency operations that can be implemented. They include conservation measures (street washing restrictions, lawn care, etc.), use of pumping stations located along aqueducts and tunnels, increase of gravity water distribution, use of additional water supplies from the Hudson River, use of wells in the southeastern part of Queens (NYC), and an increase of water supplied by the Croton system. The total current system condition is depicted daily on a web site of NYC DEP (Fig. 3). The size and connectivity of the NYC water supply was a focal point of one of the first optimization studies using a Geographic Information System (Greenberg et al ., 1971). Thus, the water supply system experiences constant management changes in response to changing population and weather events. As Major & Goldberg (2000) point out, the system is a mature infrastructure that is operated by agencies already used to dealing with weather variations; this “makes the implementation of institutional and infrastructure adjustments to increase resilience more feasible”. Population growth of NYMA and adjacent areas was always a focus of the water supply system development. Since its first structure (Croton dam) was built in 1842 the system is constantly growing in terms of size and implementation of protection measures and management policies. One of the most comprehensive studies includes a population model of conservation vs non- conservation scenarios. It was done by Hazen & Sawyer (1989). The study projected future needs increase from 1611.3 mgd (6.1 million cubic metres per day) in 1995 to 2061.4 mgd (7.8 million cubic ...