Phosphorus reduction in the New York City water supply system: A water-quality success story confirmed with data and modeling

New York City (NYC)’s drinking water reservoirs supply over four billion liters each day to over nine million consumers in NYC and upstate communities. In the last 25 years, the City has invested nearly $2 billion in watershed protection programs (WPPs) to maintain high source water quality, allowing NYC to avoid filtration for 90% of the supply. This study involves the use of a model to evaluate the impact of WPPs on phosphorus (P) loading in the Cannonsville Reservoir watershed, one of the unfiltered water supply sources. The model is SWAT-Hillslope (SWAT-HS), a modified version of the Soil and Water Assessment Tool (SWAT) that can realistically predict variable source runoff processes. We applied the SWAT-HS model to this watershed to test its ability to simulate conditions observed after the implementation of watershed protection and evaluate the impact of point and non-point source WPPs on watershed export of P. When applied to a 12-year period of WPP implementation, SWAT-HS predicted streamflow very well with a daily Nash Sutcliffe Efficiency (NSE) of 0.85 at the calibrated outlet and values ranging from 0.56 to 0.78 at six other locations within the watershed. Moreover, the monthly predictions of soluble P (total dissolved P), particulate P, and sediment (total suspended solids) were good with NSE of over 0.73. Model simulations indicated that the dominant source of soluble P was pastures while particulate P originated from both from croplands and pastures. A significant quantity of P was derived from near-stream areas, particularly from pastures where cattle spent time grazing and had access to streams. SWAT-HS was also used to estimate what the P export would have been over this 12-year period without WPP implementation. Point and non-point source programs were found to be important for P control, with non-point source controls most effective during high streamflow, and point source controls more beneficial at low flow.