Investigation and modeling of land use effects on water quality in two NYC water supply streams.

The paired watershed monitoring approach is widely used to investigate hydrologic processes and water quality, providing streamflow and water quality records for long-term trend analysis, as well as data for developing and testing hydrologic models. In this study we use 20 years of streamflow and water quality data, along with a watershed model, to examine sources of stream nutrients and their changes over time in two small streams within the New York City water supply system. We compare sources and trends in stream nitrate and dissolved phosphorus in the urbanized Amawalk watershed with those of the predominantly forested Boyd Corners watershed in the Croton system of reservoirs. Stream monitoring data reveal a decreasing trend in nitrate in both watersheds, whereas dissolved phosphorus shows a decreasing trend only in the Amawalk watershed. The decline in nitrate corresponds to decreases in atmospheric deposition of nitrogen in the region, whereas the decrease in dissolved phosphorus in the urban watershed is attributed to upgrades to wastewater treatment plants and regulations on the use of phosphorus-containing lawn fertilizers. Simulations using calibrated Soil and Water Assessment Tool (SWAT) watershed model indicate that urban land use and wastewater sources dominate nutrient loads in the Amawalk watershed, contributing 55-80% and 9-27% of the annual loads, respectively, while forested areas contribute as much as 80% of nutrients in the Boyd Corners watershed. Furthermore, we find that more than 80% of the input nitrogen from atmospheric deposition and fertilizer sources is retained in these watersheds. This study demonstrates the value of combining long-term monitoring data with watershed models to generate valuable information that could support future watershed management efforts under changing environmental conditions.