Influences of Channel Processes On Phosphorus Export From the New York City Water Supply Cannonsville Watershed.

Watershed water quality models are useful to evaluate and support nutrient management planning in agricultural watersheds. Model applications for nutrient management typically focus on controlling nutrient sources and transport to the stream channel. However, stream processes that involve nutrient release and retention may control the timing and quantity of export at the watershed outlet. The importance of these processes has received increased attention in scientific community. In this study we investigate the significance of stream channel processes in regulating phosphorus (P) loading to the NYC Water Supply Cannonsville Reservoir. Watershed P processes can include mobilization and transport of non point watershed P sources from land to the channel, and remobilization of transient stores of P from stream beds.  A mass balance based comparison of estimated P inputs to the stream channel with observed P export at the watershed outlet was done to quantify P delivery and explore the non-conservative behavior of P.  Stream channel transport of both dissolved and particulate P is found to be non-conservative, with dissolved P tending to be retained during low flows and particulate P released during high flows. The results suggest that differences in the magnitude and relative importance of in-stream biogeochemical processes under different flow regimes regulate P delivery in ways that may influence ecological impacts to downstream river reaches and reservoirs. These results are useful for quantifying the effects of nutrient management on watershed loading and for improving parameterization of water quality models.