Nitrogen loads, yields, and concentrations were predicted for 1:100,000-scale reach-based watersheds throughout New England. Presented in this dataset are the incremental loads and yields generated within the incremental watershed or catchment of the nutrient load’s origin, predicted by the New England SPARROW nitrogen model. (Catchments are the incremental watersheds or areas draining directly into a stream reach without first flowing into an upstream reach.) The U.S. Geological Survey (USGS), in cooperation with the U.S. Environmental Protection Agency (USEPA) and the New England Interstate Water Pollution Control Commission (NEIWPCC), has developed a water-quality model, called SPARROW (Spatially Referenced Regressions on Watershed Attributes), to assist in regional total maximum daily load (TMDL) and nutrient-criteria activities in New England. SPARROW is a spatially detailed, statistical model that uses regression equations to relate total nitrogen and phosphorus (nutrient) stream loads to nutrient sources and watershed characteristics. These statistical relations are then used to predict nutrient loads in unmonitored streams. The New England SPARROW models are built using a hydrologic network of 42,000 stream reaches and associated watersheds called catchments. Catchment boundaries are defined for each stream reach in the network through the use of a digital elevation model and existing digitized watershed divides.
The New England SPARROW models for total nitrogen and total phosphorus have R-squared values of 0.95 and 0.94, with mean square errors of 0.16 and 0.23, respectively. Variables that are statistically significant in the total nitrogen model include permitted municipal wastewater discharges, atmospheric deposition, cultivated agricultural area, and developed land area. Total nitrogen in-stream loss rates are significant only in streams with average annual flows less than or equal to 2.83 cubic meters per second. In streams larger than this, there is no statistically significant in-stream loss of total nitrogen in New England on an annual basis. Variables that are statistically significant in the total phosphorus model include discharges from municipal wastewater-treatment and pulp and paper facilities, developed land area, agricultural area, and forested area. For total phosphorus, in-stream loss rates are significant for reservoirs with surface areas of 10 square kilometers or less, and in streams with flows less than or equal to 2.83 cubic meters per second.
For each of the 42,000 NHD stream reaches, the New England SPARROW model results provide estimates and confidence intervals for phosphorus and nitrogen loads, area-weighted yields of nutrients, flow-weighted mean annual concentrations of nutrients, sources of nutrients, and information on the downstream movement of nutrients.