Seasonal and Annual Trends in Dynamic SPARROW Predictions of Nitrogen and Phosphorus in Streams of the Conterminous United States, December 1999 through November 2020

Annual and seasonal trends in nitrogen and phosphorus loads, mean concentrations (flow-weighted loads), mean yields (area-weighted loads), and possible related stresses on aquatic ecology were estimated for the period December 1999 through November 2020 for each of 261,247 individual stream reaches and 81,717 12-digit hydrologic units (HUC12s) in the conterminous United States (CONUS). This data release is intended to allow users to better understand nutrient changes over recent decades in streams over large areas of the CONUS. Total and incremental nitrogen and phosphorus loads in each stream reach and HUC12 during each meteorological season between winter 2000 (December 1999 through February 2000) and autumn (September through November) 2020 were estimated using dynamic spatially referenced regression (SPARROW) models (Miller and others, in press; Schmadel and others, 2026; Schwarz and others, 2026). Seasonal loads from winter through fall of each year were summed to estimate annual loads and the time-weighted mean of mean seasonal flows was computed to estimate mean annual flows. Mean yields and concentrations were estimated as the ratio of loads to watershed area and streamflow, respectively. The number of seasons during each year during which aquatic communities may be stressed by excessive nutrients in at least one stream segment within each reach or HUC12 was estimated by comparing mean concentrations to ecological nutrient criteria (Cashman and others, 2026). Seasonal and annual trends in loads, mean yields, mean concentrations, and ecological criteria exceedances were estimated using Theil-Sen slopes (Helsel and others, 2020) following methods of Stagnitta and Goodling (2025).