Incorporating 30 years of US EPA’s National Nutrient Inventory into Nationally Consistent and Spatially Explicit Catchment and Watershed Metrics
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Watershed nutrient metrics are crucial components of state-led efforts to improve nutrient reduction strategies, however, they are challenging to assemble across the scales management and research require. Therefore, we downscaled and dasymetrically allocated the EPA’s NextGen National Nutrient Inventory (NNI) to over 2.4 million subcatchments (NHDPlus V2.1) and integrated these estimates into EPA’s StreamCat database across the contiguous United States (CONUS). With the incorporation of NNI into StreamCat, we generated over 3.5 billion pieces of data, comprising three decades of nutrient estimates, resulting in a novel and robust resource to explore nutrient inputs. The new metrics cover all major sources of urban, atmospheric, wastewater, and agricultural nutrient inputs, including nitrogen and phosphorus metrics from livestock manure excretion, crop removal, fertilizer, point source loads, and atmospheric deposition. These metrics represent the highest resolution of nutrient data available across CONUS. Importantly, the watershed accumulated statistics allow seamless generation of mass balance time series that can be used to calibrate predictive models of nutrient pollution to surface and groundwater and track progress in controlling nutrient pollution sources in watersheds of interest. We will present example analyses of nutrient trends in NHDPlus V2.1 subcatchments and cumulative watersheds for surface and groundwater impaired areas. The datasets in StreamCat will be available through StreamCat’s existing Application Programming Interface and eventually accompanied by visualization functions in EPA’s StreamCatTools R package. These data provide new state-of-the-science nutrient information directly to managers and researchers aiming to understand drivers and develop solutions for improving water quality.