How did NAWQA determine regional background nutrient concentrations to compare with regional nutrient criteria?
A simulation model (USGS SPARROW model) was used to estimate background concentrations of total nitrogen and total phosphorus for all stream reaches in the conterminous U.S. (See Smith and others, 2003 for details http://water.usgs.gov/nawqa/sparrow/intro/es&t.pdf). From these estimates, the average background concentration for each of the 14 U.S. Environmental Protection Agency’s nutrient ecoregions was computed.
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