Water quality in reference watersheds in the United States: A compilation and analysis of small watershed data

The natural background contribution from grasslands and forest lands is important to consider in research and management to address the contribution of agricultural, industrial, and urban lands to water quality degradation. To our knowledge, no study has compiled and analyzed reference water quality from small reference grasslands and forests even though land use export coefficients for background water quality are assigned at that scale in decision support tools and models, total maximum daily load projects, and comparative analysis. Thus, our major objective was to summarize nitrogen (N), phosphorus (P), and sediment loads in runoff from grassland and forested reference watersheds. Measured annual nutrient loads were available from 13 grassland and nine forest reference sites in 12 North American Level II ecoregions. The grassland reference sites were relatively arid with annual runoff <353 mm (average runoff coefficient = 0.11), and forest reference sites were humid with runoff ranging from 108 to 1274 mm (average runoff coefficient = 0.34). Grassland reference watersheds tended to have higher annual sediments loads (>300 kg/ha), while forest reference watersheds tended to have higher dissolved N loads. This research provides valuable summary results and initial comparisons related to reference water quality across the United States that can serve as a benchmark to compare how anthropogenic activities affect this vital ecosystem service.