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

Authors: Harmel, Robert; AMATYA, DEVENDRA - Us Forest Service (FS); SEBESTYEN, STEPHEN - Us Forest Service (FS); SUN, GE - Us Forest Service (FS); Mott, Joshua; Schantz, Merilynn; BOGGS, JOHNNY - Us Forest Service (FS); CALDWELL, PETER - Us Forest Service (FS); CAMPBELL, JOHN - Us Forest Service (FS); HIRD, AARON - Natural Resources Conservation Service (NRCS, USDA); KEPPELER, ELIZABETH - Us Forest Service (FS); RAU, BEN - Us Forest Service (FS); JOHNSON, SHERRI - Us Forest Service (FS); Smith, Douglas

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/6/2026
Publication Date: 4/29/2026
Citation: Harmel, R.D., Amatya, D., Sebestyen, S., Sun, G., Mott, J.D., Schantz, M.C., Boggs, J., Caldwell, P., Campbell, J., Hird, A., Keppeler, E.T., Rau, B., Johnson, S.L., Smith, D.R. 2026. Water quality in reference watersheds in the United States: A compilation and analysis of small watershed data. Journal of Environmental Quality. 55(3). Article e70176. https://doi.org/10.1002/jeq2.70176.
DOI: https://doi.org/10.1002/jeq2.70176

Interpretive Summary: 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, TMDL 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 compiled from 13 grassland and 9 forest reference sites across 12 North American Level II ecoregions. The grassland reference sites were relatively arid with mean annual runoff below 353 mm, and forest reference sites were humid with runoff ranging from 108-1274 mm. Grassland reference watersheds tended to have higher annual sediments loads, while forest reference watersheds tended to have higher N and P loads. This research provides valuable summary results and initial comparisons related to reference water quality across the US that can serve as a benchmark to compare how human activities affect this vital ecosystem service.

Technical Abstract: 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, TMDL 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 compiled from 13 grassland and 9 forest reference sites across 12 North American Level II ecoregions. The grassland reference sites were relatively arid with annual runoff < 353 mm (avg. runoff coefficient = 0.11), and forest reference sites were humid with runoff ranging from 108-1274 mm (avg. 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 N and P loads. This research provides valuable summary results and initial comparisons related to reference water quality across the US that can serve as a benchmark to compare how anthropogenic activities affect this vital ecosystem service.