Historical streamflow water quality and flow data – gathered over eighty years by the U.S. Geological Survey – offer critical insights into how watersheds respond to a range of climatic disturbances. Water quality is an emerging issue in an era of global change, often reflecting changes in the watershed from hydrology, biogeochemistry, and microbial-biological couplings that control nutrient and solute cycling. In this study, we present a statistical analysis of water quality parameters across U.S. watersheds and explore factors that influence those trends. We first documented the temporal trends of nitrogen, organic carbon, dissolved oxygen, phosphorous, and temperature across 700 U.S. watersheds. Results show that there are different trends in water quality across these watersheds, but a majority show systematic declines in nutrient fluxes despite large variability in discharge trends. As documented by many studies, hydrological trends alone have failed to explain systematic and often diverging trends of stream nutrient fluxes and solute concentrations. In this study, we consider a range of factors that could contribute to the water quality trends, including changes in vegetation, atmospheric deposition, and hydrology. We used time-lapse remote sensing Normalized Differenced Vegetation Index (NDVI) and the National Atmospheric Deposition Program (NADP) to estimate temporal trends in vegetation and atmospheric deposition, respectively. We found that water quality flux trends are more often driven by changes in concentration rather than changes in flow discharge, which is an important indicator of the tight coupling to watershed biogeochemical cycling as a control on concentrations. Additionally, we found high elevation watersheds are exporting the majority of nutrients and solutes earlier in the water year (by approximately 80 days) which is an indicator of the lengthening asynchrony with the growing season.

Evolution and mechanisms driving water quality trends across United States watersheds

2019 Fall National Meeting

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