Back to the Future: Impact of Current Versus Historical Land Use on Water Quality Trends Across the Contiguous US

Anthropogenic nitrate (N) pollution is degrading water quality, changing fresh and saline ecosystem productivity, and threatening drinking water sources. Policy goals have been set to mitigate water quality by a variety of regional working groups, including the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force and the Chesapeake Bay Program, and millions of dollars have been spent to implement conservation measures. Despite such efforts, we have little understanding of how changes in management controls are impacting water quality at the watershed scale, and of the various landscape and climate controls that can mediate responses to changes in management. In the present work, we have synthesized data from over 200 watersheds across the U.S. using a novel feature-based clustering approach to group watersheds based on the features of their timeseries, including change points, magnitude of concentrations and rate of change. Watersheds are separated based on similar temporal trajectories within the last four decades which allowed for the analysis of temporal and spatial trends in N dynamics. Pairing the data with historical land-use trajectories highlights how current versus historical land use impacts water quality. The decadal trajectories of N concentrations are in many cases a consequence of the past land-use trajectory, and thus vary widely across the contiguous U.S. Changes in N concentrations over the decades can have important implications with regard to stream productivity and nutrient dynamics in downstream water bodies. An analysis of the water quality trends across the U.S. provides insight into how N levels have changed over the last 30-40 years, as well as where changes in management practices are resulting in measurable decreases in N concentrations. Mapping such progress on a large spatial scale can help to identify areas in which there are significant time lags in water quality response, and can be used to guide the setting of new water quality goals across the US.