We present a conceptual model and analytical approach for attributing water-quality (WQ) trends to two components of change: systematic changes in the streamflow regime (such as increases in low or high flows over time, or changes in timing of these flows) and changes in watershed management (changes in point- and non-point sources, or other land-surface disturbances). Using a multi-agency WQ and streamflow dataset containing 15 WQ parameters and 370 sites across the contiguous United States, we found changes in watershed management were responsible for most of the WQ changes between 1982 and 2012. However, concentration and load trends at 25% and 66% of sites, respectively, had non-negligible (&gt;7.5%) contributions from changes in streamflow. In these cases, changes in streamflow either enhanced changes in watershed management leading to a larger change in WQ than if the effects of streamflow were not considered, or opposed changes in watershed management leading to a smaller change in WQ. Changes in streamflow had larger effects on load trends compared to concentration trends, and on major ion, salinity and sediment trends compared to nutrients. Overall, the effects of changes in streamflow on WQ and how these effects interact with changes in watershed management is not as straightforward as is often assumed by the use of linear relationships. The findings of this work have important implications for analyses exploring the impact on WQ and ecology from anticipated future changes in streamflow due to factors such as water-use or climate change, and for making causal inferences about the major drivers of WQ change.

Exploring the effects of streamflow trends and watershed management on changing water quality: National perspective

2019 Fall National Meeting

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