Seasonally dynamic watershed storage lags and river corridor reactivity across basins

Abstract:

Estimation of nutrient sources and their delivery efficiencies from headwaters to estuaries requires integrated modeling at the scale of major river basins. Operating at basin scales has posed challenges related to representing the effects of dynamic reactivity, storage accumulation, and storage depletion, which are fundamental controls on hypoxia and harmful algal bloom occurrences. We have reconceptualized integrated models to capture the seasonal fluctuations in nutrient reactivity and storage with a flexible calibration framework that draws on rich dynamic datasets to improve multiscale predictions from watersheds to major river basins. Nitrogen and phosphorus models of the Northeastern U.S. are presented as test cases for the potential development of continental-scale predictions of seasonal loads and concentrations. We found that nutrient loads exiting watersheds contain a significant portion of stored mass with greater releases during shifts from drier to wetter periods while river corridors are reactive nutrient sinks during warmer, lower-flow periods. These findings implicate that current water quality is largely driven by previous season climate conditions and management decisions, setting the stage for broader advancements in predicting the role of nutrient legacies in meeting water quality targets.