Quantifying Phosphorus Balances and Water Quality Trends in the Mississippi River, 1975 - 2017
Abstract
Year-to-year variability in the watershed-scale mass-balance of water-quality constituents is a key driver of fluvial exports. Here, we consider the balance between total phosphorus (TP) inputs (fertilizer, manure, deposition, wastewater, and weathering) versus outputs (nutrient uptake and removal in harvest) in the Mississippi River Basin. We developed a multiple linear regression model to predict changes in river TP loads over time, considering legacy sources (lagged prior-year balances), current sources (current-year balances), and a latent variable representing unmeasured latent variables. We used the fitted model to develop counterfactual scenarios and compared observed to predicted river loads that would have occurred in the hypothetical absence of selected causal factors. The rate of increase in P balances slowed from 1985 onward, but river TP loads did not decrease over time. In fact, the river load response to watershed P balances changed between 1975 and 2017; annual P balances and river loads were positively correlated between 1975 to 1985, but after that P balances and river loads decoupled, suggesting that higher river TP loads may be due to one or more unmeasured factors. The effect of the latent variable changed from decreasing TP loads to increasing TP loads over the study period. The analysis suggested that unmeasured factors were more important than P balances in explaining changes in river TP loads over time. We explored this possibility qualitatively, considering the potential effects of singular or combined changes in nutrient transport due to a variety of BMPs, watershed buffering capacity, tillage practices, tile drainage, or increased precipitation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFMGC12C..07S