Monitoring Phosphorus During High Flows: Critical for Implementing Surrogacy Models
2025
Elliot S. Anderson | Keith E. Schilling | Larry J. Weber
Phosphorus (P) is a problematic waterborne pollutant, and considerable efforts have been taken to monitor its presence and transport in locales struggling with eutrophication. Most historical P datasets consist of intermittent grab samples, necessitating the construction of surrogacy models to explore P at high resolutions. In Iowa, models using historical data to relate turbidity to particulate P (PartP) have successfully been created. However, it is unknown how comprehensively historical datasets reflect Iowa&rsquo:s hydrologic conditions and how well these models perform during flows not well represented within the existing data. In this study, we analyzed historical P datasets from 16 major Iowa rivers to determine how well they captured the rivers&rsquo: full range of streamflow conditions. While these datasets contained sufficient samples during low and average flows, they typically under-sampled high flows&mdash:containing few values above the 85&ndash:95th percentiles. Therefore, we collected new data in each river during wet conditions, with ~300 samples taken from 2021 to 2024. These new sampling results largely aligned with the existing surrogacy models and slightly improved model performance, suggesting that utilizing turbidity to predict PartP is appropriate in nearly all streamflow conditions. These findings may prove consequential for robustly modeling PartP due to its dynamic nature and disproportionately high transport during wet weather events.
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