Evaluating the relation of phosphorus to turbidity during high flow events in western Iowa rivers to improve phosphorus load estimates

Sep 2022


The presence of phosphorus (P) in waterways is a major concern in the state of Iowa. Although this nutrient is essential for biological processes, excessive quantities sourced back to anthropogenic origins disrupt the ecological balance of both fresh and marine aquatic systems. Human activity has historically elevated the amount of P discharged into Iowa’s waterbodies. The state has set targets to lower this P load via the Iowa Nutrient Reduction Strategy (INRS), and a key benchmark of the INRS is the reduction of annual P load exported by Iowa’s major rivers.

Quantifying P loads in rivers remains a challenge, with estimations subject to several potential sources of error and uncertainty. Turbidity has shown promise as a surrogate for particulate phosphorus (Part P) concentrations in rivers. While monthly sampled datasets are large, peak flow conditions have not been not well represented, and this lack of data at higher flows is a shortcoming in applying turbidity as a surrogate for Part P. Additional data are needed above to better characterize the surrogacy relationship during high flows, which is essential for quantifying annual P export.

A previous, related project proved highly successful, with measurements from nine sites and over 100 new samples collected. Incorporating these new data alongside the existing monthly samples both improved the surrogacy models and validated their use during peak flows in eastern Iowa. While this project made great strides in quantifying P export, the scope was limited to eastern Iowa. The western Iowa watersheds possess a separate set of geologic and land use conditions. Similar methods will be applied to seven sites in western Iowa to expand our approach and address the estimation of Part P concentrations in these watersheds during high flows.


This project’s primary goal is to gather new data supporting the improvement of surrogacy models that predict particulate P concentrations using turbidity. The data will be collected during high flow conditions at several locations on four western Iowa rivers adjacent to existing IDNR monitoring sites and USGS stream gauges, where historical data has been lacking. Specific objectives are to:

  1. collect water samples at western Iowa’s seven terminal monitoring sites when flows exceed the predetermined thresholds;
  2. measure the Part P and turbidity levels within these samples; and
  3. compare these new data points to the established surrogacy models at each site.


Researchers plan to replicate methods that yielded success in a related, INRC-funded project in eastern Iowa. The equipment purchased as part of the previous project can be redeployed in western Iowa. Since travel times between the University of Iowa and the western watersheds are significant, local collaborators will assist in sample collection. Each sample will be analyzed by the Iowa State Hygienic Laboratory to determine its turbidity, total phosphorus (TP), and orthophosphate (OP) levels. TP and OP concentrations are needed to calculate Part P, which is the difference between TP and OP.

The resulting models will enable a scientifically defensible method for estimating Part P loads exported from the seven terminal sites in western Iowa.

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