Quantifying the Effects of BMPs on Sediment and Phosphorus Delivery to a Range of Eastern Iowa Rivers
Over the last few decades, improvements in land management and installation of best management practices (BMPs) have significantly reduced field erosion, but water quality benefits have been poorly quantified. The Revised Universal Soil Loss Equation (RUSLE) is among the most widely used empirical models to estimate gross soil erosion at the plot scale, but it does not provide an estimation of the sediment exported from a watershed. Sediment delivery ratios (SDRs) are used to estimate the fraction of gross erosion that is exported from a watershed for a given time. The SDR concept can be expanded to include a Phosphorus Delivery Ratio (PDR) if total gross erosion of soil P is compared to total P export.
The objective of this project is to develop new SDR and PDR relations for the eastern Iowa landform region that accounts for the effects of BMPs and streambank erosion. The project will quantify the effectiveness of the existing BMPs and the future potential to reduce sediment and total P export from agricultural watersheds. Obtaining these results will serve as an important demonstration of the success of BMPs, plus motivation for future implementation.
The team will examine a subset of Iowa watersheds in eastern Iowa where they already have continuous turbidity and P measurements with which to estimate total sediment and P export. The eastern Iowa basins—two Rapid Creek subbasins, Rapid Creek, Clear Creek, and English River—provide a range of watershed sizes that will allow team members to scale the SDR/PDR curves for an eastern Iowa ecoregion. At the end of the one-year project, annual sediment and total P exported from these watersheds from sheet and rill and streambank erosion will be quantified, and new SDRs will be determined that considers the effects of existing BMPs on the landscape and contributions from streambank erosion.
Significant progress was made on this project during the reporting period. Researchers successfully created correlation curves for total suspended solids/turbidity as well as phosphorus/turbidity for Old Man's Creek watershed in eastern Iowa. Turbidity data was compiled for subbasins of the Rapid Creek watershed and for the English River watershed at Kalona for 2017 and 2018. Phosphorus concentrations were modeled for Rapid Creek using loadest based on monthly measurements. Using the new correlation curves and modeled estimates of stream discharge, total suspended solids exported from Rapid Creek (and its two subbasins) and the English River were successfully quantified for 2017 and 2018. Soil erosion was also modeled for both watersheds using an updated RUSLE model that incorporated actual precipitation and management values for the watersheds in 2017 and 2018. RUSLE estimated erosion rates were compared to estimates of TSS export and new sediment delivery ratio values were determined. The new estimates for the two-year study period were 8.0, 2.4, 1.1 and 1.0% for the English River, Rapid Creek, Rapid Creek upstream subbasin and Rapid Creek downstream subbasin, respectively. These estimates are significantly less than previous estimates (old estimates exceeded 20 percent for all watersheds).
In addition, the ACPF toolbox has been further completed for each of the watersheds with comparisons of existing BMPs with potential BMPs, which will allow estimates of further reductions in sediment loss.
Characterization of the study watersheds continued during this project period and researchers continued to compile the field-edge data and land cover information to run new RUSLE soil erosion models. During this period, a model was successfully completed that estimates stream discharge based on stream stage for each watershed. Once new erosion models are finished, new correlation curves for total suspended solids in streams will be created. Researchers are also creating hydrologically corrected digital elevation models (DEMs) for the watersheds of interest and will soon run the Agricultural Conservation Planning Framework (ACPF) tool on these watersheds to relate conservation practices to SDRs.
During this first project period, characterization of the study watersheds commenced. Field-edge data and land cover information is being compiled to run new soil erosion RUSLE models. When the new erosion models are completed, new correlation curves will be created for total suspended solids in streams.