Quantification of Nutrient Reduction Practices Benefits from the Hillslope to the Watershed Scale
The Iowa Nutrient Reduction Strategy identifies the nutrient reduction potential of several Best Management Practices (BMPs) based on studies conducted, for the most part, at the plot scale. Field-scale management practices exist for reducing nutrient loads in agricultural runoff, but there is a lack of data and tools available to assess how individual and bundled management practices upscale to large watersheds.
This project involves developing a numerical tool that will quantify the benefits of soluble nutrient reduction strategies at multiple scales, from the field scale to the larger watershed. The model results will provide a science-based tool to assess the benefits of implementing a variety of BMPs.
The study area comprises three watersheds that drain into the Skunk River in southeast Iowa. A model of the study area will be created that accounts for interactions between surface and subsurface flows. A USGS streamflow station has been collecting real-time data in the area since October 2007. A water quality station will be installed at the same location to allow for estimation of loads and model performance. BMP implementation scenarios will be defined in collaboration with watershed stakeholders
FINAL REPORT: There were two objectives of this project. One was to develop a physically based coupled surface-subsurface hydrologic model of Cedar Creek, which reproduces measured discharge at the watershed outlet. The second objective was to develop a water quality model of Cedar Creek, which reproduces nitrate concentration dynamics. Both objectives were reached. The results of this project will guide future physically based hydrologic and water quality modeling in agricultural watersheds, and serve as a demonstration of the ways to simulate nutrient transport within the landscape. This will make it possible to quantify the benefits of best management practices implementation scenarios.
During this quarter, progress was made on improving the hydrologic model calibration and analyzing the predictive skill of the model. The analysis involved calculating two statistical coefficients, then matching those with USGS measured streamflow from 2007 through 2014. Results demonstrate the model has skill in predicting streamflow for average hydrologic years such as 2014, but struggles to predict streamflow in wet years (2010) and dry years (2012). Focus now shifts to the addition of the fate and transport model. This component of the hydrological model will use mathematical representations of various chemical processes to track where nutrients are coming from within the watershed, and how much are leaving. A new water quality monitor was installed in 2016 midway through this watershed. Real-time data collected by this device will be used to calibrate the nutrient model.
This quarter, progress was made on developing scenarios that investigate how certain practices can improve water quality and reduce flooding, while improving the model calibration. The scenarios include testing the 5%, 25%, 50%, 75%, and 100% adoption of cover crops on agricultural land, as well as testing the implementation of ponds and wetlands in varying size and number. Implementing cover crops, ponds, and wetlands will affect the hydrology in a way that is known to reduce peak flows during storm events, and reduce the nutrient load leaving the watershed. However, the degree to which these reductions are realized varies due to the physical properties of the watershed. The model will be used to estimate these reductions and better understand the impact of any desired changes in the Cedar Creek watershed. Improving the model calibration is an important, ongoing task.
This project will develop a physically based hydrologic model of the Cedar Creek Watershed to use in quantifying the benefits of different practices in reducing the nutrient load leaving the watershed. The modeling is conducted using MIKE-SHE, a physically based, surface-subsurface hydrologic modeling software. Inputs for the model are collected from publicly available data and characteristics of the watershed, such as surface roughness and infiltration properties. Work is underway to develop scenarios that investigate how certain practices can improve water quality and reduce flooding. This study will focus on the implementation of retention ponds, wetlands and cover crops.
During this quarter, the subsurface model was refined and the model calibrated to achieve certain hydrologic water balance targets. For the subsurface model, a bedrock layer was implemented using data from the Iowa Geological Survey to create a more realistic model and to improve routing of groundwater to the river network. Soil and evapotranspiration parameters were calibrated to ensure the annual amount of base flow, evaporation and transpiration were within an expected range. The computational cell size resolution was increased in the unsaturated zone to calibrate soil evaporation volume. Subsurface drainage and macropores, both of which likely occur in the watershed, were added to the model to improve base flow contribution.
Work is underway on a hydrologic model for the Cedar Creek watershed near Fairfield. The goal of the model is to quantify the impact certain nutrient reduction practices could have in reducing the nitrate load leaving the watershed. The focus is on gathering all necessary inputs for the model, which include elevation, land use, soil type, river network, climate and geology. Calibration of the nutrient model will use data collected at a real-time water quality station within the watershed.