In 2016 the Iowa Watershed Approach (IWA) was created to increase community resiliency against flooding, to develop hydrologic assessments that would identify strategies to reduce flooding, and to implement those strategies within nine identified watersheds that experienced flooding between 2011 and 2013. One of the nine watersheds was the Upper Iowa watershed located in northeast Iowa. This thesis focuses on the work that has been done to create a hydrologic assessment of the Upper Iowa watershed. The hydrologic assessment identifies potential conservation practices, creates a hydrologic model to assess the hydrologic cycle over the past ten years, and identifies strategies to reduce flooding within the watershed.
Many potential agricultural conservation practices within the Upper Iowa watershed were identified and trends relating to the soil, land use, and topography were determined. In addition, a methodology to compare potential conservation practices with existing conservation practices actually in place was developed including a tool to estimate the size of grassed waterways to NRCS design guidelines. The comparison validated the methodologies used to identify potential practice placements, identified locations where potential practices could be implemented, and showed how stakeholder preferences influence conservation implementation.
Additionally, a hydrologic model of the Upper Iowa watershed was developed, using the new Generic Hydrologic Overland-Subsurface Toolset model and calibrated to simulate the time period of 2007 through 2016. The model was evaluated against water balance ratios and performance statistics calculated from measured data. The model achieved Nash Sutcliffe Efficiency scores for streamflow above 0.7 and percent bias scores between ±12% for the three wettest years of 2008, 2013, and 2016. With the calibrated model, the benefits of continuous cover crop implementation were investigated under current conditions and under increased extreme precipitation intensity expected from climate change over the next half century. The results of this investigation determined that continuous cover crops increased evapotranspiration within the early half of the year creating more storage within the soil. Thus the flood risk from convective storms during the summer was lowered. In addition, the benefits from cover crops in terms of peak flow and volume reductions were cumulative increasing each consecutive year and were proportional to the percentage of cover cropped area. Lastly, a scenario using cover crops in a future extreme precipitation environment resulted in a reduction of peak discharge to current conditions. The results of this thesis will guide both future work within the Upper Iowa watershed and contribute to the knowledge of hydrologic planning and modeling within agricultural watersheds.