New applications of STEPL watershed modeling of CAFO feedlot runoff and implications of surface water impacts
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Concentrated Animal Feeding Operation (CAFO) cattle feedlot runoff is hydrologically modeled using the Spreadsheet Tool for Estimating Pollutants Load (STEPL). STEPL estimates the annual nutrient load of nitrogen and phosphorus, among other pollutants, by land use for a watershed. The Dry Creek Watershed Basin of Sioux County, Iowa, is delineated to capture runoff from large CAFO cattle feedlots. Feedlots are classified by digitizing land use data in ArcGIS, in order to estimate pollutant loads from each sub-watershed and determine the nutrient contributions from direct discharges from CAFO feedlots. Five CAFO feedlots are modeled within the Dry Creek watershed. The number of cattle confined at each CAFO ranges from 3,000 to 7,588 per sub-watershed. Cropland area comprises more than 80% of the total land use in all sub-watersheds, with the exception of one. STEPL assumes that cropland has added animal waste from land application of manure. STEPL also assumes feedlots are managed by routine scraping and removal to reduce some of the waste within the feedlots. Despite these assumptions, this modeling shows that CAFO feedlots discharge an estimated 67% to 98% of the nitrogen from the sub-watersheds compared to other land uses. Environmental impacts including dead zones, hypoxia, toxic algal blooms and other harmful aquatic impacts are attributed to excess nutrients. Many hydrological and water quality modeling studies seek to determine the source, transport and fate of nutrients in watersheds to address these environmental concerns. However, national land use data, specifically National Land Cover Dataset (NLCD) data created and managed by U.S Geological Survey does not have feedlots as a land use class (Homer et al., 2014). It would be difficult to determine nutrient loads directly from runoff without adequate land use data to represent feedlot land area. This study estimated the nutrient loads in runoff using CAFO feedlots at a small watershed scale. This method can be further applied to a larger scale to estimate the magnitude of the contribution CAFOs have on watershed contamination.
Table of Contents
Introduction -- Background and literature review -- Study area -- Methodology -- Results -- Conclusions -- Appendix A. U.S. Census Bureau 1990, Census Data for Sioux County, Iowa -- Appendix B. Data for C and P Factors from IDNR -- Appendix C. NLCD Land Cover Classification Legend -- Appendix D. STEPL Results for Watershed 1 through 4