Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/27/2007
Publication Date: 11/1/2008
Citation: Tomer, M.D., Moorman, T.B., Rossi, C.G. 2008. Assessment of the Iowa River's South Fork Watershed: Part 1. Water Quality. Journal of Soil and Water Conservation. 63(6):360-370. Interpretive Summary: Interactions between hydrology and water quality help determine the effectiveness of conservation practices in agricultural watersheds. We monitored stream discharge and amounts of nitrate, phosphorus, and E.coli in stream waters of the Iowa River's South Fork watershed, one of the ARS Watershed Assessment Studies under the Conservation Effects Assessment Project (CEAP). We found significant quantities of all three contaminants, with nitrate losses averaging >20 kg N/ha.yr, P concentrations frequently exceeding ecological thresholds, and E. coli counts frequently exceeding recommended values for contact recreation during summer. Comparison of nitrate concentrations obtained by two sampling methods provided validation statistics that could provide performance benchmarks for validation of watershed model simulations. Statistical relationships between baseflow discharge and nitrate indicate that tile drainage is the dominant loss pathway. However phosphorus concentrations showed sampling method had a large effect on sample concentrations, and relationships with hydrology were weaker. Patterns of E.coli among sub-basins defied any expectations based on distribution of confined livestock operations, indicating cattle, wildlife, and/or humans are also sources. Seasonal temperature effects and surface runoff accounted for half the variation in E. coli. Seasonal patterns varied among these contaminants highlighting the complexity of integrated water quality assessments. Results are of interest to watershed modelers, and conservation practitioners and policy makers involved in CEAP or other watershed assessment studies.
Technical Abstract: The South Fork of the Iowa River drains 78,000 ha under intensive, high-production agriculture. About 100 swine CAFOs generate manure applied onto about a quarter of the watershed annually. Hydric soils cover 54% of the watershed because the young glacial terrain is poorly dissected. Artificial subsurface drainage is extensive and SWAT modeling indicates that this dominates the hydrology. Water quality monitoring data show spring and early summer NO3-N concentrations often exceed 20 mg L-1. Loads during 2002-2005 were 16-26 kg NO3-N ha-1yr-1, but only 0.4-0.7 kg P ha-1yr-1. Nitrate concentrations were linearly related to log (baseflow), effectively a surrogate measure of tile discharge, with R2 varying between 0.67 and 0.85 among four gauging stations. Comparison of NO3-N concentrations obtained by grab and automated methods yielded an R2 of 0.94, RMSE of 2.7 mg L-1, and modeling efficiency of 0.88. Such comparisons may provide benchmark performance targets for watershed modeling studies. Mean E. coli populations in the stream exceed 500 cells 100 mL-1 during summer. Subsurface drainage plays an important role in the transport of nitrate, but is less important in direct transport of E. coli and phosphorus. The E. coli and P concentrations are governed by seasonal trends and runoff events. Results highlight that water quality in this intensively managed agricultural watershed results from a suite of processes, all varying temporally and with the contaminant of concern.