Submitted to: USDA-CSREES National Water Quality Conference
Publication Type: Proceedings
Publication Acceptance Date: 2/9/2006
Publication Date: 2/9/2007
Citation: Tomer, M.D., Moorman, T.B., James, D.E., Green, C.H., Burkart, M.R. 2007. Water quality and conservation practices in the Iowa River's South Fork Watershed. Proceedings of USDA-CSREES National Water Quality Conference. p.17. Interpretive Summary:
Technical Abstract: The Iowa River's South Fork, one of 12 ARS CEAP watersheds, drains 87,000 ha (218,000 ac) in Iowa. The area is intensively managed for high-production agriculture. About 85% of the area is under corn-soybean rotations. There are about 100 confined feeding operations generating manure that is applied before corn, onto about a quarter of the watershed annually. Artificial drainage is extensive, and hydric soils occupy 54% of the watershed. Monitoring of discharge quantity and quality is conducted at four stream-gauging stations and two tile-gauging stations; grab sampling at 15 additional sites supplements the water quality data. Nutrients, sediment, and pathogen indicators (E. coli) are monitored. Objectives are to determine spatial and temporal patterns of pollutants, and provide a database to support model validation and evaluation of conservation practices. A conservation- and tillage-practices survey was conducted by NRCS in 2005, which is being combined with NASS crop cover imagery to identify dominant management systems. Water quality results show NO3-N concentrations often exceed 20 mg L-1 during spring and early summer. Phosphorus and sediment concentrations increase during runoff events. Loads during 2002-2004 were 16-26 kg NO3-N ha-1yr-1 and 0.4-0.7 kg P ha-1yr-1. Contaminant hydrographs show NO3-N delivery lags sediment and P during events. Median E. coli populations in streams exceed the recreational-contact standard of 125 cells/100 ml. There is seasonal variation to these populations that follow temperature, but increases also occur during runoff events. Multiple sources are suspected, including resident populations in streambed sediments. About 10% of the agricultural land is in no- or strip-tillage. On remaining fields, adequate residue cover was usually found following corn, with poor residue cover (<30%) usually after soybeans. Given these early results, practices to reduce N loss and options to manage risk of runoff when applying manure following soybean are key areas to encourage new practices.