Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 30, 2007
Publication Date: November 1, 2008
Citation: Tomer, M.D., Moorman, T.B., James, D.E., Hadish, G., Rossi, C.G. 2008. Assessment of the Iowa River's South Fork Watershed: Part 2. Conservation Practices. Journal of Soil and Water Conservation. 63(6):371-379. Interpretive Summary: The effectiveness of conservation practices in agricultural watersheds depends on the types and extent of practices implemented. An inventory of conservation practices and land use was conducted for the Iowa River's South Fork watershed in 2005, under a Watershed Assessment Study of the Conservation Effects Assessment Project (CEAP). The watershed is characterized by intensive agricultural production, with about 85% of the land in corn and soybean rotations and more than 100 livestock production facilities, most producing swine. Results indicated that erosion control practices were installed on 90% of land subject to USDA Conservation Compliance, i.e. field with >33% highly erodible land (HEL). Inadequate residue cover (<30%) dominantly followed soybean, regardless of proximity to livestock facilities and expected fertilizer source. However, maintaining residue when applying manure after soybean is a key management challenge if receiving soils are accumulating P. Targeting of conservation practices near streams was not apparent. But fields with >33% HEL that were near streams had increased frequency of multiple practices and rotations including third crops, compared to watershed averages. The inventory’s results, considered with water quality issues identified in a companion paper, show most practices are aimed at controlling runoff, whereas tile drainage is the dominant hydrologic pathway. Implications for future conservation programs in tile-drained areas are apparent. For example, nutrient removal wetlands, now uncommon, would help remove nitrate discharged via artificial drainage. But ongoing impacts of past erosion/sedimentation and implications for stream corridor management need to be better resolved. Results are of interest to watershed modelers, and conservation practitioners and policy makers involved in CEAP or other watershed assessment studies.
Technical Abstract: Assessments of conservation practice effects on water quality in watersheds require the types and extent of practices implemented to be known. This paper assesses results from a conservation-practices inventory for the South Fork of the Iowa River (78,000 ha), conducted in 2005. Agricultural management systems in the watershed were determined using maps of crop cover and livestock facilities. About 85% of the watershed is in corn and soybean rotations, with only 7% of cropland in no-tillage. Based on the extent of livestock (swine) facilities and expected manure amounts and characteristics, about 30% of the watershed’s cropland receives manure annually, in rotation prior to corn. Inventory results showed inadequate residue (<30% cover) dominantly followed soybean; maintaining residue during manure application is therefore a key management challenge. The conservation inventory further showed 90% of fields with >34% highly erodible land, subject to USDA conservation compliance, do indeed have erosion-control practices installed. Grassed waterways and riparian buffers are common edge-of-field practices. While targeting (preferential installation) of conservation practices near streams (within 200 m) was not apparent, >34%-HEL fields that were near streams had increased frequency of multiple practices and rotations including third crops compared to watershed averages. The inventory’s results, considered with water quality issues identified in a companion paper, show most conservation practices are aimed at controlling runoff, whereas tile drainage is the dominant hydrologic pathway. Implications for future conservation programs in tile-drained areas are apparent. For example, nutrient removal wetlands, now uncommon, would help remove nitrate discharged via artificial drainage.