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WHAT DOES DRAINAGE DENSITY MEAN IN TILED LANDSCAPES?
Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org
Citation: 2016 10th International Drainage Symposium Conference, 6-9 September 2016, Minneapolis, Minnesota .(doi:10.13031/IDS.20162492895)
Authors: Keith E Schilling, Calvin F Wolter
Keywords: Drainage density, Iowa, stream channels, tile drainage
Abstract. Drainage density has long been considered a fundamental watershed property to describe how well or how poorly a watershed is drained by stream channels. But where do stream channels begin in watersheds that have been hydrologically altered and drained by extensive subsurface tile networks? In this presentation, the evolution of drainage density in two small Iowa watersheds from pre-settlement times to current conditions is discussed. One watershed (Bear Creek) is located in the recently glaciated Des Moines Lobe (DML) region where tile drainage is widespread and intense. In contrast, the Walnut Creek watershed is located in an older glacial landform region (Southern Iowa Drift Plain, SIDP) where tile drainage is concentrated in swales and grass waterways. While the number of streams per watershed area and total stream length is greater in older landscapes (SIDP) compared to younger landscapes (DML), in all regions, drainage density increases with expansion of tile drainage. In Bear Creek, drainage density increased from 0.0006 m-1 based on the currently delineated perennial stream network to 0.04 m-1 if drainage density from tiles is considered. Increasing drainage density reduces groundwater travel times and decreases opportunities for landscape-scale nutrient processing. Tile drainage reduced average groundwater travel times by 50% in Walnut Creek, and in Bear Creek, average groundwater travel times were reduced from approximately 80 years to 1 year with increasing tile drainage density. With expansion of tile drainage networks in many Midwestern agricultural watersheds, the natural evolution of stream channel has been bypassed and the traditional concept of drainage density does not apply. Does drainage density apply to only mappable surface stream systems or should it include the interconnected subsurface drainage network? It is clear that tile drainage is a fundamental watershed property of many agricultural landscapes and efforts should be directed at better characterizing its spatial extent and capacity to assess the contribution of tile drainage to watershed export.
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