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Exploring Agricultural Drainage's Influence on Wetland and Watershed Connectivity
Christensen, J. Exploring Agricultural Drainage's Influence on Wetland and Watershed Connectivity. US-IALE 2017, Baltimore, MD, April 09 - 13, 2017.
Presentation for IALE on agricultural drainage estimation 2017
Artificial agricultural drainage (i.e. surface ditches or subsurface tile) is an important agricultural management tool. Artificial drainage allows for timely fieldwork and adequate root aeration, resulting in greater crop yields for farmers. This practice is widespread throughout many regions of the United States and the network of artificial drainage is especially extensive in flat, poorly-drained regions like the glaciated Midwest. While beneficial for crop yields, agricultural drains often empty into streams within the natural drainage system. The increased network connectivity may lead to greater contributing area for watersheds, altered hydrology and increased conveyance of pollutants into natural water bodies. While studies and models at broader scales have implicated artificial drainage as an important driver of hydrological shifts and eutrophication, the actual spatial extent of artificial drainage is poorly known. Consequently, metrics of wetland and watershed connectivity within agricultural regions often fail to explicitly include artificial drainage. We use recent agricultural census data, soil drainage data, and land cover data to create estimates of potential agricultural drainage across the United States. We estimate that agricultural drainage in the US is greater than 31 million hectares and is concentrated in the upper Midwest Corn Belt, covering greater than 50% of available land for 114 counties. Estimated drainage values for numerous counties indicate that even moderately well drained soils are subject to artificial drainage. Agricultural drainage influences measures of hydrologic and biological connectivity when considering individual wetland metrics and watershed metrics. When exploring selected regions in the Corn Belt where subsurface tile drainage predominates, artificial drainage is negatively correlated with wetland density, and remaining wetlands have higher distances to nearest neighbors which has consequences for biological connectivity. Meanwhile, artificial drainage increases the contributing area for watersheds potentially influencing the timing of peak flows, base flows and pollutant loads. In connecting more historically isolated uplands and wetlands to the natural drainage network, artificial drainage illustrates the importance of both connectivity and disconnectivity to the integrity of healthy watersheds.