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Tile discharge contributions deduced from high frequency stream nitrate data

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.20162492825)
Authors:   Christopher S. Jones, Caroline Davis
Keywords:   Nitrate; supply-limited; transport-limited; turning point

Abstract.

In many Iowa watersheds, tile drainage serves as the headwater source for second- and third-order streams. Corn and soybean are the dominant crops in these tilesheds which are very vulnerable to losses of soluble nitrogen and other pollutants through the drainage network. As a result, higher order streams such as the North Raccoon, Des Moines, and Boone Rivers often suffer from elevated concentrations and loads of nitrate-nitrogen (NOx-N). Long-term river NOx-N trends are largely driven by precipitation and discharge, and thus N loss is considered to be transport limited in this region. This study examines daily average discharge and NOx-N data collected over multiple years using continuous monitors. Careful examination of this high-frequency river nitrate data shows that the discharge associated with peak nitrate concentration varies from year to year, and that discharge at peak nitrate concentration tends to be similar between watersheds of varying size when discharge is normalized to drainage area. Concentration differences between nearby watersheds at the same normalized discharge show that the supply of nitrate that is vulnerable to loss varies, indicating differences in soil type and possible farming practices. From these data we also estimate the maximum subsurface flow possible from the studied basins. This work demonstrates that N loss is not completely transport limited and that some focus on managing the N supply on the landscape is merited.

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