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MASGC - Decreasing nitrate-N loads to coastal ecosystems with innovative drainage management strategies in agricultural landscapes

Robert Kröger, Jerry Farris (ASU), Matt Moore (USDA/ARS)

Graduate Student(s):  Alex Littlejohn

This project is funded through the Mississippi Alabama Sea Grant Consortium and is the first project that has been funded through Sea Grant away from the coast and in agriculture. It started in April of 2010, and is expected to be completed in April of 2012.

Agricultural drainage ditches are integral components and ubiquitous features of the agricultural landscape and act as major conduits of surface and subsurface flow N from agricultural lands to receiving waters. Drainage ditches are wetlands that are the forgotten links between agricultural fields and receiving waters. They possess hydric soils, support a diverse community of hydrophytes, and are subject to the unpredictable changes in soil saturation as a result of hydrological variability. Controlled drainage within drainage ditches has been proposed as a best management practice primarily aimed at reducing nutrient concentrations and loads in drainage ditches reaching receiving waters by reducing total drainage outflows. A commonly used practice for controlled drainage involves the use of a variable height riser in the drain or ditch outlet. This concept relies on the ability to control drainage intensity by determining the height of the riser and thus control volume of outflow and load of solutes. The variable height of the riser can also be used to increase groundwater levels during times of water stress and drought. For the most part, riser controlled drainage occurs seasonally when fields are fallow. Taking into consideration that certain surface drainage ditches are hundreds of meters long with variable slopes, would a temporally continuous stepwise increase of water levels improve retention and controlled drainage? An innovative, and cost effective drainage strategy proposes the installation of low-grade weirs (henceforth referred to as weirs) within the drainage ditch at multiple spatial locations within the agricultural landscape. This innovative concept provides drainage management on an annual and spatially gradated basis, rather than a single slotted riser occurring during the dormant season. Spatial allocation of weirs has some significant theoretical improvements over conventional slotted board risers. Water table depths should increase uniformly throughout the agricultural landscape, rather than just around the outflow. The very important service of first flush capture of non point source contaminants would occur at multiple locations and entry points along the drainage ditch, rather than just at the outflow. Multiple weirs will increase chemical residence time within each drainage ditch, and provide multiple sites for magnified microbial transformations, nutrient adsorption sites, and improved sedimentation. Spatially orientated weirs show promise to significantly improve N reductions by expanding and creating synergistic aerobic and anaerobic soil conditions through decreased flow rates and increased water levels and volumes.

The goal of this project is to identify the contribution an innovative, cost effective drainage management technique plays in decreasing source N concentrations and loads to coastal ecosystems. The rationale that underlies this research program is that understanding the intricate complexity between hydrology, N inputs, and N dynamics within managed drainage ditches within the agricultural landscape is expected to lead to new knowledge of the contribution innovative controlled drainage systems make to N loads to downstream aquatic ecosystems (i.e., coastal ecosystems). This, in turn, is expected to lead to new models, and better strategies for preventing nutrient contamination and impairment from source production agriculture to receiving coastal systems.


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