A field-scale investigation of nutrient and sediment reduction efficiencies of a low-technology best management practice: Low-grade weirs
Introduction
Global crop demands are forecasted to increase between 100–110% from 2005 to 2050 (Tilman et al., 2011), requiring the intensification of agricultural production to meet demands. As agricultural intensification in the last 35 years has already resulted in a 6.87- and 3.48-fold increase in nitrogen (N) and phosphorus (P) fertilization, linear expansion of past trends project approximately 3-fold increases in N and P associated with continued population expansion (Tilman, 1999). Increased fertilizer applications to agricultural landscapes result in higher nutrient loading to receiving surface waters (Donner, 2003) leading to eutrophic conditions in coastal ecosystems. This issue is of particular concern in the Gulf of Mexico, where nutrient losses from agricultural landscapes in the Mississippi–Atchafalaya River Basin have led to increased nutrient loading to downstream surface waters and the occurrence of an annual coastal hypoxic zone in the Gulf of Mexico (Rabalais et al., 2002, Rabalais, 2011). The need to reduce nutrient loading to the Gulf of Mexico has led to the establishment of a goal to reduce riverine total N and P loads by 45% (USEPA, 2008, GOMA, 2009) and an increased allocation of resources and research initiatives to implement innovative management strategies to achieve those nutrient reductions.
Serving as primary conduits for agricultural runoff, ditch systems, are essential features for adequate drainage to maintain crop production. Found throughout the Lower Mississippi Alluvial Valley, ditch systems have been shown to transport nutrients and other agricultural pollutants to downstream waters (Moore et al., 2001a, Moore et al., 2001b, Randall and Vetsch, 2005, Needelman et al., 2007, Smith et al., 2008). Additionally, drainage ditches function to control water Table levels, influencing chemical and biological processes (Needelman et al., 2007) and have been examined as a management practice for nutrient remediation (Moore et al., 2001a, Moore et al., 2001b, Cooper et al., 2002, Moore et al., 2010). Recent studies have demonstrated the ability of drainage ditches to mitigate nutrient runoff (Kröger et al., 2007, Kröger et al., 2008a, Moore et al., 2010). Furthermore, utilizing controlled drainage structures within ditch systems has been identified as having additional benefits for nutrient management (Evans et al., 1995).
Best management practices (BMPs) in the form of controlled drainage have been implemented on the agricultural landscape to reduce nutrient concentrations and loads to receiving waters by reducing drainage outflows (Gilliam et al., 1979, Evans et al., 1992, Evans et al., 1995, Borin et al., 2001). Controlled drainage is a practice in which a structure, such as a variable-height riser, is used to manage the water level in a drainage outlet (Evans et al., 1995). Low-grade weirs (hereafter referred to as weirs) have been utilized in lieu of traditional control drainage practices (e.g., variable-height risers) to reduce effluent nutrient loads in recent studies. Kröger et al. (2008a) demonstrated that weirs had the potential to facilitate more efficient management of systems via flexible spatial configurations within the drainage ditch rather than stationary outflow control structures. Semi-controlled experiments showed increases in hydraulic residence time (HRT) (Kröger et al., 2008b) and reduced nitrate concentrations (Kröger et al., 2011, Kröger et al., 2012) in systems with weirs. Results of a field-scale investigation of a single drainage ditch with weirs installed showed similar effects on HRT and nutrient reductions of N and P constituents ranging from 14 to 67% (Littlejohn et al., 2014). Additional investigation of the potential use of weirs for controlled drainage at the field scale is warranted for replication.
The objective of this study was to quantify nutrient reduction efficiencies of weirs in drainage ditches receiving agriculture runoff at the field-scale. The experiment was conducted in the Mississippi Delta, a highly productive agricultural region of large economic importance to the State of Mississippi. The Mississippi Delta is located in northwestern Mississippi and is part of the larger Mississippi River Basin. The hypothesis of this study was that the implementation of weirs in agricultural drainage ditches would alter hydraulic residence time and denitrification, resulting in enhanced nutrient load and concentration reductions compared to ditches without weirs. Nutrient concentrations and loads were monitored in drainage ditches with and without weirs within working row-crop agricultural landscapes. Goals of weir implementation and nutrient monitoring aim to highlight the effectiveness of these structures for water quality improvement. This investigation also aims to demonstrate how innovative, low-technology nutrient reduction strategies can decrease nutrient contributions to coastal ecosystems.
Section snippets
Materials and methods
Study sites were located in in the Mississippi Delta, an area formed by alluvial deposits of the Mississippi River and its tributaries (Fisk, 1951). Sub-watersheds were chosen because of their priority status within a Mississippi River Basin Healthy Watersheds Initiative and listing on the Mississippi Department of Environmental Quality 303(d) list of impaired waters (Upper Yazoo-Hydrologic unit code 08030206). A total of six agricultural drainage ditches were chosen for experimental sites
Results
Ditch physical characteristics, watershed drainage area, drainage ditch area, watershed to drainage ditch ratio, and soil types were determined; all characteristic surveys were conducted following reconstruction and weir implementation (Table 1). Ditches Ref-A and W4-B had the smallest watershed to drainage ditch ratios at 72:1 and 74:1, respectively, and ditches W2-A and W4-B had the greatest watershed to drainage ditch ratios of 146:1 and 144:1, respectively. Ratios for ditches Ref-B and W2-B
Discussion
Understanding the nutrient reduction efficiencies of agricultural best management practices is necessary to adequately reduce nutrient loads to downstream aquatic ecosystems as agriculture production continues to intensify to sustain expanding global populations. This study aimed to quantify nutrient reduction efficiencies of weirs implemented in drainage ditches at the field-scale. Results of this three-year investigation in the Mississippi Delta yielded data supporting: (1) the nutrient
Acknowledgements
The authors would like to gratefully acknowledge funding from the Environmental Protection Agency Gulf of Mexico Program Office (Grant No. MX-95460110), Delta F.A.R.M., and study location land owners. Authors would also like to thank the Mississippi Agricultural Forestry and Experiment Station, and Forest and Wildlife Research Center for support.
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