Effectiveness of low-grade weirs for nutrient removal in an agricultural landscape in the Lower Mississippi Alluvial Valley

New best management practices (BMPs) are needed to provide enhanced water quality improvements to downstream aquatic systems from agricultural landscapes. In Mississippi, a simple process of controlled surface drainage is being advocated in drainage ditches within agricultural landscapes. Low-grade weirs (hereafter called “weirs”) are low, check-dam structures where water is held in drainage ditches at multiple locations. This spatial arrangement of controlled drainage results in increases in hydraulic residence time, decreases in flow velocities at multiple locations, and potential decreases in nutrient concentrations and sediment loads. This study was the first field-scale evaluation of weirs toward storm event nutrient (nitrate – NO3−, nitrite – NO2−, ammonia – NH3, dissolved inorganic P and total inorganic P) removal within a single ditch, Terrace, over an 18-month period. Individual sites within Terrace were monitored on rising and falling limbs of the storm hydrograph for changes in nutrient concentrations. A Hydrologic Engineering Centers River Analysis System (HEC-RAS) model was setup to calculate load derivations and differences. Overall there were very few statistical differences (P>0.05) between inflow and outflow concentrations due to their nascent variability in concentration between seasons, hydrology and runoff volume. However, median mass kg/ha as well as percentage nutrient (NH3, NO2−, dissolved inorganic P, and total inorganic P) load reductions were positive ranging from 14% (dissolved inorganic P) to 67% (NH3 and NO2−), with the exception of a median percent increase in NO3− load from inflow to outflow, likely resulting from the influence of two outlying storm events. Results indicate that at the field scale, weirs within a ubiquitous landscape feature such as an agricultural drainage ditch can reduce nutrient loads moving downstream. Weirs could be considered a viable BMP in agricultural landscapes aiming to control surface nutrient runoff; however, additional research of nitrogen dynamics is warranted to ensure their efficacy.