Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/20/2007
Publication Date: 3/20/2008
Citation: Kroger, R., Holland, M.M., Moore, M.T., Cooper, C.M. 2008. Agricultural drainage ditches mitigate phosphorus loads as a function of hydrological variability. Journal of Environmental Quality. 37:107-113. Interpretive Summary: Excess phosphorus in agricultural runoff water can be damaging to fish, plants, and other non-target organisms in downstream receiving systems. Two experimental ditches were sampled for two years for baseflow and stormflow orthophosphorus concentrations. The vegetated drainage ditches reduced the load of total phosphorus leaving the fields 47% over two years. Information on how nutrients are transported from fields through ditches will aid farmers and other conservationists in adequately designing strategies to mitigate potential downstream effects of excessive phosphorus runoff.
Technical Abstract: Phosphorus loading from non-point sources such as agricultural landscapes contributes to downstream aquatic ecosystem degradation. Specifically within the Mississippi watershed, enriched runoff contributions have far reaching consequences for coastal water eutrophication and Gulf of Mexico hypoxia. Through storm and overland surface runoff events, the phosphorus mitigation capacity of agricultural drainage ditches under no-till cotton was determined for natural and variable rainfall conditions in north Mississippi. Two experimental ditches were sampled monthly for TOP concentrations in baseflow, and on an event-driven basis for stormflows. Sediments were sampled for TP on a monthly basis. Phosphorus concentrations, Manning’s equations with a range of roughness coefficients for changes in vegetative densities within the ditches, and NRCS dimensionless hydrographs combined to determine maximum storm load effluent from the farms. Baseflow regressions and percentage reductions with storm loads show that the ditches alternated between being a sink and source for dissolved phosphorus and particulate phosphorus concentrations throughout the year. Storm event loads resulted in 31% of the annual applied fertilizer (9.1 kg/ha/yr) to be transported into the drainage ditches. The ditches mitigated 47% of the total effluent phosphorus load, leaving 1.8 kg/ha/yr to enter receiving waters. Agricultural drainage ditches were exhibiting a strong phosphorus mitigation potential, and warrant future work on controlled drainage to improve mitigation capacity.