Assessing Water-Quality Changes in Two Oxbow Lake Tributaries of the Mississippi Delta

Author(s): Murphy, J.; Hicks, M.; Stocks, S.

Many best management practices (BMPs) have been implemented across the Mississippi Delta in an effort to reduce the amount of nutrients and sediment leaving agricultural fields. However, it has been difficult to assess the influence of BMPs because there is often a lack of monitoring of downstream water quality. To this end, the U.S. Geological Survey collected approximately 8-9 years of water-quality and hydrology data at two agricultural ditches that drain row-crop fields and have a variety of BMPs in place in the ditches or fields. These sites discharge into separate oxbow lakes in the Mississippi Delta and previously had been identified as having excessive nutrient and sediment runoff. Using an event-based dataset and bootstrapping techniques, we tested for differences in flow-weighted mean concentrations of nutrients and sediment between an early and late period at each site. Most of the major BMP implementations occurred during in the early period whereas the late period had some additional implementations but typically at a lower intensity. We tested for differences and equivalences in median early and late concentrations and also for differences in the concentration-streamflow relationship between periods. We found several statistically significant decreases in nutrients and sediment at one site but none for the other. Nutrients and sediment were also not found to be equivalent between the early and late periods at either site. This means, while one site had substantial decreases in event concentrations for some constituents, results at the other site were generally inconclusive. These mixed results are likely due to differences in BMP implementation, farming practices, and the data characteristics at these sites.

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