| Popis: |
Quantifying the effectiveness of conservation practices at the watershed scale throughout the nation has been identified as a critical need. Our objective was to determine the effectiveness of these conservation practices for reducing sediment yield. The Topashaw Canal watershed (TCW), an 11,000-ha (27,181-ac) area in northcentral Mississippi, exhibits flashy stream response to storms with mean sediment concentrations (117 mg L -1 (117 ppm)) almost double the median sediment concentration (60 mg L -1 ). The most prevalent conser- vation practice imposed by acreage, since 1985, is enrollment in the Conservation Reserve Program (e.g., planting of pine trees). Grade-stabilization structures (e.g., drop pipes) are the most common conservation practice used to control gully erosion within the TCW. These structures are estimated by the USDA Natural Resources Conservation Service to reduce annual sediment yield from 11.5 to 0.1 Mg ha -1 yr -1 (5.13 to 0.05 tn ac -1 yr -1 ), but measure- ments have not been made to determine the accuracy of these estimates. Nonetheless, an average of 58 drop pipes have been installed annually within the TCW using Environmental Quality Incentives Program funds, and an additional 5.4 large drop pipes have been installed each year using US Corps of Engineers funds. Annual gully erosion accounted for 54% of the total sediment yield of over 73,000 Mg (80,445 tn) from TCW. The shift in land use to Conservation Reserve Program, combined with channel incision, has resulted in stream- bank failure and gully erosion being the primary sources of sediment currently leaving the watershed. |
