Flow dynamics of three experimental forested watersheds in coastal South Carolina (USA)

Three first-, second- and third-order experimental forested watersheds located within the Francis Marion National Forest in the lower coastal plain of South Carolina were monitored for rainfall and stream outflows. The largest watershed (WS 78) with some open lands, roads and wetlands gave higher annual water yields compared to the two other smaller ones (WS 79, WS 80) with mostly forest vegetation, possibly due to a difference in land use, soils and topography as well as increased base flows. Flow duration curves yielded largest flow rates exceeding 4% of the time for the second-order watershed (WS 79). As expected, the daily flows persisted for 79% of the time in the largest 3rd-order watershed (WS 78) with a larger storage compared to only 65 and 60% in the 2nd- and 1st-order watersheds. The flow frequency analysis of peak flows, employing Pearson III-type distribution, revealed the peak flows for 100-, 50-, 25-, 10- and 5-year return periods as 1805, 1565, 1326, 1009 and 769 cfs (cubic feet per second) for WS 78; 379, 325, 272, 200 and 146 cfs for WS 79; and 73, 63, 54, 41 and 32 cfs for WS 80. These results are in good agreement with the data calculated using the USGS-developed formulae for the South Carolina Lower Coastal Plain and have implications for the design of engineering structures, water and nutrient management, as well as evaluation of the impacts of development and natural disturbances on the forested lands of the Atlantic Coastal Plain.