Long-term flow dynamics of three coastal experimental forested watersheds
Three 1st2nd, and 3rd order experimental forested watersheds located within Francis Marion National Forest in Coastal South Carolina were monitored for rainfall and stream outflows. These watersheds were WS80, a pine-hardwood forest (206 ha); WS79 a predominantly pine forest (500 ha); and WS78, a mixed land use dominated by ine-hardwood forests (5,000 ha). The mean runoff coefficients for the 13-yr period (1964-76) for WS78, for the eight-yr period (1 966-73) for WS79, and for the eight-yr period (1969-76) for WS80 were 38%, 25%, and 21 %, respectively. However, when the same five-year period ( I 969-73) was considered for all watersheds, the mean runoff coefficients for WS78, WS79, and WS80 were 44%, 27% and 22%, respectively. The largest watershed consistently yielded higher annual water yields compared to the two other smaller ones. Flow duration curves were derived to examine the exceedance probabilities of daily stream flow regimes on each of the watersheds. The flow frequency analysis with 13, 7 and 13 years of peak flows for WS78, WS79 and WS80, respectively, employing Pearson Ill-type distribution revealed the peak flows for loo-, 50-, 25, lo-, and 5-year return periods as 1805, 1565, 1326, 1009, and 769 cfs for WS78,379,325,272,200, and 146 cfs for WS79, and 73,63, 54,41, and 32 cfs for WS80. These results are in good agreement with data calculated using USGS deveioped formulae for the South Carolina Lower Coastal Plain. These results have implications in design of engineering structures, water and nutrient management as well as in evaluation of impacts of development and natural disturbances on the forested lands of the Atlantic Coastal Plain.