Hydrology and water quality of two first order forested watersheds in coastal South Carolina
Two first-order forested watersheds (WS 80 and WS 77) on poorly drained pine-hardwood stands in the South Carolina Coastal Plain have been monitored since mid-1960s to characterize the hydrology, water quality and vegetation dynamics. This study examines the flow and nutrient dynamics of these two watersheds using 13 years (1 969-76 and 1977-81) of data prior to Hurricane Hugo (1 989) and nearly 10 years (1 990-1 992, 1996-99, and 2003-06) immediately after Hugo. WS 80 remained as a control throughout the study period, whereas WS 77 underwent several treatments including prescribed burning, partial harvest, salvage logging and prescribed fire for red-cockaded woodpecker habitat management. Depending upon the antecedent moisture conditions, both the watersheds were highly responsive of rainfall events throughout the periods. Accordingly, annual outflows varied from 5% in 1981 to 59% in 1998 with an average of 22% of the annual precipitation for the control (WS 80) and from 9% in 2004 to 44% in 1991, with an average of 27% for the treatment watershed (WS 77). The coefficient of variation (COV) on WS 80 was higher (55%) compared to 36% for the WS 77. Annual rainfall variation was much lower (COV = 14%) than the variation in stream outflows. Post Hugo average outflow from WS 77 increased relative to WS 80 until 1992. By the regeneration period of 1996 reversal in outflow was noticed with the higher outflows on WS 80 than on the WS 77. While prescribed burning of WS 77 in a course of five years (1977-81) did not affect on stream outflows and chemistry, mastication in course of nine months in 2001 followed by another prescribed burning of 84% of WS 77 on May 10,2003 seemed to have increased the outflows on WS 77 both in 2004 (64%) and 2005 (70%). Average nutrient concentrations were similar on both watersheds although there was a wide variability in NH4-N on the treatment watershed (WS 77) compared to WS 80. pH was slightly lower on the WS 77 (5.4) than on WS 80 (6.8). Both NO3-N and NH4-N concentrations were very low for both the watersheds, before and after Hugo, with organic nitrogen as the dominant factor on both watersheds. Phosphate was also very low (0.02 mg L-I, on average) on both the watersheds during both the periods. Hurricane Hugo substantially increased the nutrient loads primarily due to increase in outflows. Although data presented herein may serve as baseline information for assessing impacts of both the developments and natural disturbance in the region, further studies and analysis with additional data should be conducted to verify some results such as the reversal of flow pattern after the hurricane Hugo that may have changed the dynamics of regenerated vegetation after Hugo possibly affecting stream outflows via evapotranspiration (ET) on these humid coastal plain watersheds.
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