UNCERTAINTY IN ESTIMATING STORM FLOW AND BASE FLOW RESPONSE TO EXTREME WEATHER EVENTS IN COASTAL FORESTED WATERSHEDS
CALLAHAN, Timothy J.1, AMATYA, Devendra M.2, GRIFFIN, Michael P.3 and VULAVA, Vijay M.1, (1)Geology and Environmental Geosciences, College of Charleston, 66 George Street, Charleston, SC 29424, (2)Center for Forested Wetlands Research, USDA Forest Service, 3734 Highway 402, Cordesville, SC 29434, (3)Environmental Studies Graduate Program, College of Charleston, 66 George Street, Charleston, SC 29424, callahant@cofc.edu
There is large uncertainty in measuring effects of extreme weather events (drought; flooding) on stream flow and groundwater recharge in coastal regions. The role of these hydrological processes is not very well-understood in the low-gradient coastal region of the southeastern USA, an area vulnerable to extreme events. Our objective was to characterize the uncertainty in stream flow dynamics in response to drought and large storm events. The study site was the US Forest Service Santee Experimental Forest (SEF), 2600 hectares in area in the Francis Marion National Forest in Berkeley County, South Carolina, and where nearly all forest types on the lower coastal plain can be found. Watersheds here are wetland-dominated and the region has historic examples of impacts from extreme weather events, including Hurricane Hugo in 1989; in the past decade there have been two extended drought periods classified as severe on the Palmer Index. Stream flow and groundwater data have been collected within the SEF and the adjacent 7260-ha, third-order Turkey Creek watershed, part of the headwaters of the East Branch of the Cooper River, a headwaters of the estuary at Charleston, SC. This region has seen rapid urban development and land use change over the past three decades and continued population growth is a motivator to understand how future extreme events may affect hydrology and water quality of the region.
We hypothesize that the storm flow and base flow ratios on these systems will vary substantially during extreme weather events. Through ongoing research on the Turkey Creek watershed, we have used four methods to better understand its storm flow and base flow response from varying weather conditions: (1) field-based water-table fluctuation methods provided point information on shallow groundwater response to storm events; (2) hydrograph separation of stream gauge data and (3) end-member mixing analysis methods allowed for partitioning of base flow and storm flow during both drought and large rain events; and (4) an automated base flow separation technique for stream flow records. Existing modeling results show large uncertainty in large-storm event prediction, which we intend to reduce using site-specific information to better calibrate the models.
