A study of coastal headwater stream hydrology: Bannockburn plantation, Georgetown County, SC

Coastal headwater streams in undeveloped forested landscapes function as a natural storage and conveyance mechanism for rainfall and groundwater. Stream flows are often driven by groundwater table depth, while soil saturation with a high water table may drive rainfall response. Evapotranspiration also plays a significant role in groundwater levels and influences stream flows both seasonally and diurnally. The dynamics of coastal forested watersheds are complex, and water budgets of these low gradient headwater streams are difficult to quantify.;Understanding these hydrologic dynamics is critical to water resources protection and flood prevention in coastal landscapes, especially as forested areas are converted to residential and commercial developments. A benchmark for baseline hydrology must be established for sustainable development goals over the course of land use change. Understanding short- and long-term hydrologic responses from the conversion of forest lands to urban areas can minimize negative effects in terms of water quantity and quality. Toward the goal of establishing a hydrology benchmark in a pre-development forested watershed, three approaches were taken: quantifying the water budget, determining runoff: rainfall relationships through hydrograph separation and curve number modeling. Stream flows, groundwater levels, and rainfall were measured in an approximately 400- acre coastal watershed, Upper Debidue Creek, on Bannockburn Plantation in coastal South Carolina, USA. Evapotranspiration rates were also estimated with sensors located nearby on Oyster Creek which is maintained by NOAA. Water quality measurements (temperature, specific conductivity) and water quantity (stage and flow rate) were collected. In balancing the coastal watershed's water budget parameters, evapotranspiration was found to drive runoff through groundwater and soil storage depletion during the growing season and recharging during the dormant season. Analysis from hydrograph separation methods used for establishing rainfall-runoff relationships, showed runoff to rainfall ratios increasing during the dormant season and decreasing in the growing season, largely affected by the groundwater table position. Rainfall-runoff ratios for the fall, winter and spring month in the current data set were 0.14, 0.56, and 0.19, respectively. During the summer months and peak growing season, the watershed experienced rainfall but no measurable runoff. Predictions of runoff using the SCS Curve Number method in comparison with observed data from Upper Debidue Creek, were observed to better estimate runoff when groundwater elevation and antecedent rainfall conditions were taken into account when back calculating curve numbers. The results from these simulations showed that there is no one Curve Number value for the lower coastal plain to predict runoff. The implications of the varying Curve Number are grave when the protocol for post-development water management to reflect pre-development calls for the use of one Curve Number value. The Curve numbers ranged from 96 to 35 for rainfall events during different antecedent moisture conditions on the same watershed.