The objective of this study was to assess curve number (CN) values derived for two forested headwater catchments in the Lower Coastal Plain (LCP) of South Carolina using a three-year period of storm event rainfall and runoff data in comparison with results obtained from CN method calculations. Derived CNs from rainfall/runoff pairs ranged from 46 to 90 for the Upper Debidue Creek (UDC) watershed and from 42 to 89 for the Watershed 80 (WS80). However, runoff generation from storm events was strongly related to water table elevation, where seasonally variable evapotranspirative wet and dry moisture conditions persist. Seasonal water table fluctuation is independent of, but can be compounded by, wet conditions that occur as a result of prior storm events, further complicating flow prediction. Runoff predictions for LCP first-order watersheds do not compare closely to measured flow under the average moisture condition normally associated with the CN method. In this study, however, results show improvement in flow predictions using CNs adjusted for antecedent runoff conditions and based on water table position. These results indicate that adaptations of CN model parameters are required for reliable flow predictions for these LCP catchments with shallow water tables. Low gradient topography and shallow water table characteristics of LCP watersheds allow for unique hydrologic conditions that must be assessed and managed differently than higher gradient watersheds.(KEY TERMS: surface water/groundwater interactions; runoff; stormwater management; watershed management; curve number method; first-order streams.)

Hydrologic monitoring was conducted in two first-order lower coastal plain watersheds in South Carolina, United States, a region with increasing growth and land use change. Storm events over a three-year period were analyzed for direct runoff coefficients (ROC) and the total storm response (TSR) as percent rainfall. ROC calculations utilized an empirical hydrograph separation method that partitioned total streamflow into sustained base flow and direct runoff components. ROC ratios ranged from 0 to 0.32 on the Upper Debidue Creek (UDC) watershed and 0 to 0.57 on Watershed 80 (WS80); TSR results ranged from 0 to 0.93 at UDC and 0.01 to 0.74 at WS80. Variability in event runoff generation was attributed to seasonal trends in water table elevation fluctuation as regulated by evapotranspiration. Groundwater elevation breakpoints for each watershed were identified based on antecedent water table elevation, streamflow, ROCs, and TSRs. These thresholds represent the groundwater elevation above which event runoff generation increased sharply in response to rainfall. For effective coastal land use decision making, baseline watershed hydrology must be understood to serve as a benchmark for management goals, based on both seasonal and event-based surface and groundwater interactions.(KEY TERMS: surface water ⁄ groundwater interaction; runoff; watershed management; streamflow; coastal watershed hydrology; first-order stream; hydrograph separation; South Carolina.)

Permeable pavements are implemented to provide at-source treatment of urban stormwater runoff while supporting vehicular and pedestrian use. Studies on these systems have mainly focused on those treating only direct rainfall and installed atop well-drained soils which typically provide substantial hydrologic mitigation through exfiltration that may not be representative of more hydrologically taxing conditions.A single lane parking area retrofitted with permeable interlocking concrete pavement in Vermilion, OH, USA was monitored over a 15-month period to quantify its hydrologic performance under such conditions. The 470 m 2 permeable pavement was underlain by silt loam soils and a shallow bedrock layer and treated run-on from the adjacent 324 m 2 asphalt drive lane. Observed data were compared to a calibrated SWMM model developed to simulate the pre-retrofit conditions of the site (i.e., a completely impervious parking lot). Cumulative runoff volumes were reduced by 43% across all events in the monitoring period compared to a fully impervious parking lot.While median peak flows were reduced by 75%, substantial mitigation was limited to smaller, lower intensity events with longer antecedent dry periods (i.e., non-flood producing events). The permeable pavement significantly delayed the occurrence of peak flows from the site following peak rainfall intensity by a median 29 min. Results from this study demonstrate that permeable pavements which receive run-on from adjacent imperious cover and are installed atop poorly drained soils can significantly reduce runoff volumes and peak flow rates and delay the occurrence of peak discharge. The modelling approach implemented can provide a better estimation of diffuse inflows to green infrastructure stormwater controls and aid in refining design features which enhance the hydrologic performance in systems underlain by poorly drained soils.

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