Seasonal relationships between precipitation and stream flow patterns related to watershed characteristics of two third-order coastal plain watersheds in South Carolina

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La Torre Torres, Ileana Bagnamanay
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Hydrological studies in the Southeastern U.S. have primarily focused on runoff generation processes in Piedmont and mountainous areas; much less is known about hydrological processes in Coastal Plain watersheds. These two areas differ considerably in climate, topography and soil composition. Because of population growth in the last few decades in the Lower Coastal Plain (LCP) of South Carolina, it is important to fully understand natural hydrologic processes in LCP in order to predict impacts of land management activities on watershed hydrology. This study describes the relationships between rainfall patterns and stream flow for a third-order watershed, Turkey Creek. Runoff-rainfall ratios for several storm events were used to describe baseline runoff as a function of seasonal storm event and rainfall. It was hypothesized that runoff rainfall ratios are smaller during the summer season and greater in the winter due to generally reduced flows as a result of increased evapotranspiration (ET) from the forests during summer fall, and saturated/sustained flows in winterspring. Alternatively, runoff-rainfall ratios may be directly proportional to the antecedent soil moisture condition (as estimated by 5 day prior rainfall). To complement the objectives of this study the Soil Conservation Service-Curve Number (SCS-CN) and Rational methods were used to predict runoff volumes and peak flow rates for simulated and design storms. Results from this study showed that runoff-rainfall ratios were greater for wet (winter-spring) periods compared to dry (summer-fall) periods. Also, runoff response was related to antecedent soil moisture conditions for wet and dry conditions. Results obtained from SCS-CN and Rational method overestimated runoff volumes and peak flow rates for the Turkey Creek watershed. These results were influenced by the design limitations of the methods, thus, the application of these methods in similar watersheds is not recommended.
Watershed hydrology; Streamflow -- South Carolina