Rainfall-Streamflow Relations for Coastal Plain Watersheds

The objective of this article was to develop hydrologic information needed for nonpoint source water quality screening and assessment on agricultural watersheds in the Gulf-Atlantic Coastal Plain of the southeastern U.S. The USDA-Agricultural Research Service (ARS) has instrumented study areas in the Coastal Plain that provide rainfall and streamflow data from seven agricultural watersheds ranging from 15.7 to 334 km2 (6.0-129 mi2). Hydrologic budgets were developed for Little River experimental watersheds using long-term (up to 25 yr) water balance analyses and results of previous USDA-ARS research. The primary water balance loss component was evapotranspiration, which accounted for 69% of annual rainfall. Streamflow, the other major loss component, accounted for 30% of annual rainfall. Water balance analyses provide insight into the relative magnitudes of hydrologic budget components and their interactions that is useful for understanding landscape and watershed scale hydrologic and water quality processes on Coastal Plain watersheds where hydrologic response is influenced by riparian storage. Two forms of simple rainfall-streamflow relationships were evaluated for estimating annual water yields: constant percentage (runoff coefficient) relations and linear rainfall-streamflow regressions. Annual runoff coefficients, which averaged 0.30 for the study areas, were related to relative watershed riparian area. Watershed riparian area was more effective than upland land use for predicting annual runoff coefficients. Simple linear rainfall-streamflow regressions were found to more adequately represent year-toyear variability in streamflow production. Use of linear regression models eliminates the bias in water yield estimates inherent to constant percentage relations. Averages of 716 to 772 mm (28.2-30.4 in.) of rainfall were required annually to meet evapotranspiration and other losses on study areas before significant streamflow was produced. About 75% of rainfall in excess of the threshold was measured as streamflow. The effect of rainfall distribution on annual streamflow volume was evident, with two distinct seasons for production of streamflow noted. Rainfall-streamflow relations developed provide regional hydrologic information needed for nonpoint source water quality screening and assessment, and for water resources planning and management on low-gradient, humid region watersheds of the Coastal Plain of the southeastern U.S.