Rainfall-runoff relationships for small, mountainous, forested watersheds in the eastern United States
Tedela, Negussie Hailu
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Runoff is a complex interaction between precipitation and landscape factors. While some of these factors (e.g., land use and cover, topography, soil characteristics, and drologic condition) have been defined for urban, rangeland, and agricultural drainages, runoff from mountainous, forested watersheds is poorly understood, especially in the eastern United States. This study investigated the response of streamflow to rainfall on ten gaged, small watersheds in the mountainous forests of the eastern United States using two methods to estimate runoff; the semi-empirical curve number method, and the semi-distributed TOPMODEL. Alternative techniques for calibrating watershed curve numbers were first assessed to determine whether these methods provide acceptable estimates. Runoff estimated using tabulated curve numbers was assessed separately and provided very poor, inadequate runoff estimates for all ten watersheds. Curve numbers calibrated using rainfall-runoff observations provided adequate estimates for only four of ten watersheds. Even calibrated curve numbers contain large uncertainties, thus requiring statistical proof that estimated runoff adequately agrees with observations for use in critical designs. For ungaged, forested watersheds, estimated curve numbers should be independently confirmed using data from gaged watersheds with similar hydrologic conditions. The effects of seasonal variation, forest harvesting, and return period frequencies on curve numbers were evaluated, and all affect curve numbers under some circumstances. Design engineers and analysts should consider using these factors to adjust curve numbers; otherwise, runoff calculations are even poorer estimates. Watershed runoff responses also were evaluated using the TOPMODEL, which uses topography to simulate runoff based on the concepts of saturation excess overland flow as controlled by subsurface processes. The results showed that the TOPMODEL best estimated runoff at three of the four locations. Results were in general agreement with other the TOPMODEL studies. The timing, shape and magnitude of the simulated hydrograph during the, rising, and recession periods of each storm events was very well reproduced by the model. The relationship between the TOPMODEL topographic index and the curve number for a given watershed may provide a useful procedure for better estimating runoff from small, mountainous, forested watershed in the eastern United States.