ANALYSES OF INFLUENTIAL FACTORS FOR ACCURATE DETERMINATION OF PEAK RATE FACTORS AND TIMES TO PEAK OF UNIT HYDROGRAPHS
McCuen, Richard H.
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Despite the availability of a number of sophisticated hydrologic models, the unit hydrograph (UH) is still one of the most widely used models for computing runoff hydrographs. The two-parameter gamma UH can be fully characterized by two parameters: the peak rate factor (PRF) and the time to peak (tpUH). Currently, obtaining accurate estimates of UH parameters is still a problem, especially for ungauged watersheds. The goal of this research was to analyze factors that influence estimates of UH parameters and to develop general guidelines that can assist in estimating UH parameters more accurately. A calibration model was developed for evaluating PRFs and tpUHs simultaneously from rainfall-runoff data. The effects of various influential factors were identified and investigated based on analyses of both synthetic and measured rainfall-runoff data. Results showed that the accuracy of calibrated UH parameters is affected by the rainfall characteristics, the time offset, the nonuniformity of rainfall, the extent of nonlinear watershed processes, and the flexibility of the gamma probability distribution function. Guidelines were developed to assist UH users in interpreting the calibration results and calibrating UH parameters more accurately.