Statistical Distributions, Direct Runoff, and Baseflow

For design projects in the water resources area, a hydrograph is commonly used for the determination of flood volume generated as a direct runoff by a drainage basin. The division of total flow into direct runoff and baseflow is an essential process for these types of investigations. A large sample of hydrographs from different geographical areas of 48 contiguous States is considered in this study. The two-parameter distributions, such as Gamma, Beta, Weibull, Fréchet, and Lognormal, are fitted to each one of the sample hydrographs. The two parameters of each distribution are related to the dimensionless shape parameter and storage time constant of the same drainage basin. Both parameters for each distribution are determined through analysis of the measured data at a stream gaging station. The calibrated forms of the five distributions describe the natural hydrograph curve well and furnish accurate estimates of direct runoff and baseflow for a given drainage basin.