Graphical Calculation of Small Catchment Peak Discharge

A graphical procedure for calculating peak discharge from small catchments is developed on the basis of the standard Natural Resources Conservation Service (NRCS) rainfall-runoff hydrologic methods, in which rainfall excess obtained using the curve-number approach applied to 24-h design storms is transformed to runoff by means of triangular unit hydrographs. The solution is made dimensionless by grouping parameters, and, as a result, can be condensed into uncomplicated graphical relations that provide numerically precise peak flow rates. Mathematical expressions are also given that provide accurate approximations to the peak-discharge graphical relations. An additional result of the analysis is a comprehensive means of estimating rational-formula runoff coefficients for any location in the United States.