Test of Trapezoidal Hyetograph Method with Laboratory Watershed Data

Abstract

Trapezoidal hyetographs are common in both real-world small watersheds and laboratory watersheds because (1) real-world hyetographs are often represented by line charts that can be divided into multiple trapezoidal rainfall pulses, and (2) laboratory rainfalls are usually generated by rainfall simulators uniformly in space and time, but when considering infiltration loss, the excess hyetographs may be approximated to be trapezoidal. Particularly, a trapezoidal hyetograph can be converted to a runoff hydrograph simply, accurately, and theoretically based on the general unit hydrograph model. The objective of this research is thus to test the theoretical trapezoidal hyetograph method with laboratory watershed rainfall-runoff data. The results showed that this theoretical method agrees very well with laboratory watershed data, with determination coefficients r2>0.93 for both constant and variable (incident) rainfall intensities. Therefore, the trapezoidal hyetograph method can be used to convert any line-chart hyetograph to a runoff hydrograph analytically, where rectangular and triangular hyetographs are special cases. Finally, the trapezoidal hyetograph method for a constant excess rainfall intensity (or the general rational method) is comparable to the classic kinematic wave theory for overland flow, but it is much simpler in applications.