Comparison of rational formula alternatives for streamflow generation for small ungauged catchments

Abstract

A Unit Hydrograph can be derived using the peak discharge obtained from the Rational Equation and using unit depth of rainfall in calculating the Rainfall Intensity. Possible alternatives for selection of sub catchments, selection of a guideline to determine the Runoff Coefficients and the Time of Concentration were taken into account. A significant variation of the results was observed with the selected alternatives. In order to select a better alternative comparison of generated stream flows with the observed values is required. Unit hydrograph derivation using the Rational method would yield a lesser peak flow value and a higher time to peak when spatially distributed approach using sub catchments is used instead of a lumped single catchment The guideline used for determination of Runoff coefficients plays very important role in Rational Method and it showed considerable variation of the results in Welipenna River Basin. The time of concentration estimation method is also important when the catchments become larger. In the Welipenna River Basin the Lumped Parameter method of deriving Unit Hydrograph taking Runoff Coefficient from based on Slope, Land Use and Soil Type with Tc from ID Guidelines a Qp estimate of 0.49m3/s, while the peakflow estimate computed with the distributed parameter method using sub catchments was 0.51m3/s.