Computation and Optimization of Snyder’s Synthetic Unit Hydrograph Parameters

Abstract

In Sri Lanka, the availability of Snyder’s Synthetic Unit Hydrograph (SSUH) parameters are reported only in the Irrigation Department Guidelines and those are limited to only 19 locations. The present study is to determine the SSUH parameters and their applicability to the Karasnagala watershed (52.58 KM2). 60 events corresponding to both North East and South West monsoons during the 1971-1989 period were selected for the model calibration and verification. Considering a balanced representation of both seasons, 30 events were selected for model calibration while the rest were taken for model verification. Events were separated using a minimum inter-event time of 2 days. Effective rainfall corresponding to each event was determined using Phi-Index and incorporating the baseflow separation with the use of Concave method. A one day triangular SUH computed for each event was then converted to curvilinear SUH with the help of SCS dimensionless hydrograph. Mean Ratio of Absolute Error (MRAE) was chosen as the objective function for the evaluation of the total, high, intermediate and low flow estimated by the model. Model verification used the averaged parameter values optimized for each event during model calibration. Averaged calibrated parameters Ct and Cp for Attanagalu Oya Basin at Karasnagala were 3.75 and 0.38 respectively with MRAE value of 0.2. The results obtained were further compared with the recommended ID guideline parameters. The value of Ct and Cp can be applied to the other ungauged areas of the Attanagalu river basin and regions having similar characteristics and consider as the basis for further studies with shorter temporal data resolution.