An application of Equivalent Frequency Transfer Function (EFTF) was applied in this study. This EFTF has originally been used among control engineers to analyze non-linear elements; for example threshold and saturation elements. Previous study by the authors had successfully derived the EFTF between rainfall input and discharge for real catchment area based on kinematic wave model. This study developed application of EFIT by using diffusion wave model with zero gradient of depth downstream

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... ary condition. Gain, time lag characteristics and vector locus are proposed in order to investigate efficiency and effectiveness of EFTF. It is also able to extend this technique to multi element basin.