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Abstract
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Derivation of the unit hydrograph for a given watershed is impotent task in water resources management. Rainfall, as the most important input, is variable in time and space. Considering this variability’s can be effective in adjust unit hydrograph derivation errors. In small watershed, spatial variability of rainfall can be neglected and temporal variability can be assessed by unit hydrograph derivation methods from complex storms. Collin’s method, a simple trial and errors method of unit hydrograph derivation can be used for this purpose. This study challenges advantages and disadvantages of Collin’s method in derivation of the unit hydrograph from complex storms. After derivation the unit hydrograph, direct run off hydrograph were synthesized. Results show that Collin’s method, as a simple method, is more effective when the biggest rainfall block is bigger than the others but it is unable to ensure the non-negative values of unit hydrograph ordinates for any rainfall event. These negative values can be related to data collection errors, non linear rainfall- runoff relationship and the errors in hydrograph analysis calculations.
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