Khaled Osati - PHD Student, Watershed Management, University of Tehran and member of Young-Researchers Club of Kurdistan
Ali Salajegheh - Assistant Professor, Faculty of Natural Resources, University of Tehran
Mohammad Mahdavi - Professor, Faculty of Natural Resources, University of Tehran

Knowledge of rainfall-runoff relationship is one of the most important subjects in hydrology, watershed management, water resources and etc. Hydrograph is effective way in flood studying. Derivations of the unit hydrograph from complex storms can be effective in compensate ideal-single event shortage and calculating an exact unit hydrograph, although it’s complicated calculations. Differences of complex storms and single storms in base flow separation and excess rainfall calculations are discussed. This paper also discusses about a common mistake in unit hydrograph derivation studies in Iran, i.e. using the word “matrix method” instead of the word “least square method “. Least square method is a method derived from matrix method and is based on rainfall-runoff relationship mathematic basics and simultaneously solution a set of equation derived from Convolution equation, as noted in performed studies. Both LP and least square methods are the best linear method in derivation of the unit hydrograph from complex storms but matrix method is less effective than least square method.

matrix method, least square method, unit hydrograph, complex storms, convolution equation