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Nakayasu synthetic unit hydrograph is one of some unit hydrograph methods that is widely used in the analysis of design flood in Indonesia. The Nakayasu synthetic unit hydrograph method requires relatively a little data and the results still need to be tested regarding to the reality. Following the various studies related to alfa parameter that have been carried out, these parameters, are still not optimal and only valid in the certain locations, so the formulation is needed to determine the value of alfa parameter that is adjusted to the physical watershed characteristics in Java. This study aims to develop a model of alfa parameter that is related to the characteristics of watersheds in Java Island. The watershed characteristics consist of watershed shape, watershed area, slope, and river typology (length of river and bed slope). The alfa parameter development is generated from the statistical analysis for several watershed characteristic variables that have a significant effect. The model of alfa parameter is hoped to be a function of the watershed characteristic variables with the general equation is as follow: alfa = f (river length, slope, area, perimeter, watershed shape).