Factor H is introduced for direct head loss calculation and sizing in dual-diameter laterals of sprinkler and trickle irrigation systems when the Hazen-Williams equation is to be applied. Application of this factor prevents trial and error and uses fewer head-loss equations in calculating the total friction loss and designing dual-diameter laterals. The proposed factor is a function of total number of outlets on the lateral, number of outlets on the smaller-diameter section of the pipe and

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... of smaller diameter to the larger diameter pipe. By solving two practical examples, the advantage of this factor over other approaches was shown. It was also demonstrated that design of lateral sizing by the given equations is simpler, easier and more accurate than the previous methods.