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<br />The hydrometeorological approach to analyzing flood events and <br /> <br /> <br />predicting the magnitude of floods requires a firm estimate of the <br /> <br /> <br />relationship between precipitation and resulting runoff. There is the caution <br /> <br /> <br />in Design of Small Dams (p 591, "Extreme caution should be used in drawing the <br /> <br /> <br />isohyetal pattern in mountainous areas where the orographic effect is an <br /> <br /> <br />important factor in the areal distribution of rainfall. For example, if there <br /> <br />is a precipitation station in a valley on one side of a mountain range and <br /> <br /> <br />another station in a valley on the opposite side of the range with no <br /> <br /> <br />intervening station, it cannot be assumed that the rainfall during a storm <br /> <br /> <br />would vary linearly between the two stations. It is likely that the rainfall <br /> <br /> <br />would increase with increases in elevation on the windward side of the divide, <br /> <br /> <br />whereas on the leeward side, precipitation would decrease rapidly with <br /> <br /> <br />distance from the divide. This type of distribution can uSLlally be verified <br /> <br /> <br />in mountainous areas where there are sufficient precipitation stations to <br /> <br /> <br />define the isohyetal pattern accurately." The ability to obtain sufficient <br /> <br /> <br />data from precipitation stations near the stream of interest being the major <br /> <br /> <br />disadvantage. <br />Various models for the rainfall runoff method are available. Regression <br /> <br /> <br />equations can then be calibrated for the rainfall-runoff method, one of these <br /> <br /> <br />being the rational method. The rational method assumes discharge is a <br /> <br /> <br />function of rainfall and area and also that the discharge flood frequency is <br /> <br /> <br />equal to the input rainfall frequency. Unit hydrographs are an example of <br />rainfall runoff models and so are synthetic unit hydrograph models. All <br /> <br />models assume that the recurrence interval of the runoff variable is equal to <br /> <br /> <br />the input rainfall frequency. Other examples of rainfall runoff models <br /> <br /> <br />include the ses method, the Storm Water Management Model (SWMMI developed by <br /> <br />6 <br />