INTERMEDIATE-DURATION-RAINFALL INTENSITY EQUATIONS

A procedure is described for obtaining rainfall-intensity-duration equations for durations ranging from 1 to 24 hr. The method uses readily available isopluvial maps prepared by the U.S. National Weather Service (NWS) for four geographic regions: the contiguous United States, Alaska, Hawaii, and Puerto Rico and the U.S. Virgin Islands. However, because more up-to-date maps for durations from 1 to 24 hr have been prepared for the 11 states west of the 105th meridian, the procedure does not apply to the western United States. Dimensionless forms of four types of commonly used intensity-duration equations were fit to standard ratios of t-hr to l-hr rainfall depths for t = 1, 2, 3, 6, 12 and 24 hr. For a given ratio of 24-hr to l-hr rainfall depths, the NWS finds the four intermediate ratios to be nearly constant within each of the mapped regions. Optimal parameter values for each of the four equation types were found by an intensive pattern search covering the entire feasible range of each parameter, and are graphed as functions of the 24-hr to l-hr rainfall-depth ratio for a specific return period. The best equation for a particular site is found from graphs of the error sum of squares as a function of the 24-hr to l-hr rainfall-depth ratio. Based on these graphs, a simple equation needing only a single parameter is suggested that provides a good fit for all the geographic regions covered. The procedure described here will be useful for developing precipitation patterns (i.e., design storms) used in designing hydraulic structures.