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intensities versus drainage area size on semi-logarithmic paper. 5.1.3. Interpolate for the desired storm area. determine PMP intensities for each duration and plot on ordinary graph paper. 5.1.4. Determine rainfall in each 6.llour tirne interval by interpolation, then increment the rainfall by successive subtractions. The PMP will be the maximum value for the selected storm area only; areas greater or less than the selected storm area will show smaller values of PMF'. Considerable trail an,j error computations will normally be required to determine the storm area which maximizes average precipitation in the study watershed. 5.1.5. Studies for HMR-52 have found ':Ilat major storms have a dominant orientation. which mayor may not be similar to the general orientation of the watershed. The PMP will often be reduced. depending on the drainage arE'a size and the an!lle between the storm and watershed orientation. No reduction is taken for orientation differences less than j:40.', regardless of area, or for drainage areas less than 300 square miles, regardless of orientation. Maximum PM!' reduction due to orientation is 15 percent. 5.1.6. Storm shape is given by criteria in HMf-I-52, with the PMS having a general elliptical isohyetal pattern with a ratio between major and minor axes ranging from two to five. Areas less than 300 square miles may use a circular ,;hape, i'l desired. the adopted ratio of the axes will be that which gives the most hydrologically-severe storm (one which maximizes volume) within 1I-e study watershed. This usually requires significant trail and error work. 5.1.7. With the maximized storm pattern established, the spatial variability 0' the precipitation is determined, again by HMR-52, through figures and nomographs. Spatial variability is greatest forthe maximum six,hour period. diminishes for the second and third greatest six-hour periods, and Ilas no variability for the remaining six-hour periods. Precipitation profiles are given in HM R-52 to develop the spatial variation in the peal, 18 hours of the PMS. 5.1.8. Final development of the PMS Calls for a temporal arrangement to give tile most critical hydrologic response (maximum runoff).. the six-hour values of the PMS are arranged such that they decrease progressively to either side of the greatest six-Ilour value. the four greatest six.hour increments are placed at any position in the sequence except during the first 24 hours. Only the general outline of PMS development has been given in this chapter. Tile engineer should refer to HMR-51 and HMR-5;, for detailE,d guidance in determination of the PMS. Colorado Flood Hydrology Manual DRiU'1' 7.12