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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-hour time 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 PMP, Considerable trail and 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 that 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 area size and the angle between the storm and watershed orientation, No reduction is taken for orientation differences less than :1:40.0. regardless of area. or for drainage areas less than 300 square miles, -regardless of orientation, Maximum f'MP reduction due to orientation is 15 percent, 5.1.6. Storm shape is given by criteria in HMR-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 shape. if desired, the adopted ratio of the axes will be that which gives the most hydrologically-severe storm (one which maximizes volume) within the study watershed, This usually requires significant trail and error work, 5.1.7. With the maximized storm pattern established, the spatial variability of the precipitation is determined, again by HMR-52. through figures and nomographs, Spatial variability is greatest for the maximum six-hour period, diminishes for the second and third greatest six-hour periods. and has no variability for the remaining six-hour periods, Precipitation profiles are given in HMR-52 \0 develop the spatial variation in the peak 18 hours of the PMS, 5.1.8. Final development of the PMS calls for a temporal arrangement to give the 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-hour 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, The engineer should refer to HMR-51 and HMI'l-52 for detailed guidance in determination of the PMS, Colorado Flood Hydrology Manual DRAFI' 7,12