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39 the longer record station was used in the studies for determining the magni- tude and regional and seasonal variation of convergence PMP. Additional data were sought from major storms of record for which there were large rainfalls in least-orographic regions. Almost all major storms in the Southwest have their centers in orographic regions; thus, it is difficult to obtain large amounts (more than one inch in 24 hours) in least-orographic regions. Data from the August 1951 and the northern center of the September 1970 storms along with seven lesser nonsunnner storms were considered for guid- ance in establishing the seasonal variation of durational relations. The latter storms are listed in table 2.5. Table 2.5.--Nonsummer storms in the Southwest and the number of stations with relatively large rainfalls in least-orographic regions, used in duration analysis of convergence PMP. Date No. of stations Location Dec. 14-17, 1908 4 W. Cent. Arizona Dec. 17-24, 1914 6 S. Arizona Jan. 14-20, 1916 5 S. Arizona Feb. 01-07, 1905 5 SE Calif., S. Ariz. Feb. 10-22, 1927 3 S. Utah Mar. 11-17, 1941 3 SE Calif., S. Ariz. Apr. 05-10, 1926 2 S. Arizona 2.4.2 Depth-Duration Relation A depth-duration relation of PMP for an area size indicates the relation- ship between PMP values for various durations. It can qe specified by a smooth curve of duration vs. depth (either in inches or percent of the value for a selected duration) or mathematically by ratios of the depths for var- ious durations to that say of 24 hours. A PMP depth-duration relation is based on the concept that the average intensity of rainfall decreases with increasing duration. This concept is analogous to that in depth-area rela- tions of PMP in which precipitation decreases with increasing area size. It might be well to point out that a depth-duration relation of PMP does not specify the time sequence in which incremental rain will fall. A smooth depth-duration relation can be quite well defined by the 6/24- and 72/24-hr ratios of rainfall. Some regional PMP studies have used one depth-duration relation for the en- tire region. From preliminary examination of 6/24-hr ratios of rainfall, it was apparent that seasonal and regional variations precluded use of a single relation for the Southwestern States. As an alternative, a concept of a family of smooth depth-duration relations was envisioned that would cover the range of probable relations required. When expressed in percent of the 24-hr amount, the concept of a smooth family of curves that require a continually decreasing rate of rainfall intensity in- volves an inverse relationship: Where the short-duration value is high, the long-duration value with which it is associated is low, and vice versa. In effect, this implies that high 6/24-hr ratios relate to low 72/24-hr ratios, and that low 6/24-hr ratios relate to high 72/24-hr ratios.