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36 Columbia River drainage. The adopted variation with elevation, which is proportional to the variation in precipitable water, is consistent with the method used 'for moisture-maximizing the greatest observed least-orographic rains for guidance in setting the level of 1000-mb (lOO-kPa) convergence PMP. The maximum l2-hr persisting 1000-mb (lOO-kPa) dew points for August gen- eral storms (Schwarz and Hansen 1978) of 730 (230C) were used for determining the percent reduction due to effective barriers and elevations. The August dew points tend to give less reduction than winter dew points. High-eleva- tion rainfall would be unreasonably reduced if winter dew points were used, particularly because the use of a single moisture chart does not allow for the high wind and therefore higher rainfall capability at the higher eleva- tion in the cool season. Figure 2.18 shows the reduction (in percent) of 1000-mb (lOO-kPa) conver- gence PMP for effective barrier and elevation over the Southwestern States. There is agreement between the patterns shown in figures 2.17 (barrier/eleva- tion) and 2.18 (reduction of 1000-mb (lOO-kPa) convergence PMP) with one exception. Figure 2.18 contains a large area of 45% reduction in north- eastern Arizona, to the lee (northeast) of the Mogollon Rim. A continuous approximate 8,000-ft (2,440-m) barrier does not exist to support the 45% feature directly. We believe this factor is justified, since the effect of downslope motion behind the major barrier is to produce additional drying of the air which is equivalent to a higher effective barrier. Further downwind, the 45% reduction line has been closed off to indicate the gradual influence of recharge of moisture below 8,000 it (2,440 m). When using figure 2.18 to determine a percent of convergence PMP for a specific basin, interpolate between the isopleths. However, for locations that lie within closed contours or at the end of gradients, (within the 95% con- tour in southern California, and within the 50% contour in north-central Nevada, for example), the correct value is that of the last identified contour, i.e., do not extrapolate. 2.4 Depth-Duration Variation The 24-hr mid-month convergence PMP values can be extended to other dura- tions through application of rainfall depth-duration relationships. Durations between 6 and 72 hours are required. Relationships were developed from 6/24- hr, 48/24-hr and 72/24-hr ratios of rainfall in selected severe storms and from maximum rainfalls of record at recorder stations. Seasonal and regional variations of depth-duration relations are given. 2.4.1 Data Hourly precipitation data for up to 25 years (1948-72) were available on magnetic tapes for recorder stations listed in table 2.4. These stations are located in the least-orographic regions shown in figure 2.1. Stations A, B, C, D, and F in table 2.4 are geographically close to stations 3, 10, 11, 13, and 23, respectively, in table 2.2. An additional station at Baker, Cali- fornia (station E in table 2.4) was included in the southern Nevada subregion. Although some of these stations (A to F) had records exceeding 20 years, only