ral variations. The total PMP represents a combination of orographic and conver-gence PMP.

Estimates of local storm PMP for the Colorado River and Great Basin drainages and for California can be determined using HMR 49. Estimates of local storm PMP for the northwestern states are described in HMR 43. To derive a local storm for areas less than 500 mi2 and durations less than 6 h, the average 1-h 1-mi2 PMP is chosen from regionalized charts in the appropriate HMR. These values are then adjusted for basin elevation and size and are distributed over time. Elliptically shaped isohyetal patterns are used to account for basin shape and storm center location in the estima-tion of local storm PMP.

PMP Estimates for California.

Procedures for estimating PMP in California are described in HMR 36 [15]. The total PMP consists of two parts: (1) orographic component and (2) restricted convergence component.

The orographic component is based on the following:

  1. orographic PMP index map (Fig. 14-3),

  2. orographic PMP computation areas (Fig. 14-4),

  3. basin-width variation (Fig. 14-5),

  4. seasonal variations (Table 14-1), and

  5. durational varia-tions (Table 14-2).

The restricted convergence component PMP is based on the following: (1) con-vergence PMP index map (Fig. 14-6) and (2) variation of convergence PMP with basin size and duration (Fig. 14-7). The PMP is the greatest of either (a) the sum of oro-graphic plus restricted convergence PMP or (b) the unrestricted convergence PMP, calculated as four-thirds of the restricted convergence PMP. A detailed step-by-step procedure to develop a PMP estimate for California follows.

OROGRAPHIC PMP

  1. Use Fig. 14-3 to determine a basin-average orographic PMP index (a grid average is adequate).

  2. Determine the representative basin width, measured perpendicular to the parallel sides of one of the orographic computation areas shown in Fig. 14-4 that is closest to the basin under study.

  3. Enter Fig. 14-5 with the representative basin width to determine the basin-width adjustment factor, in percent.

  4. The adjusted basin-average orographic PMP index is obtained by multiplying the basin-width adjustment factor by the basin-average orographic PMP index and dividing by 100. The January 6-h orographic PMP is equal to the adjusted basin-average orographic PMP index.

  5. Use Table 14-1 to determine the monthly 6-h orographic PMP for the months of October to April.

  6. Use Table 14-2 to determine the 6-h orographic PMP incremental values (in percent of first 6-h period) for durations ranging from 12 to 72 h.

  7. For small basins, the 1-h and 3-h orographic PMP values are 20 and 54 percent, respectively, of the 6-h orographic PMP.
Figure 14-3. California: Orographic PMP Index, 6-h January (in.) [15]. Figure 14-4. California: Orographic PMP Computation Areas [15]. Figure 14-5 California: Basin-width adjustment [15]. TABLE 14-1 SEASONAL VARIATION OF OROGRAPHIC PMP IN CALIFORNIA1 [15] (in percent of adjusted basin-average orographic PMP index) 1 The Sierra range values are for basins located to the east of a line through the middle of the Central Valley between Redding and Bakers-field. Coastal range percentages apply to the remaining areas of interest in California. TABLE 14-2 DURATIONAL VARIATION OF OROGRAPHIC PMP IN CALIFORNIA [15] (in percent of first 6-h period) CONVERGENCE PMP (Restricted convergence PMP to be combined with orographic PMP)

  1. Use Fig. 14-6 to determine a basin-average value of convergence PMP index (a grid average is adequate).

  2. Use Fig. 14-7 to determine a table of 6-h PMP increments, in percent of conver-gence PMP index, for the months of October to April. After the third or fourth
Figure 14-6 California: Convergence PMP Index, 6-h 200-mi2 January (in.) [15]. Figure 14-7(a) California: Variation of Convergence PMP with Basin Size and Duration, October and November [15]. Figure 14.7(b) California: Variation of Convergence PMP with Basin Size and Duration, December, January and February [15]. Figure 14-7(c) California: Variation of Convergence PMP with Basin Size and Duration, March and April [15].
    increment, the 6-h increments are independent of basin size, as indicated in Fig. 14-7. The 1-h and 3-h convergence PMP percentages are also included in Fig. 14-7.

  1. Use the percentages obtained in the previous step to determine the 1-h, 3-h, or the 6-h incremental restricted convergence PMP, on a monthly basis.

    For durations in excess of 6-h, the 6-h increments are summed to obtain the PMP values for desired durations. For each duration, the all-season PMP is the high-est monthly total. See [151 for additional temperature and wind criteria for determin-ing snowmelt contribution to PMP.

    Example 14-1. Compute the PMP for a 1540-mil basin located in the large Sierra Nevada slope basin, California, with coordinates 38° N, 119° 45' W (adapted from [15]).

    OROGRAPHIC PMP

    1. From Fig. 14-3, the basin-average orographic PMP index is 4.86 in.

    2. From Fig. 14-4, the representative basin width is measured perpendicular to the parallel sides of Area No. 26. Assume 30 mi for the basin of this example.

    3. From Fig. 14-5, the basin-width adjustment factor is 100 percent.

    4. The adjusted basin-average orographic PMP index is 4.86 X 100/100 = 4.86 in. Therefore, the 6-h January orographic PMP is 4.86 in.

    5. From Table 14-1, the monthly 6-h orographic PMP values (Sierra Range), in inches, are: October, 4.71; November, 4.76; December, 4.81; January, 4.86; February, 4.86; March, 4.67; and April, 4.37. 6. From Table 14-2, for 38° N latitude, the 6-h incremental orographic PMP values are obtained, as shown in Table 14-3.

    6. It is not necessary to compute 1-h and 3-h PMP values for this large-area basin.
    CONVERGENCE PMP (Restricted convergence PMP to be combined with orographic PMP)

    1. From Fig. 14-6, the basin-average value of convergence PMP index is 2.26 in.

    2. From Fig. 14-7, a table of 6-h PMP increments, in percent of convergence PMP index, for the months of October to April, is developed (see Table 14-4). It is not necessary to compute 1-h and 3-h PMP values for this large-area basin.
    TABLE 14-3 6-H INCREMENTAL OROGRAPHIC PMP: EXAMPLE 14-1 (in.)

    1. Table 14-5 shows the 6-h incremental restricted convergence PMP, in inches, calculated by multiplying the values in Table 14-4 by the convergence PMP index obtained in step 1 (2.26 in.).

    Because of the pronounced orographic PMP component for this basin, it is not necessary to calculate the unrestricted convergence PMP. The total PMP for this basin is obtained by adding the values of Tables 14-3 and 14-5. Table 14-6 shows the 6-h incremental con-vergence and orographic PMP values. Table 14-7 shows the cumulative values of PMP for durations every 6-h interval. For each duration, the all-season PMP is the highest monthly total shown in Table 14-7.

    Other PMP Estimates. HMR 51 and HMR 52 (United States east of the 105th Meridian) and HMR 36 (California) are typical of the methodologies used to obtain generalized PMP estimates. However, other HMR publications may differ from these, if not in principle, at least in detail. For a basin located in a given geo-graphic region, an estimate of PMP should be based on the methodology outlined in the applicable HMR reference [15-30]. Time Distribution of PMP. In calculating the 6-h PMP increments, the first 6-h increment is interpreted as the first in magnitude rather that the first in sequence. The 6-h PMP increments are used to develop a critical storm pattern, i.e., a probable maximum storm. In turn, this probable maximum storm is used to calculate the PMF.

    Critical storm patterns are obtained by ordering 6-h PMP increments based on the following sequencing rule: For each duration, the second-largest 6-h increment TABLE 14-4 6-H CONVERGENCE PMP INCREMENTS: EXAMPLE 14-1 (in percent of convergence PMP index) TABLE 14-5 6-H INCREMENTAL RESTRICTED CONVERGENCE PMP: EXAMPLE 14-1 (in.) TABLE 14-6 6-H INCREMENTAL RESTRICTED CONVERGENCE PLUS OROGRAPHIC PMP: EXAMPLE 14-1 (in.) TABLE 14-7 CUMULATIVE PMP FOR INDICATED DURATIONS: EXAMPLE 14-1 (PMP in inches and duration in hours)

    must be adjacent to the largest in order to provide the most critical 12-h combination; the third-largest increment should be positioned immediately before or after this 12-h sequence in order to provide the most critical 18-h combination; and so on. Sample 24-h storm patterns following this critical sequencing rule are shown in Fig. 14-8(a) and (b).

    The examination of 72-h hyetographs has shown that storms of this duration typically consist of two or more peaks or bursts. Time sequence patterns which show more than one peak are obtained by the following procedures:

    1. Group the four largest 6-h increments of the 72-h PMP in a first 24-h sequence, the middle four increments in a second 24-h sequence, and the smallest four increments in a third 24-h sequence.

    2. Within each of these three 24-h sequences, arrange the four increments in accor-dance with the critical sequencing rule, i.e., the second largest next to the larg-est, the third largest adjacent to these, and the fourth at either end.

    3. Arrange the three 24-h sequences in accordance with the critical sequencing rule, that is, the second-largest 24-h period next to the largest, and the third at either end. Sample 72-h storm patterns obeying this critical sequencing rule are shown in Fig. 14-8(c) to (e). In practice, it may be necessary to experiment with several possible sequences in order to determine the most critical sequence for a particular basin.