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OOOHl B. DIFFERENCE BETWEEN AVERAGE AND MEDIAN PRECIPITATION TOTALS IN SEMI-ARID CLIMATES It has been the policy in most climatological data publications. including this one, to present precipitation quantities as average precipitation by monthly totals for any particular location. This average (mean) is obtained by the simple mechan- ics of adding together all of the monthly totals for the series of record available and dividing that total number by the quantity of months used in the sample. This is a very easy method for obtaining a general indication of the precipitation that may be expected in a given area, but it can be definitely misleading if the array of precipitation quantities throughout the record is made up of a few .very high monthly totals and the majority of the monthly totals ranging around a much smaller value. The median value of monthly precipitation gives a better indicator of what to expect in the semi-arid region from which the Colorado River obtains its runoff . The median is defined as the point in a total sample which has half the number of individual values above it and half below it. In any semi-arid region which has many small storms and few large ones. the median value is consistently below the mean value. This fact is illustrated in Table IV I which shows the difference between monthly mean and monthly median in the three elevation groups used in Figure 2. The difference between the average and the median at high level stations per month is O. 24 inch. The difference at the middle level stations is 0.20 inch, and at low level stations. 0.18 inch. The most extreme case of relative importance is the month of June at low elevation stations when the arithmetic average is 0.61, while the median TABLE N 9 is only 0.40. Even at the high elevation stations the difference between average and median is generally greater than 10 per cent of the monthly values. C. PERCENTAGE OF STORM PERIODS GIVING VARIOUS FRACTIONS OF TOTAL ANNUAL RAINFALL 1. Percentage of Storm Periods Giving 25 Per Cent of the Annual Rainfall for the Water Year The percentage of the number of storm periods required to give one-fourth of the annual rainfall for the year is shown in Figure 7. Figure 7 shows the skewed nature of the annual precipita- tion amounts. In every case approximately 65 per cent of the storm periods are required to produce 25 per cent of the annual rainfall. Conversely. 75 per cent of the annual rainfall is contributed by only 35 per cent of all storms. Fort Collins. a station on the eastern slope of the Continental Divide, requires an exceptionally high percentage, 74.6 per cent of all storms. to produce 25 per cent of its annual precipitation. 2. Percentage of Storm Periods Giving 50 Per Cent of the Arumal Rainfall for the Water Year For all the stations analyzed, approximately 85 per cent of the storm periods are required to produce 50 per cent of the annual rainfall for the water year. The other 50 per cent is produced by only 15 per cent of all storms. (Figure 8). 3. Percentage of Storm Periods Giving 75 Per .Cent of the Annual Rainfall for the Water Year Approximately 95 per cent of the storm periods are required to produce 75 per cent of the COMPARISON OF GROUP MEAN OF AVERAGE MONTHLY PRECIPITATION AND GROUP MEAN OF MEDIAN MONTHLY PRECIPITATION FOR THREE ELEVATION GROUPS (See Fig. 2) Oct Nov Dec Jan Feb Mar Apr May June July Aug Sept High Level Stations Average 1. 69 1. 36 1.77 1. 94 1. 86 2. 18 2.04 1. 65 1. 34 2.25 2. 16 1.59 Median 1. 45 1. 18 1. 47 1. 64 1. 59 1. 95 1.76 1. 51 1.03 2.10 1. 82 1. 37 Difference .24 . 18 .30 .30 .27 .23 .28 .14 .31 . 15 .34 . 22 Middle Level Stations Average 1.36 .99 1.29 1. 35 1. 24 1. 37 I. 36 1. 40 1.03 1. 47 1. 62 1. 18 Median 1. 16 .88 1. 06 1. 10 1. 03 1.23 1. 19 1. 26 .77 1.23 1. 39 ,96 Difference .20 .11 .21 .25 .21 .14 . 17 .14 .26 .24 .23 , 22 Low Level Stations Average 1. 17 .74 .93 .95 .84 .92 .95 .92 .61 1. 03 1. 35 1. 08 Median .99 .54 .77 .78 .73 .78 .79 .70 .40 ,88 1.17 , 69 Difference . 18 .20 .16 . 17 .11 . 14 . 16 .22 .21 .15 . 18 , 19