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38 Chapter 2 Water Sources: Surface Water
more often than once in 20 years. In still drier years, it may be necessary to curtail the use
of water by limiting or prohibiting, for example, lawn sprinkling and car washing.
Restricting water use is irksome to the public and a poor way to run a public utility. As
a practical matter, moreover, use must be cut down well in advance of anticipated exhaus-
tion of the supply. It would seem logical to consider not only the frequency of curtailment
but also the depletion point at which conservation should begin. In practice, the iron ration
generally lies between 20% and 50% of the total water stored. Requiring a 25% reserve for
the drought that occurs about once in 20 years is reasonable. An alternative is a storage al-
lowance for the drought to be expected once in 100 years. This is slightly less in magnitude
than the combination of a 25% reserve with a once-in-20-years risk.
In undeveloped areas, few records are even as long as 20 years. Thus, estimation of the
5%, 2%, and 1% frequencies, or of recurrence intervals of 20, 50, and 100 years, requires ex-
trapolation from available data. Probability plots lend themselves well to this purpose.
However, they must be used with discretion. Where severe droughts in the record extend over
several years and require annual rather than seasonal storage values to be used, the resulting
series of storage values becomes nonhomogeneous and is no longer strictly subject to ordi-
nary statistical interpretations. They can be made reasonably homogeneous by including, be-
sides all truly seasonal storage values, not only all true annual storage values, but also any
seasonal storage values that would have been identified within the periods of annual storage
if the drought of the preceding year or years had not been measured. Plots of recurrence in-
tervals should include minor storage capacities as well as major ones. The results of these
statistical analyses are then conveniently reduced to a set of draft-storage-frequency curves.

EXAMPLE 2.2 DESIGN OF STORAGE REQUIREMENT FOR VARIOUS FREQUENCIES
Examination of the 25-year record of runoff from an eastern stream shows that the storage amounts
2
listed in Table 2.3 are needed in successive years to maintain a draft of 750,000 gpd/mi (1,096,000
2
L/d/km ). Estimate the design storage requirement that is probably reached or exceeded but once in
20, 50, and 100 years.
Table 2.3 Storage Requirements (Example 2.2)

Order of year 1 2 3 4 5 6 7 8 9 10 11 12 13
Calculated storage, MG 47 39 104 110 115 35 74 81 124 29 37 82 78

Order of year 14 15 16 17 18 19 20 21 22 23 24 25
Calculated storage, MG 72 10 117 51 61 8 102 65 73 20 53 88

Conversion factors: 1 MG 1,000,000 gal 3.785 ML 3,785,000 L.
Solution:
1. The 25 calculated storage values arrayed in order of magnitude are plotted on arithmetic-
probability paper in Fig. 2.8 at 100n/n 1; 100 1>26 3.8, 100 2>26 7.7,
100 3>26 11.5%, and so forth. A straight line of best fit is identified in this instance,
but not necessarily others, the arithmetic mean storage being 67 MG (254 ML) and
the standard deviation 33 MG (125 ML).
2. The storage requirements reached or exceeded once in 20, 50, and 100 years, or 5%, 2%,
and 1% of the time, are read as 123, 137, and 146 MG, respectively (466, 519, and 553
ML, respectively). Probability paper is used because it offers a rational basis for projecting
the information beyond the period of experience. The once-in-20-years requirement with