manual for estimation of probable maximum precipitation - WMO

INTRODUCTION 3
intervals. In addition to their use in PMP studies, these analyses can also
be useful in a study of the hydrologic characteristics of a region.
1.2
1.2.1
LrnER AND UPPER LllIITS OF PMP
Accuracy of PMP estimates
There is no objective way of assessing the accuracy of the
magnitude of PMP estimates derived by the procedures described here or by
any other known procedures. Judgement of meteorologists, based on
meteorological principIes and storm experience, is mos t importan t ..
Obviously, estimates subsequently exceeded by Dbserved storm rainfall were
too low. The accuracy of PMP estimates may be generally assessed by
consideration of the following factors: (1) excess of estimated PMP over
the maximum observed rainfall values for the surrounding meteorologically
homogeneous region; (2) number and severity of recorded storms in the region
considered, if a long record is available; (3) limitations on storm
transposition in the region; (4) number, character and interrelationship of
maximizing steps; (5) reliability of any model used for relating rainfall
to other meteorological variables; and (6) probability of exceeding the
individual meteorological variables used in such models, with care being
taken to avoid excessive compounding of probabilities of rare events. That
the procedures described here for deriving estimates of PMP yield results to
the nearest millimetre or tenth. of an inch should not be taken as an
indication of the degree of accuracy of the estimates.
Subsequent chapters show that various steps in the procedures
require meteorological judgement. Consequently, the resulting estimates are
dependent on decisions affecting the degree of maximization used in their
derivation. It is the task of the meteorologists to balance these decisions
to make a realistic estimate. Since alternate decisions are possible at
some steps, lower and upper limits to PMP can be estimated, although in
practice these limits are not determined and only· one set of values, or
"best estimate," is usually derived.
1.2.2 Confidence bands
The delineation of lower and upper limits to PMP estimates is
somewhat analogous to the confidence bands used in statistics. It would be
convenient if a confidence band could be placed about a PMP estimate in an
objective manner, similar to the standard statistical method, but this is
not possible because PMP is not estimated by formal statistical
procedures. This limitation, however, does not invalidate the" concept of a
confidence band about the estimate, but it means that such limits must be
based in considerable measure on judgement, as is the PMP estimate itself.
Factors influencing such judgement are the same as those for assessing the
appropriate magnitude of PMP listed in the preceding paragraph.
1.3
1.3.1
THE MANUAL
Purpose
The following statement, published in a UN/WHO report [United
Nations/WMO,1967J, was cited in the original manual and is repeated here: