A probabilistic approach to the concept of Probable Maximum Precipitation

Abstract

The concept of Probable Maximum Precipitation (PMP) is based on the assumptions that (a) there exists an upper physical limit of the precipitation depth over a given area at a particular geographical location at a certain time of year, and (b) that this limit can be estimated based on deterministic considerations. The most representative and widespread estimation method of PMP is the so-called moisture maximization method. This method maximizes observed storms assuming that the atmospheric moisture would hypothetically rise up to a high value that is regarded as an upper limit and is estimated from historical records of dew points. In this paper, it is argued that fundamental aspects of the method may be flawed or inconsistent. Furthermore, historical time series of dew points and "constructed" time series of maximized precipitation depths (according to the moisture maximization method) are analyzed. The analyses do not provide any evidence of an upper bound either in atmospheric moisture or maximized precipitation depth. Therefore, it is argued that a probabilistic approach is more consistent to the natural behaviour and provides better grounds for estimating extreme precipitation values for design purposes.

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Advances in Geosciences, 7, 51–54, 2006
SRef-ID: 1680-7359/adgeo/2006-7-51
European Geosciences Union
© 2006 Author(s). This work is licensed
under a Creative Commons License.
Advances in
Geosciences
A probabilistic approach to the concept of Probable Maximum
Precipitation
S. M. Papalexiou and D. Koutsoyiannis
Department of Water Resources, School of Civil Engineering, National Technical University of Athens, Greece
Received: 8 November 2005 – Revised: 12 December 2005 – Accepted: 14 December 2005 – Published: 24 January 2006
Abstract. The concept of Probable Maximum Precipitation
(PMP) is based on the assumptions that (a) there exists an up-
per physical limit of the precipitation depth over a given area
at a particular geographical location at a certain time of year,
and (b) that this limit can be estimated based on deterministic
considerations. The most representative and widespread esti-
mation method of PMP is the so-called moisture maximiza-
tion method. This method maximizes observed storms as-
suming that the atmospheric moisture would hypothetically
rise up to a high value that is regarded as an upper limit and
is estimated from historical records of dew points. In this pa-
per, it is argued that fundamental aspects of the method may
be flawed or inconsistent. Furthermore, historical time series
of dew points and “constructed” time series of maximized
precipitation depths (according to the moisture maximization
method) are analyzed. The analyses do not provide any ev-
idence of an upper bound either in atmospheric moisture or
maximized precipitation depth. Therefore, it is argued that
a probabilistic approach is more consistent to the natural be-
haviour and provides better grounds for estimating extreme
precipitation values for design purposes.
1 Introduction
The Probable Maximum Precipitation (PMP) is defined as
“theoretically the greatest depth of precipitation for a given
duration that is physically possible over a given size storm
area at a particular geographical location at a certain time
of year” (WMO, 1986). Even though, the PMP approach
has been widely proposed and used as design criterion of
major flood protection works (Schreiner and Reidel, 1978;
Collier and Hardaker, 1996), severe criticism has been made
by hydrologists not only to the concept of the PMP, which
practically assumes a physical upper bound of precipitation
amount, but also to the fact that this limit can be estimated
Correspondence to: S. M. Papalexiou
(sp@itia.ntua.gr)
based on deterministic considerations (Benson, 1973; Kite,
1988; Dingman, 1994; Shaw, 1994; Koutsoyiannis, 1999).
The main scope this paper is to apply the PMP estimation
method and to make a probabilistic analysis of its results. An
additional objective was to find an appropriate probabilistic
model capable of describing the empirical distribution of the
monthly maximum daily dew points, in order to be used in
the application of the method. Last, but not least, an exclu-
sively probabilistic approach was applied to the annual max-
imum rainfall depths.
The methodology was applied to four stations in Nether-
lands (De Bilt, Den Helder, Groningen, Maastricht) and
the station of the National Observatory of Athens, Greece
(NOA).
2 Method overview
Techniques used for estimating PMP have been listed by
Wiesner (1970), as follows: (1) the storm model approach;
(2) the maximisation and transposition of actual storms; (3)
the use of generalised data or maximised depth, duration and
area data from storms; these are derived from thunderstorms
or general storms; (4) the use of empirical formulae deter-
mined from maximum depth duration and area data, or from
theory; (5) the use of empirical relationships between the
variables in particular valleys (only if detailed data are avail-
able); (6) statistical analyses of extreme rainfalls.
In this study, the most representative and widely used esti-
mation method of PMP, the so-called moisture maximization
method, is examined. The method is based on the simple
formula
hm=Wm
Wh, (1)
where hmis the maximized rainfall depth, his the observed
precipitation, Wis the precipitable water in the atmosphere
during the day of rain, estimated by the corresponding daily
dew point Td, and Wmis the maximized precipitable water,

52 S. M. Papalexiou and D. Koutsoyiannis: The concept of Probable Maximum Precipitation
0.00
0.05
0.10
0.15
0.20
0.25
-0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30
L-skewness
L-kurtosis
NOA st atio n Netherlands station
s
Average Gumbel
Weibull max n=5 Weibull max n=30
Weibul 3P GEV
Gum b e l n=5
n=30
Fig. 1. L-moment ratio diagram of maximum daily dew points for
each month.
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
NOΑDe Bilt Den Helder Groningen Maastricht
Stations
L-skewness
Ave rage L-skew ness of monthly
max imu m rai nf all dep th s
Ave rage L-skew ness of monthly
maximized r ainfall depths
Fig. 2. Average L-skewness of monthly maximum and maximized
daily rainfall depths.
estimated by the maximum daily dew point Td,m of the cor-
responding month.
The term Td,m is estimated either as the maximum histor-
ical value from a sample of at least 50 years length, or as the
value corresponding to a 100-years return period, for samples
smaller than 50 years (WMO, 1986).
3 Statistical analysis of dew points
The Gumbel distribution (Gumbel, 1958), which is the
most common probabilistic model for hydrological extremes,
proved inadequate for describing the empirical distribution
of the monthly maximum daily dew points (Papalexiou,
2005). Therefore, the probability theory of maxima was ap-
plied. According to that theory, given a number of ninde-
pendent identically distributed random variables, the largest
of them (more precisely, the largest order statistic), i.e.
X=max(Y1, . . ., Yn), has probability distribution function
Hn(x)=[F(x)]n,(2)
where F(x)=P(Yi≤y)is the probability distribution func-
tion of each Yi, referred to as the parent distribution.
-10
40
90
140
190
240
290
0 20 40 60 80 100 120 140
Raifall number
Rainfall depht ( mm)
maximized r ainfall depht (mrd)
daily rainf all depht correspon ding to mrd
maximum monthly precipitable water cor responding to mrd
daily prec ipitable w ater cor responding to mrd
PMP
Fig. 3. Maximized rainfall depths of the NOA station and related
quantities.
The frequency analysis for the daily dew points indicated
the three-parameter Weibull model as a sufficient probabilis-
tic model for describing their empirical distribution (Papalex-
iou, 2005). As a result, it can be used as parent distribution,
so that the theoretical maximum distribution of the monthly
maximum daily dew point can be described by Eq. (2), where
nstands for the days of each month.
Since the condition of independence of random variables
does not hold, as shown from the high values of autocorre-
lation coefficients, the exponent nwas expected and proved,
indeed, lower than the number of days in a month (Papalex-
iou, 2005). Moreover, given the uncertainties related to the
estimation of the three parameters of the parent distributions,
as well as the uncertainty of the value of the exponent n, we
implemented a “parallel” optimization approach, by simulta-
neously fitting the theoretical models F(x)and Hn(x)to the
empirical distributions of daily and monthly maximum daily
dew points. The objective function is written as
LSET ot al =LSE (F(x)) +[LSE (Hn(x))]2,(3)
where LSE is the least square error between the theoretical
and the empirical distributions. This strategy helped to better
fit the theoretical maximum distribution derived by the parent
distribution.
The L-moment ratio diagram (Vogel and Fennessey, 1993;
Stedinger et al., 1993) shown in Fig. 1, illustrates that the
theoretical maximum distribution derived from the parent
three-parameter Weibull distribution is more appropriate than
the asymptotic ones (Gumbel, Generalized Extreme Value or
GEV).
4 Application of the PMP estimation method
The maximized precipitation time series were analysed in
comparison to the observed ones. It was concluded that the
maximization process causes sometimes a disproportional
increase in the range of the values of recorded rainfall and
that the maximized samples exhibit higher skewness than

S. M. Papalexiou and D. Koutsoyiannis: The concept of Probable Maximum Precipitation 53
1000050002000100050020010050201052
0
50
100
150
200
250
300
350
400
450
Return period (years)
Rainfall depht (mm)
E.D. of annual maximum
GEV MLM of max imu m ann ual
E.D. of max imiz ed mo nth ly
GEV MLM of mon thl y ma ximi zed
E.D. of monthly maximum
GEV MLM of mon thl y ma ximu m PM P
Fig. 4. Probability plot of annual maximum, monthly maximum
and monthly maximized daily rainfall depths of NOA (on Gumbel
probability paper).
0
50
100
150
200
250
300
350
100 1000 10000 100000 1000000
Return period of monthly maximum dew point (years)
PMP (mm )
NOA
De Bi lt
Den He lder
Groningen
Maastricht
Fig. 5. PMP estimations for various return periods of monthly max-
imum daily dew point.
the recorded ones (Fig. 2), especially when the sample L-
skewness values are low.
Figure 3 illustrates the values of maximized rainfall depths
of the station of NOA in descending order, the concurrent
daily-recorded rainfall depths, the concurrent daily precip-
itable water and the maximum monthly precipitable water.
The 120 maximized rainfall depths that are illustrated in
Fig. 3 (the highest is the estimated PMP), are the result of the
merging of the 10 maximized rainfall events of each month,
produced by the 10 maximum rainfall events of the respective
month. It is evident that the estimated PMP point is located
in a very uncertain area of the curve, where the slope is very
high.
In addition, as shown in Fig. 3, if the record was shorter
or had a missing data point (the most left in Fig. 3) the PMP
value would be downgraded from 240 to about 190mm.
Moreover, the sample of the 120 maximized rainfall
depths was analyzed in the same probabilistic manner as the
maximum monthly and annual rainfall depths were analyzed.
Figure 4 depicts the relevant data of NOA. The fitted distri-
butions (GEV) to the maximum monthly and annual rainfall
depths, as Fig. 4 suggests, have no upper bound, so it is very
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45
L-skewness
L-kurtosis
Gumbel
GEV
NOA
De Bi lt
Den Helde r
Groningen
Maa s t r ic h t
Gum b e l
Fig. 6. L-moment ratio diagram for annual maximum daily rainfall
depths.
% probability o f excee dence
0.01
0.02
0.05
0.1
0.2
0.5
1
2
5
10
20
30
50
70
90
95
99
99.9
99.99
2 5 10 20 50 100 200 500 1000 2000 500010000
0
50
100
150
200
250
300
350
400
450
500
Return period (years)
Rainfall depth ( mm)
PMP
Gu mbel L -mo ment s
GEV L- momen ts
GEV Least s quare error
GEV Max imum l ikel iho od Estimated PMP
Fig. 7. Probability plot of annual maximum daily rainfall values of
NOA station on Gumbel probability paper.
likely that if a longer rainfall record were available, the esti-
mate of the PMP would be higher.
Furthermore, if solely the distribution of maximized rain-
falls is taken under consideration, then the estimate of PMP
is the outcome of an extremely uncertain estimation of the
first order statistic.
Finally, the PMP estimation method was applied with re-
gard to the monthly maximum daily dew point for a wide
range of return periods, admitting that the WMO suggestion
to use a 100 year return period is arbitrary and this return
period could be well assumed greater. The distribution of
the monthly maximum daily dew point was the one derived
by the three-parameter Weibull parent distribution. The es-
timate of the PMP, as illustrated in Fig. 5, is an increasing
function of the monthly maximum daily dew point.
5 A probabilistic approach for the annual maximum
daily rainfall depth
If one abandons the effort to put an upper limit to precipi-
tation and the deterministic thinking behind it, the next step
is to model maximum rainfall probabilistically. As the L-
moment ratio diagram of the annual maximum daily rainfall

54 S. M. Papalexiou and D. Koutsoyiannis: The concept of Probable Maximum Precipitation
(Fig. 6) suggests, the GEV distribution describes sufficiently
the empirical distribution of the annual maximum daily rain-
fall depths.
The GEV model was fitted on the historical data with three
different methods, the method of Least Square Error (Pa-
poulis, 1990), the method of L-Moments (Hosking, 1990)
and the method of Maximum Likelihood (Fisher, 1922). Ac-
cording to the fitted probabilistic model, the estimate of the
PMP value is associated with a return period or probability
of exceedence. It can be concluded that this probability is
not negligible. The above analysis was also conducted us-
ing the Gumbel model, which obviously underestimates the
exceedence probability of the PMP (Fig. 7).
6 Conclusions
In the above analysis, no evidence for an upper bound of dew
point and of precipitation was found. The estimation of the
PMP based on the moisture maximization concept is consid-
erably uncertain and was proven to be too sensitive against
the available data.
The study showed that the existence of an upper limit on
precipitation, as implied by the PMP concept, is statistically
inconsistent. Moreover, such a limit cannot be specified in
a deterministic way, as the method asserts; in reality, from a
statistical point-of-view, this “limit” tends to infinity.
According to the probabilistic analysis on the annual daily
maximum rainfall depths, the hypothetical upper limit of the
PMP method corresponds to a small, although not negligible,
exceedence probability. For example, this probability for the
Athens area is 0.27%, a value that would not be acceptable
for the design of a major hydraulic structure.
A probabilistic approach, based on the GEV model,
seems to be a more consistent tool for studying hydrological
extremes.
Edited by: V. Kotroni and K. Lagouvardos
Reviewed by: A. Koussis
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