Annual Journal of Hydraulic Engineering, JSCE, Vol.52, 2008, February
Annual Journal of Hydraulic Engineering, JSCE, Vol.52, 2008, February

DEVELOPMENT OF REGIONAL RAINFALL
INTENSITY-DURATION-FREQUENCY CURVESBASED ON SCALING PROPERTIESLe Minh NHAT 1, Yasuto TACHIKAWA2, Takahiro SAYAMA3 and Kaoru TAKARA4
1Student Member, Graduate student, Dept. of Urban and Environmental Eng., Kyoto University(Kyoto 615-8540, Japan) E-mail: nhat@flood.dpri.kyoto-u.ac.jp
2Member of JSCE, Dr. Eng., Associate Professor, Dept. of Urban and Environmental Eng., Kyoto University(Kyoto 615-8540, Japan)
3Member of JSCE, Dr. Eng., Assistant Professor, DPRI, Kyoto University (Uji 611-0011, Japan)
4Fellow of JSCE, Dr. Eng., Professor, DPRI, Kyoto University (Uji 611-0011, Japan)

This study developed a regional Intensity–Duration–Frequency (IDF) relationship for ungauged locations
based on the scaling theory. The scaling properties of extreme rainfall are examined to establish scaling
relationship behaviors of statistical moments over different durations. The results show that a rainfall
property in time does follow a simple scaling process. A scale invariance concept is explored for
disaggregation (or downscaling) of rainfall intensity from low to high resolution and is applied to thederivation of scaling IDF curves. These curves are developed for ungauged sites based on scaling of theExtreme Value type 1 (EV1) or Gumbel probability distributions. The spatial distribution maps of threeparameters: the scaling exponent and two statistical parameters are constructed. By using these maps, theIDF relationships are deduced from only daily rainfall with the scaling approach, which shows good
agreements in comparison with the IDF curves obtained from traditional techniques.
Key Words:

regional analysis, scale invariance, IDF curves, design rainfall, ungauged.

1. INTRODUCTION
The knowledge of rainfall Intensity Duration
Frequency (IDF) relationships is of fundamental
importance in hydrology. The IDF curves are usedin developing design storms and are also needed inmany hydrologic models and procedures incomputation of water quantity and quality
characteristics. For the site for which sufficient
rainfall data available, frequency analyses arecommonly used for design of various hydraulicstructures. In particular, precipitation frequency
analysis studies are necessary for development of adesign storm. There have been considerable
attentions and researches on the IDF relationship.
Hershfield1) developed various rainfall contour maps
to provide the design rain depths for various return
periods and durations. Koutsoyiannis et al.2) citedthat the IDF relationship is a mathematical
relationship between the rainfall intensity, theduration and the return period. Sivapalan and
Blöschl3) proposed an approach for constructing
catchment IDF curves based on the spatial
correlation structure of rainfall. Nhat et al.4)
estimated IDF curves, constructed the parameter
contour maps of IDF functions and generalized IDF
formulas for the monsoon area of Vietnam.
However, these techniques have limitations toestimate the IDF characteristics for ungauge sites.
For the site where rainfall record is unavailable or
the data sample at the site is limited,“Regionalization” and “Scaling” methods should bestudied more to transfer the rainfall informationfrom one location to others.
Over the last two decades, concepts of scale
invariance have come to the fore in both modelingand data analysis in hydrological precipitation
research. Gupta and Waymire5) studied rainfall
spatial variability by introducing the concepts ofsimple and multiple scaling to characterize theprobabilistic structure of the precipitation processes.
Burlando and Rosso6) showed that both the simple
scaling and multiscaling lognormal models can beused to derive Depth Duration Frequency (DDF)
curves of point precipitation. Nguyen et al.7)
proposed a Generalized Extreme Value distribution
model for regional estimation of short duration
rainfall extremes based on the scaling theory.
Menabde et al.8) developed a simple scaling
methodology to use daily rainfall statistics to infer
the IDF curves for rainfall duration less than one
day. Pao-shan, et al. 9) is an example ofmethodology in which the theories of scaling
properties are employed to infer the IDF
characteristics of short-duration rainfall from daily
data. Kuzuha et al. 10) showed the scaling framework- 85 -

and regional flood frequency analysis. Nhat et al.11)
showed the existence of the simple scaling in timeand space within some ranges.
The purpose of this study is to investigate ‘scale
invariance’ or ‘scaling’ properties of rainfall forderivation of IDF relationships at ungauged site. At
first, the scaling exponent for each gauge and thespace variation of the scale exponents were
investigated by using the annual maximum rainfall
intensity series for various duration and return
periods at the selected twenty stations in the
Yodo River catchment. The IDF formulas can bederived based on three parameters: the scale
exponent and two statistical parameters of 24 hours
rainfall data only. Secondly, three parameters areinterpolated from the contour maps of theseparameters and the IDF at any sites are generated.
Finally, the IDF relationships at the ungauged sites
with the regional scaling model are examined,
which shows good agrrements with the IDF curves
obtained from conventional techniques.

2. THEORETICAL FRAMEWORKTHE SIMPLE SCALINGOF

The scaling or scale-invariant models enable us
to transform hydrologic information from one
temporal or spatial scale to another one, and thus,
help overcome the difficulty of inadequatehydrologic data. A natural process fulfills the simple
scaling property if the underlying probability
distribution of some physical measurements at one
scale is identical to the distribution at another scale.
The basic theoretical development of scaling has
been investigated by many authors, including Gupta
and Waymire5) and Kuzuha et al.10)
Rainfall intensity I(d) with duration d, exhibits
a simple scale invariance behavior ifdist
I (λ d ) = λ H I ( d )
dist

(1)

holds. The equality “ = ” refers to identicalprobability distributions in both sides of theequations; λ denotes a scale factor and H is a scaling
exponent. From equation (1), it leads to a simple
scaling law in a wide senseqq(2)
E[{ I (λ d )} ] = λ qH E[{ I ( d )} ]where E[] is the expected value operator and q is themoment order. The random variable I(d) exhibits a
simple scale invariance in a wide sense if Equation(2) holds. If H is a non-linear function of q, the I(d)is a general case of multi-scaling. The moments E[]
are plotted on the logarithmic chart versus the scale
λ for different moment order q. The slope functionof the order moment K(q) is plotted on the linear
chart versus the moment order q. If the plotted
results are on a straight line and through the origin,

the random variable shows simple scaling, while inother cases, the multi-scaling approach has to beconsidered5).
The IDF curves are often fitted to the extreme
value type I (EVI) distribution developed byGumbel and it is still the most often used
distribution by many national meteorologicalservices in the world to describe rainfall extremes. It
will also be used in this study along with the methodof moments. The annual maximum rainfall intensity
I(d) has a cumulative probability distribution CDF,
which is given by1= exp[− exp{− [ I (d ) − μ ] / σ }]TI (d ) = μ − σ ln(− ln T )F [ I (d )] = 1 −(3)where the location parameter μ and scale parameterσ to be calculated from data series based on L
moment method 12).
According to the scaling theory, the IDF formulacan be derived 8), 11) with
⎧ I = μ + σ [− ln(− ln(1 − 1/ T )]⎪ d ,T
(4)
d −H
⎨
−
−
H
H
⎪ μ = μ .(λ d ) ; σ = σ .(λ d )⎩24
24where the μ24 and σ24 are parameters of 24 hour data
series. It is worthwhile to note that the simple
scaling hypothesis leads to the equality between thescale factor and the exponent in the expression
relating rainfall intensity and duration. The IDF
relationship can be derived from 24 hours data
series based on three parameters: scale exponent, thelocation and scale parameters of EVI distribution.

3. REGIONAL SCALING MODEL TODERIVE RAINFALL IDF FORMULAS(1) Study area and rainfall data
The Yodo River catchment (approximately 7281
km2) was selected as the study area. The rainfall
data for analysis herein were collected from twenty
hourly rain gauges of which locations are indicatedin Fig. 1. The name and record length for recording
rain gauges are listed in Table 1. The Annual
Maximum Rainfall Intensity (AMRI) series forvarious durations, including 1, 2, 3...24 hours weretaken from Automated Metrological Data
Acquisition System (AMeDAS).
(2) Spatial distribution of scale exponent
The formula of IDF curves (Equation 4) can bederived from scaling invariance of rainfall. It is afunction of scaling exponent H, and the twostatistical parameters: Location parameter μ24 of 24
hour rainfall and scale parameter σ24 of 24 hour
rainfallI (d , T ) = f ( H , μ24 ,σ 24 )(5)
where T is return period and d is duration of rainfall
intensity.

- 86 -

135.462

135.837

136.213IMAJYOU#{TURUGA1 !
(#NAKANOKAWATI
35.53735.537

MIHAMA#OBAMA#
MAKINO#ITIBA#SEKIGAHARA#HIKONE#
35.17435.174MIYAMA#SYUUZANN#!
(

KATADA

KATURA
#

NOSE
#

!
(

OOTORII#

KURODU34.812HIRAKATA#!
(MIYAMURA

Fig. 2 Relationship between moment of order q andduration (hour) at Ootorii station.

OOGAWARA#KOUGA#

KAMEYAMA
#

TARAO
#

34.812(!

KUMOGAHATA

KAMO#

Elevation
IENO
#

Value
High : 1372.000000(!

KASAMA#

Stations for interpolation!
(

6 rainfall stations for validation
135.4620135.837

5

10

20 Km

34.44934.449Low : 3.900000

136.213

Fig. 3 Slope of the line provides an estimate of scaling
exponent for the Ootorii station in the Yodo area.

Fig. 1 Study area and rain gauge locations for analysis.
Table 1 The stations and length of data.
No
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

Name of stations
Ieno
Itiba
Oogawara
Ootorii
KaturaKamo
Kouga
Syuuzann
Tarao
HikoneHirakata
MakinoImajouTurugaMihamaObama
MiyamaNose
Kameyama
Sekigahara

Length of data
1982-1996
1982-2002
1982-2002
1982-2002
1982-2002
1982-2001
1982-2002
1982-2002
1982-1997
1982-2002
1982-2002
1982-2002
1976-2002
1976-2002
1976-2002
1976-2002
1976-2002
1976-2002
1976-2002
1976-2002

The regional IDF formula can be developed
based on the three parameters above. First, thescaling exponent H was to be examined for thetwenty stations in the Yodo River catchment. Basedon the testing results, it is likely to conclude theexistence of the simple scaling for area or region of
the whole. Second, the two parameters of statisticalanalysis: μ24 and σ24 were derived from distributionof 24h ARMI data series.
With the values of the three parameters of all
stations, spatial distributions of three parameters areconstructed with the GIS interpolations technique.
The IDF relationship for any point (ungauged) canbe derived based on these maps and Equation (4).
To this end, after verification of scaling of the
IDF estimates at Hikone stations in the previous
studies11), the assumption of scaling was examinedat a representative sample of stations throughout the
Yodo River catchment. The available hourly data atthese stations were recorded by AMeDAS. An
AMRI data set was derived from the data, and thefirst five moments were calculated for durations of 1hour up to 24 hours. These moments were plottedagainst duration on a log–log scale to determine
whether scaling could be assumed.
In Figure 2 the qth moment of the intensity isplotted against the duration for the Ootorii station incentral of the Yodo River catchment. It is clear thatthe data is linearly related, and there is not a break
between 1 hour and 24 hour duration data11). The
linearity of the moments seen here is similar to theother nineteen stations that were examined,
indicating that for durations greater than 1 hour,scaling appears to be applicable throughout the
Yodo River catchment. For each station, the scaling
exponent, Hq, was plotted versus the moment, q, todetermine if the simple scaling is applicable and to
estimate the scale exponent for stations throughout
the catchment. As shown in Fig. 3, the scaling
exponent coefficient for the Ootorii station is foundto be 0.55, with an R2 value of 0.9989. This
indicates that the simple scaling could be assumed at
this station instead of the more complex
multi-scaling.

- 87 -

Data from other gauges showed a similar scaling
relationship, indicating that simple scaling may be
applicable in the Yodo River area for the durations
considered. In the Table 2, the results of the scaling
exponent factor H of the twenty stations in the Yodo
River catchment are shown with the highcoefficients of determination (R2) for each station
ranging from 0.98 to 1.0. The result indicates astrong validity of the simple scaling property of theextreme rainfall in time series. The scalingcoefficient for the Ootorii station of 0.55 was typicalfor the twenty stations examined, with a mean valueof 0.61 and a standard deviation of 0.06. However, a
few stations showed significantly higher scale
exponents, with the highest being 0.69 (Kamostation). These stations were generally located on
the southern part of the Yodo River catchment, asseen in Fig. 4, indicating that the effect of
topography should be studied further. It can also beseen in Fig. 4 that the scaling coefficients weregenerally lower in the northern part of the Yodo
River catchment.

distributions of the scaling exponent parameters
generated by the twenty stations (p=1). The location
parameter μ24 and the scale parameter σ24 of the EV1(Gumbel) distribution of 24 hours of AMRI can bederived by statistical analysis based on L-moment
method as the results are shown at Table 2. With thesame technique, two maps of statistical parameters
can be constructed as shown in Fig. 5 and Fig. 6(p=1.7 and 1.6). It is expected that those three maps
can be applicable for any ungauged locations within
the catchment. It is required to verify these maps forregional IDF relationships.
Table 2 The parameters at recording rain gauged.
Scale
Scale
Location
Name
Parameter
exponent
Parameterof stations
(σ24)
(-H)
(μ24)
1
Kamo
0.69
3.80
0.82
2
Hirakata
0.68
4.04
1.26
3
Ieno
0.67
3.88
1.02
4
Katura
0.66
4.58
1.50
5
Hikone
0.61
3.83
1.36
6
Makino
0.60
4.34
1.38
7
Ootorii
0.55
4.43
1.44
8
Kouga
0.55
4.05
1.59
9
Oogawara
0.55
6.44
3.06
10 Tarao
0.53
4.93
1.76
11 Syuuzynn
0.53
4.41
1.52
12 Itiba
0.46
4.75
2.20
13 Imajou
0.67
4.06
0.92
14 Turuga
0.62
3.46
1.22
15 Mihama
0.62
3.82
1.73
16 Obama
0.60
3.70
1.64
17 Miyama
0.70
3.91
1.28
18 Nose
0.61
4.15
1.50
19 Kameyama
0.58
5.55
1.84
20 Sekigahara
0.60
5.53
2.04
Mean
0.61
4.38
1.55
Std.
0.06
0.74
0.49

(3) Estimation of the parameters at ungauged
sites
The three maps can be constructed by using
Inverse-Distance Weighted interpolation method in
ArcMap. The being estimated values were obtained:nn(6)
Z * = ∑ Z i di − p / ∑ d i − p
i =1135.837

136.213135.486135.862

136.237

4.06

0.672

0.623

#

IMAJYOU

{

0.632
0.624
&
(
!
#
NAKANOKAWATI
35.537

MIHAMA
3.7

#

#

OBAMA

OBAMA

0.603

4.34

#

#

0.46 MAKINO

0.581
&
!
(
KATADAHIKONE

0.675

#

4.41

#

4.367
#
(
!
KUMOGAHATA

#
#

0.581

#

KAMEYAMA

0.689

4.04

#

HIRAKATAOOGAWARA
4.05

#

5.55

KOUGA

#

KAMEYAMA

4.93

#

TARAO

Mu-parameter
0.669

3.88

Value

#
0.649IENO&(!
KASAMA

#

4.045IENO#(!
KASAMAHigh : 6.43613

Low : 0.460135

135.837

5

10

20 Km

34.446

Low : 3.80062
034.4496 rainfall stations for validation

3.8

#

KAMO

High : 0.688942

Stations for interpolation

4.330
#
(
!
MIYAMURA

#

Value

!
(

6.44

#

4.411 4.43##!
(
KURODUOOTORIIKAMOScaleExponent#

KATURA

NOSE

TARAO

HIRAKATA

#

4.15

#

OOGAWARA

0.552

KOUGA

4.394
#
(
!
KATADA

4.58

0.55

0.584 0.554#&!
(
KURODUOOTORII
0.605
&
(
!
0.532
MIYAMURA#

34.808

KATURA#SYUUZANN

0.661

#

0.615

#

NOSE

34.449MIYAMA
35.17135.174

0.588&(!
KUMOGAHATA

34.81235.1740.526#

135.462

3.83

#

#

HIKONESYUUZANN

SEKIGAHARA

3.91

0.605

#

MIYAMA#ITIBASEKIGAHARA

0.699

5.53

#

#

ITIBA

34.8124.75 MAKINO

0.604

#

35.17135.537

MIHAMA

0.603

3.899
3.46
#
(
!
TURUGA1 #NAKANOKAWATI
3.82#

35.533#35.533{#IMAJYOU

136.213#

Stations for interpolation(!

6 rainfall stations for validation
135.486

Fig. 4 Spatial distribution maps of the scale exponent (H).

0135.862

5

10

20 Km

34.446135.46234.808i =1where di is the distances from each of the nobserved locations to the location being estimated,
Zi are the observed values at those locations. The
parameter is the power value (p), determined byminimizing the RMSE, using the cross validation
technique. Figure 4 displayed maps of spatial

136.237

Fig. 5 Spatial distribution maps of the statisticalparameters: the location (μ) of 24h rainfall AMRI.

- 88 -

135.472

135.848136.223

0.92#

IMAJYOU

1.252
1.22
(
!
TURUGA1 #NAKANOKAWATI
1.73

{

#

#

35.52035.520MIHAMA
1.64

#

OBAMA

1.38

#

2.19 MAKINO

2.04

#

#

ITIBA

SEKIGAHARA

1.28

1.36

MIYAMAHIKONE#

35.15835.158#

1.52#

SYUUZANN

1.49(!
KUMOGAHATA
#

1.52

(
!
KATADA#

1.5

KATURA

#

1.448 1.44

OOGAWARA#!
(
KURODUOOTORII

#

1.334

KOUGA

#

(
!
MIYAMURA

1.59
1.84

#

#

34.795#HIRAKATA

0.822

KAMEYAMA

1.75#

34.795

1.25

Fig. 7 Comparison the IDF cuvers derived from regional
scaling model and IDF cuver with conventional method(EV1) at Kumogahata station.

3.06

#

1.5

#

NOSE

TARAO

#

Table 3 RMSE obtained for the estimation of rainfall
intensity of 7 return periods (2-200yrs) and 24 durations(1-24h) for 6 stations with 21 years of observation data.

KAMO

XIMA-parameter

1.02

#

Value

1.15IENO(!
KASAMAHigh : 3.05874#

Stations for interpolation(!

6 rainfall stations for validation
135.4720135.848

5

10

20 Km

34.43334.433

Low : 0.822476#

Years Kuro- Miyareturn dumura

Kasa- Nakano- KumoKatadamakawati gahata

200
100
50
25
10
3
2

3.8
3.5
3.1
2.8
2.3
1.9
1.7

136.223

Fig. 6 Spatial distribution maps of the statisticalparameters: the scale parameter (σ) of 24h rainfall AMRI.

4. APPLICATION AND VALIDATION OF
THE REGIONAL SCALING MODEL
The model can be applied at any site in the
study region for storm duration d equal to 1, 2..., 24
hours. For each storm duration d and return period T,
the design storm I(d,T) can be evaluated as follows:
1) Estimate the local scale exponent (H) value forthe site of interest. If this site is ungauged aspatial interpolation procedure can be used (seee.g., Fig. 4).
2) Estimate the statistical parameters: the locationand the scale parameters of 24 hours of
EV1(Gumbel) distribution by L-moment
method. If the site is ungauged a spatial
interpolation procedure can be used (see e.g.,
Figs. 5–6).
3) Compute an estimate of the design storm I(d,T )for ungauged site as a function of the scale
exponent, the location parameter (μ24) and thescale parameter (σ24) of 24hour data only,
where T is the return period in the years; d is
duration in the hours.
Figure 7 shows the IDF curves derived fromthe regional scaling model in comparison with IDF
curves derived from conventional method by EV1
distribution.
For demonstrating the performances of regional
IDF model, which is developed in this study, thecalculated rainfall intensities of the 7 return periods(2 to 200 years) and 24 durations (1-24hours) byusing the regional IDF model were compared with

4.8
4.2
3.6
3.0
2.2
1.5
1.0

2.3
2.0
1.7
1.4
1.0
0.7
0.5

6.8
6.0
5.3
4.6
3.6
2.8
2.3

2.4
2.1
1.9
1.6
1.2
1.0
0.7

3.0
2.6
2.2
1.8
1.3
1.0
0.7

* Unit of RMSE value is mm/hr.

the I(d,T) values estimated by conventional method,a frequency analysis based on the EV1 distribution
with the observed data. Six stations, namely Kurodu,
Miyamura, Kasama, Nakanokawati, Kumogahata
and Katada, were chosen from the study area forvalidation. These validation gauges were consideredas the ungauged sites (Regional scaling model did
not use dada from these stations).
The values of the local scale exponent (H) andthe two parameters μ24 and σ24 were estimated fromisoline maps of the study area. Then the Root Mean
Square Errors (RMSE) and Mean absolute Relative
Percent Errors(MRPE) estimation values
1 n
2
RMSE (d , T ) =∑ ( I (d , T )* − I i (d , T )) (7)n i =1 i
1 n I (d , T )* − I i (d , T ).100% (8)MRPE (d , T ) = ∑ in i =1I i (d , T )were obtained, where the I(d,T)* indicates therainfall intensity of duration d and return period T
estimated by the regional scaling model and I(d,T)
indicates the rainfall intensity from EV1 distribution.
Table 3 shows the RMSE values that were obtainedfor 7 return periods (2-200yrs) and 24 durations(1-24h) for six stations.
Figure 8 displays the comparison results of thetwo stations in the Yodo River catchment. The same
calibration results were found in other four stations.

- 89 -

Fig. 8 The validation results for a) Nakanokawati station b) Kumogahata station.

The calibration results for all six stations revealthat the regional scaling models have good
performances for Yodo River catchment and themean relative percentage errors are less than 20
percent for the six ungauged stations.

5. CONCLUSION
The major findings of the present study can besummarized as follows: The properties of the timescale invariance of rainfall quantiles are examined in
the Yodo River basin: for time scaling, rainfall
properties follows the simple scaling. The
hypothesis of piecewise simple scaling combinedwith the Gumbel distribution was used to develop
the IDF scaling formulas depending on the threeparameters: the scaling exponent, and the twostatistical parameters, location and scale parametersof 24 hour rainfall. The spatial distribution maps ofthese parameters are used to derive the rainfall
intensity duration frequency at ungauged points
which were interested for designing analysis.
This study established the regional rainfall
intensity duration frequency relationship for
ungauged catchments based on three spatial
distribution maps from scale invariance of rainfall intime. The paper presents a regional frequency
analysis of annual maximum rainfall intensity forstorm duration ranging from 1 hour to 24 hours,
observed for a dense network of raingauges locatedin Yodo River cathment of Japan. The study
investigates the scaling properties of rainfall
extremes using the EV1 distribution (Gumbel) basedon L-moments.
We developed a regional scaling model thatenables one to estimate design storm at any
ungauged sites of the study area. The uncertainty ofthe estimates for ungauged sites is quantified byusing the RMSE. We obtained for the study area
resampled estimates of the design storm
characterised by mean relative errors, generally

lower than 10% and never higher than 20%. The
methodology should be applied in the different areasand climatology in order to generalize the results.
ACKNOWLEDGMENT:This study was supported by Grant-in-Aid for
Scientific Research (C) 18560497 (PI: Assoc. Prof. Y.
Tachikawa, Kyoto Univ.).
REFERENCES
1) Hershfied, D. M.: Estimating the Probable Maximum
Precipitation, Journal of the Hydraulic Division, Proceedingof the ASCE, HY5, pp. 99-116, 1961.
2) Koutsoyiannis, D., Manetas, A.: A mathematical frameworkfor studying rainfall intensity duration frequency
relationships, Journal of Hydrology, 206, pp.118–135, 1998.
3) Sivapalan, M. and Blöschl, G.: Transformation of point
rainfall to areal rainfall: intensity–duration–frequency
curves. J. Hydrol, Vol. 204(1/2), pp. 150–167,1998.
4) Nhat, L. M., Tachikawa, Y. and Takara, K.: Establishmentof Intensity-Duration-Frequency curves for precipitation in
the monsoon area of Vietnam, Annuals of Disas. Prev. Res.
Inst., Kyoto Univ., No. 49B, 2006.
5) Gupta, V. K. and Waymire, E.: Multiscaling properties ofspatial rainfall and river flow distributions. Journal ofGeophysical Research, 95(D3), pp. 1999-2009, 1990.
6) Burlando, P. and Rosso, R.: Scaling and multiscaling
models of depth-duration-frequency curves for storm
precipitation. Journal of Hydrology, 187, pp. 45-65, 1996.
7) Nguyen, V. T. V, Nguyen, T.-D. and Ashkar, F.: Regionalfrequency analysis of extreme rainfalls, Water Sci. Technol.
45(2), pp. 75–81, 2002.
8) Menabde, M., Seed, A. and Pegram, G.: A simple scaling
model for extreme rainfall. Water Resources Research, Vol.
35, No.1, pp. 335-339, 1999.
9) Pao-shan Y., Yang, T.C. and Lin, C. S.: Regional rainfall
intensity formulas based on scaling property of rainfall
Journal of Hydrology, 295(1-4), pp. 108-123, 2004.
10) Kuzuha, Y., Komatsu, Y., Tomosugi, K. and Kishii, T.:
Regional Flood Frequency Analysis, Scaling and PUB,
Journal Japan Soc. Hydrol. and Water Resources Vol. 18,
No. 4, pp. 441-458, 2005.
11) Nhat, L. M., Tachikawa, Y., Sayama, T., and Takaka, K.: A
simple scaling characteristic of rainfall in time and space toderive intensity duration frequency relationships, Annual
Journal of Hydaulic Engineering, JSCE, Vol. 51, pp. 73-78,
2007.
12) Hosking, J. R. M., and Wallis, J. R.: Regional frequency
analysis: An approach based on L-moments, Cambridge
University Press, Cambridge, UK, 1997.

- 90 -(Received September 30, 2007)