Annual Journal of Hydraulic Engineering, JSCE, Vol.54, 2010, February

AN ASSESSMENT OF PREDICTIVE ACCURACY
FOR TWO REGIONAL FLOOD-FREQUENCYESTIMATION METHODS
Binaya Kumar MISHRA1, Kaoru TAKARA2, Yosuke YAMASHIKI3and Yasuto TACHIKAWA4
1Student Member of JSCE, Dr. Eng., Dept. of Urban and Environmental Eng., Kyoto University(Kyoto 615-8540, Japan) E-mail:mishra@flood.dpri.kyoto-u.ac.jp
2Fellow of JSCE, Dr. Eng., Professor, DPRI, Kyoto University (Uji 611-0011, Japan)
3Member of JSCE, Dr. Eng., Associate Professor, DPRI, Kyoto University (Uji 611-0011, Japan)
4Member of JSCE, Dr. Eng., Associate Professor, Dept. of Urban and Environmental Engineering,
Kyoto University (Kyoto 615-8540, Japan)

Direct-regression and index-flood methods are the two major types of regional flood-frequency
estimation methods. While the former method is well-established for flood-frequency estimation inpractice in many countries, the popularity of latter method is limited among the researchers i.e.,
universality of the latter method has not been established. In this regard, this study has attempted to assess
the prediction accuracies in design floods for the two regional flood-frequency estimation methods. The
design floods were assessed on 11 example Nepalese river basins using the Jackknife technique. The
index-flood method was found to have slightly better prediction accuracies over the direct-regression
method.
Key Words: Direct-regression, frequency, index-flood, Nepalese river basins

streamflow-based equations are reliable for theregions with not many flow-control structures. In
streamflow based method, no assumption is required
regarding the relationship between the probabilities
of rainfall and runoff.
This study is related with streamflow-based
method of design flood estimation. The streamflowmethods are mainly based on the analysis ofstreamflow data. These methods include empiricalequations, and at site or regional statistical analyses.
Regional analysis methods may be used to estimate
design floods at locations with inadequate
streamflow data or no data.
Direct-regression and index-flood methods are
the two major approaches of regional floodfrequency analysis. Delineation of hydrologic
homogeneous regions is common major step of any
regional flood-frequency analysis. Regionalizationis performed to transfer the hydrologic
characteristics from gauged basins to ungauged
basins. In the previous study3), Nepalese river basins
were grouped into five hydrologic regions (Fig. 2).

1. INTRODUCTION
Design flood (maximum discharge of a specificreturn period) estimations are required for various
hydraulic works such as design of weir, barrage,
dam, irrigation facilities, flood control measures etc.
Over/under-estimates of design floods result losses
like waste of resources, infrastructural damage,human life and many others. Research in design
flood estimation is on the decline and there is a large
gap between design flood research and practice1).
This needs redress if improvements to design flood
estimation practice is to be made.
Several techniques are available for estimating
design floods2). The estimation methods can bebroadly classified into two groups (Fig. 1): rainfallbased methods and streamflow-based methods.
Rainfall-based methods are more scientific and
can account easily the changes of climate, landuse,
etc. However, the rainfall-based methods aredata/skill intensive. On the other hand, streamflowbased methods are relatively simpler. The-7-

Fig. 1 Methods for estimating design flood.

WECS method has considered all the Nepalese river
basins as of one hydrologic region.
In the present study, the direct-regression method
was used for deriving regional flood-frequency
relationships in an example hydrologic Region 4.
The hydrologic Region 4 was selected since thisregion has enough hydrometric (discharge) stations,and hence better regression relationships can bederived. The study investigated the predictive
accuracies of the direct-regression and index-flood
based regional flood-frequency equations. While
investigating the predictive accuracies, the design
floods predicted by the two regional equations werecompared with that of at-site flood estimates (trueestimates). The design flood predictive accuracies
were tested at 11 river basins of Region 4. The
predictive accuracies were assessed in term of mean
and median errors in flood estimates.

Fig. 2 Map showing the hydrologic homogeneous regions with
11 test basins outlet inside the hydrologic Region 4.

In developing the regional flood-frequency
relationships, direct-regression based method has
been commonly used in the previous works4),5).
Index-flood based method, with the use of Lmoments, can result flood predictions as good as or
better than those based on the direct-regression
method of regional flood-frequency analysis6),7),8).
The index flood is expected to have better
predictive because the index-flood method provides
an appropriate procedure for statistical flood
estimation of extreme events and also better
represents the basin characteristics. Consequently,
the index-flood based regional flood-frequency
relationships were developed for the Nepalese using
the flood data of 49 Nepalese river basins9).
In the previous work, the distributions: GEV,
lognormal and Pearson type III were found to bereasonably fitting in all of the hydrologic regions.
The drainage area was found to be mainly governing
the value of index flood10). The index-flood based
regional flood-frequency relationships were foundto have far-better predictive accuracy over the
WECS (Water and Energy Commission Secretariat,
Nepal) method4). The WECS method is frequently
used for estimation of return period floods inungauged basins of Nepal and have been developed
using the direct-regression method. However, the

2. REGIONAL FLOOD-FREQUENCYANALYSIS
Regional flood-frequency analysis is an
important method for estimating flood peaks within
specified probabilities of exceedance at ungauged
sites or enhancing estimation at gauged sites where
historical records are short. It is a means oftransferring flood-frequency information fromgauged basins to ungauged basins on the basis ofsimilarity in basin characteristics. Regional
relationships can also mitigate the effect of outliersand can lead to more reliable extrapolation2). Directregression and index-flood methods are the two
major types of regional flood frequency analysis. A
brief description on two major methods is given in
the following sub-sections.
(1) Direct-regression method
In this method, the regression models may be
used in the following form as Eq. (1):-8-

QT = aX 1bX 2c X 3d ...

relationships can be used for estimating design floodof any intermediate values of return periods.

(1)

where QT is the peak discharge for the T-year return
period, X1, X2, X3…are physiographic/climaticcharacteristic variables of the gauged basins, and a,b, c, d... are regression parameters. Since it isunknown in advance what physiographic/climaticcharacteristics may have significant impact on
flood-frequency estimates, a number of parameters
are calculated and investigated as possible
predictors of T-year discharges. Then the variables
with the smallest significance are removed until thestatistically significant terms remain. In the present
study, use of drainage area was limited as regression
variable since the drainage area was only found tobe effective in governing the flood in the previous
study10).

3. METHODOLOGY
Evaluation of the developed regional floodfrequency relationships is an important aspect. The
accuracy of two regional flood-frequency
relationships has been assessed on 11 independent
gauged stations (Fig. 2). All these test basins havetheir catchment boundary inside the hydrologic
Region 4. The basins which possessed at-least 20
years of observation flood data series were selected
for the assessment since reliable at-site estimates(assumed as true estimates here) can be expected
only for the stations having longer observation flood
series.
The regional flood-frequency relationships havebeen tested by comparing the return period floods ofregional flood-frequency equations with that of atsite flood-frequency analysis method. Jackknife
technique was employed for assessing the design
flood estimates at each of the test stations. In thistechnique the station, at which assessment is to be
performed, is excluded in deriving the regional
flood-frequency relationships.
For illustration, let us consider the test station
409.5 of Region 4 for evaluating the predictive
accuracy. The drainage area of this basin is 113.51
km2. Out of total 24 stations in the region, thestation 409.5 was excluded and the remaining 23
stations were considered for deriving the regional
flood-frequency equations for the two methods. A
brief detail on the development of flood-frequency
relationships for evaluating the design flood at thisstation is presented below.

(2) Index-flood method
The index-flood method8),11) assumes that, within
a hydrologic homogeneous region, the exceedance
probability distribution of annual peak discharge isidentical except for a site-specific scaling factor
called the index flood. This index flood parameterreflects the important physiographic andmeteorological characteristics of a basin. In thismethod, a relationship is established for estimating
the flood quantile QT of return period T at site i as
the product of index flood (average likely flood) μi,
which is the function basin area, slope etc., andregional growth factor, qT. The growth factor is adimensionless frequency distribution quantitycommon to all sites within a hydrologic
homogeneous region. The design flood estimation
relationship may be expressed by Eq. (2).
QT = qT μ i(2)Index-flood method
Design flood, QT (m3/s) of T year return period inbasin i is (Eq. (3)):

For the estimation of index flood, a relationshipin term of basin characteristics is established basedon available information gathered from the gauged
sites. Regional growth curves showing therelationship between qT and T are derived once anappropriate frequency distribution has been foundwithin a hydrologic region with N sites that fits all
the gauged flood series.
In simple words, index flood based regional
flood-frequency analysis method can be said of
three major steps: hydrologic homogeneous
regionalization, selection of regional frequency
distribution and estimation of index flood
relationship.
Unlike the direct-regression based regional
flood- frequency analysis which is derived for a
fixed values of return periods (e.g. T = 2, 5, 10, 20,
50, 100 ……years), the index flood-based regional

QT = 6.23 Ai

0.68⎡
T − 1 −0.137 ⎫⎤ (3)⎧⎬⎥⎢0.726 − 2.73⎨1 − (− ln T )⎩⎭⎦⎣where Ai is drainage area in km2.
250

Floods, QT (m3/s)

200

QT= 47.97ln(T) + 50.292
R² = 0.9779

150
100
50
0
1

10

100

1000

Return period, T-years

Fig. 3 Plot of floods against their return periods at station 409.5.

-9-

At-site method
The annual maximum flood series were arranged indescending order. Return periods were computed forthe ordered values using the Weibul’s plotting
position formula (Eq. (4)):

T=

n +1
m

Table 1 Values of regression parameters for T-years
T
2
5
10
20
50
100
200

(4)

where n is sample size and m is rank of the floods.

QT = 47.97 log(T ) + 50.29

Direct-regression method
Firstly, maximum discharges of 2, 5, 10, 20, 50,
100 and 200-years return period were computed
using the method of at-site flood-frequency at theremaining 23 stations in the region. As mentioned
earlier that the catchment area mainly govern theflood values in the delineated hydrologic
homogeneous regions, a simple regression technique
was applied for each of the return period floods as
dependent variable and the drainage area as
independent variable (Fig. 4). The general form ofregional flood-frequency estimation relationshipmay be expressed by Eq. (6):b(6)
where a and b are regression parameters (Table 1).
The similar process was repeated at each of thetest stations. Assessment of the two regional floodfrequency methods has been made in terms of mean
and median absolute error in design flood estimatesof different return periods (Eqs. (7-8)):
ΔQTIF =QTIF − QTAtsite
QTAtsite(7)

2-year flood, Q2 (m3/s)

4000
3500
3000
2500
2000

Q2 = 2.41Ai0.7996
R² = 0.8198

1500
1000
500
0
10

100

1000

b
0.80
0.76
0.74
0.73
0.73
0.72
0.72

R2
0.81
0.75
0.72
0.70
0.68
0.67
0.66

(5)

The at-site flood-frequency equation (Eq. (5))
corresponds to the line fitting the plotted points ofthe floods versus return periods (Fig. 3).

QT = aAi

a
2.41
5.09
7.19
9.31
12.15
14.30
16.47

10000

Drainage Area, Ai (km2)

Fig. 4 Illustrative regression plot of 2-year flood against
corresponding drainage area of 23 river basins.

ΔQTDR =QTDR − QTAtsite
QTAtsite(8)

where, ΔQTIF is relative absolute error in indexflood based estimates for T-year return period;
ΔQTDR is relative absolute error in direct-regression
based estimates for T-year return period;
QTIF is index-flood based estimates of T-year return
period;QTDR is direct-regression based estimates of T-year
return period; andQTAtsite is at-site flood-frequency analysis estimatesfor T-year return period.

4. RESULTS AND DISCUSSION
The assessment work started with the estimationof return period floods for the at-site floodfrequency analysis, direct-regression and indexflood methods. Considering the estimates of at-site
flood-frequency analysis method as true estimates,
the error in direct-regression and index-flood
estimates were projected.
Figs. 5-11 show the comparative plot of flood
estimates for 2, 5, 10, 20, 50, 100 and 200-years
return periods respectively obtained using the atsite, direct-regression and index-flood methods. In
these figures, some stations are found to have larger
predictive discrepancies. These stations are situatedat the boundary of the hydrologic regions, hence
may be influenced by other region. This may result
bigger discrepancies in the estimated values. Fromthese plots, it is difficult to distinguish the predictive
superiority of either method over another. The
predicted floods seem closely similar at most of thestations for both direct-regression and index-flood
methods.
To identify which regional method is better,
relative absolute error in the estimates of directregression and index-flood methods were evaluatedby considering the at-site flood estimates as true
estimates. Using the Eqs. (7-8), relative absolute
error at each of the test stations were computed forboth the regional methods.

- 10 -

1000

620

470

465

460

600

Fig. 9 Comparison of design floods for T = 50 years

300

True (at-site)

2000
1000

620

470

465

460

448

447.9

1600

446.8

409.5

0

430

Direct-regression

Direct-regression

3000

428

Index-flood

2000

Hydrometric station indices

1200
800

Fig. 10 Comparison of design floods for T = 100 years

400
0

1000

1500

620

470

465

460

Hydrometric station indices1000

Fig. 11 Comparison of design floods for T = 200 years

500

Table 2 Average absolute mean and median error in design
flood estimates.
620

470

465

460

448

447.9

446.8

430

428

417

409.5

0

Returnperiod, T
years

Hydrometric station indicesFig. 7 Comparison of design floods for T = 10 years

3000

448

0

2000

447.9

Direct-regression

446.8

Index-flood

2000

430

True (at-site)
2500

3000

428

Fig. 6 Comparison of design floods for T = 5 years

Direct-regression

4000

417

Hydrometric station indices

Index-flood

5000

409.5

200-years flood (m3/s)

620

470

465

460

448

447.9

446.8

430

428

417

True (at-site)
409.55-years flood (m3/s)

True (at-site)

Index-flood

4000

417

100-years flood (m3/s)

620

470

465

460

448

447.9

446.8

430

428

417

409.5

Fig. 5 Comparison of design floods for T = 2 years

3448

Hydrometric station indicesHydrometric station indices10-years flood (m /s)

447.9

3

900

0

True (at-site)

Index-flood

2
5
10
20
50
100
200
Average

Direct-regression

2000
1000

`

620

470

465

460

448

447.9

446.8

430

428

417

0

409.5

20-years flood (m3/s)

430

0

1200

428

Direct-regression

Direct-regression

2000

417

50-years flood (m3/s)

Index-flood

Index-flood

3000

409.5

2-years flood (m /s)

True (at-site)

True (at-site)
4000

446.8

The maximum absolute percentage error between
the at-site flood-frequency analysis estimates andthe index-flood based regional estimates at any
stations was found to be 72.72%. In contrast, themaximum absolute percentage error in directregression regional estimates was found to be
68.58%.

Hydrometric station indicesFig. 8 Comparison of design floods for T = 20 years- 11 -

Mean absolute error (%)
Index
24.75
26.90
28.14
30.80
28.25
29.06
31.28
28.46

Direct
24.78
27.49
30.15
29.43
29.81
31.45
33.06
29.45

Median absolute error (%)
Index
16.15
19.19
20.32
20.89
20.44
29.06
22.55
21.23

Direct
23.73
26.02
26.28
26.37
26.39
31.45
27.47
26.81

Mean error (%)

Index-flood

35

Direct-regression

30
25
20
15
1

10

100

1000

Return period, T years

Median error (%)

Fig.12 Plot of mean error for the direct-regression and indexflood methods.
Index-flood

35

Direct-regression

30
25
20
15
10
1

10

100

1000

Return period, T years

Fig. 13 Plot of median error for the direct-regression and indexflood methods.

Mean and median absolute error was used toshow the trend in error for the two methods. Table 2
shows the average mean and median absolute error(%) for the two regional methods. The respective
graphical plot has been made in Figs. 12-13. Thesetables and figures show that the absolute percentage
error in index-flood method is relatively smaller
than that of direct-regression method.
5. CONCLUSIONSThe overall objective of this study was to assess
the methods of index-flood and direct-regression
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based
regional
flood-frequency
estimation
techniques for better estimation of return period
floods. The index flood-based regional flood
frequency method was expected to have better
predictive accuracies than the direct-regression
method because the index-flood method provides anappropriate procedure for statistical flood estimationof extreme events and better represents the local
characteristics. The objective was achieved, at first,
by deriving the direct-regression based regional
flood-frequency estimation relationships in one of
the hydrologic homogeneous regions of Nepaleseriver basins and then comparing the estimated return
period floods of direct-regression and index-flood
methods with that of at-site method.
The plot of predicted floods for different return
periods at the 11 test basins do not point out any
clear-cut advantage/disadvantage of either regional
flood-frequency methods. Comparative analysis on
flood estimates in term of mean and median error
for the index-flood and direct-regression methods
point out that the index-flood method has slightly
better predictive accuracy over the direct-regression
method. These lead to conclude that index-flood
based regional flood-frequency estimation method isbetter than the direct-regression based regional
flood-frequency estimation method.
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basins, the degree of assessment may not be
considered well-enough. Therefore, the study
recommends performing the flood predictive
accuracy assessment in additional hydrologic
regions to give more reliable conclusion.
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- 12 -(Received September 30, 2009)