Diapositive 1

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Regional analysis for the
estimation of low-frequency daily rainfalls
in Cheliff catchment
-Algeria- BENHATTAB Karima 1 BOUVIER
Christophe 2 MEDDI Mohamed 3
1USTO Mohamed
Boudiaf-Algérie 2 Hydrosciences
Montpellier-France 3 ENSH BLIDA-Algérie
FRIEND project - MED groupUNESCO
IHP-VII (2008-13) 4th International Workshop on
Hydrological Extremes 15 september 2011
LGEE
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Introduction

• Sizing of minor hydraulic structures is based on
  design Rainfall quantiles (QT) of medium to high
  return periods (T).
• If the length of the available data series is
  shorter than the T of interest, or when the site
  of interest is ungauged (no flow data available)
  obtaining a satisfactory estimate of QT is
  difficult.
• Regional flood Frequency analysis
  is one of the approaches that can be used in
  such situations.

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The Cheliff watershed, Algeria
46 rainfall stations located in the northern part
of the basin daily rainfalls records from 1968
to 2004
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• The Cheliff watershed, Algeria

• 2 main topographic regions valley and
  hillslopes influence on mean annual rainfall

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Why L-moment approach?

• Able to characterize a wider range of
  distributions
• Represent an alternative set of scale and shape
  statistics
• of a data sample or a probability
  distribution.
• Less subject to bias in estimation
• More robust to the presence of outliers in the
  data

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Brief
Intro to L-Moments

• Hosking 1986, 1990 defined L-moments to be
  linear combinations of probability-weighted
  moments

Let x1 ? x2 ? x3 be ordered sample . Define
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Estimating L-moments
where then the L-moments can be
estimated as follows l??????b0 l?2????2b1
- b0 l?3????6b2 - 6b1 b0 ?4????20b3 - 30b2
12 b1 - b0 L-CV l?2??/ l?1??
(coefficient of L-variation) t3 l?3??/
l?2?? (L-skewness) t4 l?4??/
l?2?? (L-kurtosis)
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Steps for success of Regionalisation
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Heterogeneity test (H)
H is the discrepancy between L-Moments of
observed samples and L-Moments of simulated
samles Assessed in a series of Monte Carlo
simulation

H?
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Heterogeneity test (H)
The performance of H was Assessed in a series
of Monte Carlo simulation experiments
H?2 Region is definitely
heterogeneous. 1 Hlt2 Region is possibly
heterogeneous . Hlt1 Region is acceptably
homogeneous.
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Delineation of homogeneous groups
Hlt1
Dendrogram presenting clusters of rainfall
originated in Cheliff basin
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Delineation of homogeneous groups
Dendrogram presenting clusters of rainfall
originated in Cheliff basin
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Delineation of homogeneous groups
Dendrogram presenting clusters of rainfall
originated in Cheliff basin
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Delineation of homogeneous groups
Group1
Group2
Group3
Dendrogram presenting clusters of rainfall
originated in Cheliff basin
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Clusters pooling
The stations located in the valleys
correspond to the group 1 (downstream valley) or
3 (upstream valleys) whereas stations located
on the hillslopes correspond to the group 2.
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Estimation of the regional frequency distribution
Hypothesis
The L-moment ratio diagram

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Estimation of the regional frequency distribution
LCsLCk moment ratio diagram for group 1.
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Estimation of the regional frequency distribution
LCsLCk moment ratio diagram for group 2.
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Estimation of the regional frequency distribution
LCsLCk moment ratio diagram for group 3.
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The goodness-of-fit measure ZDist

Dist refers to the candidate distribution,
t4 DIST is the average L-Kurtosis value
computed from simulation for a fitted
distribution. t4 is the average
L-Kurtosis value computed from the data of a
given region, ß4 is the bias of the
regional average sample L-Kurtosis, sv is
standard deviation. A given
distribution is declared a good fit if
ZDist1.64
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Distribution selection using the goodness-of-fit
measure
Groups Number of stations Regional frequency distribution Zdist
1 17 Generalized Extreme Value 0,51
2 16 Generalized Extreme Value 0,97
3 9 Generalized Extreme Value -0,84
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Estimation of precipitation quantiles
Generalized Extreme Value (GEV) distribution
Quantile is the inverse
k shape ? scale, ? location
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Estimation of precipitation quantiles
Regional Estimation
Local Estimation
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The regional and at-site
annual rainfall
group 1
At-site and regional cumulative distribution
functions (CDFs) for one representative station
at each group
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The regional and at-site annual rainfall
group 2
Tissemsilt station
Teniet El Had station
we observe a reasonable underestimation or
overestimation of quantiles estimated for the
high return periods .
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Reliability of the regional approach

group1
The values of RMSE is greater and the
discrepancy is growing when Tgt 100 years.
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Conclusions and Recommendations

• the regional approach proposed in this study is
  quite robust and well indicated for the
  estimation of extreme storm events
• L-moments analysis is a promising technique for
  quantifying precipitation distributions
• L-Moments should be compared with other methods
  (data aggregation for example).

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THANK YOU