Analysis of a Sahelian annual rainfall index from

Hydrological Sciences-Journat-des Sciences Hydrologiques, 47(4) August 2002
Analysis of a Sahelian annual rainfall index from
1896 to 2000; the drought continues
YANN L'HOTE
IRD, Maison des Sciences de l'Eau, BP 64501, F-34394 Montpellier cedex 5. France
vann.lhote@msem. univ-montp2.fr
GIL MAHÉ
IRD Hydrologie, 01 BP 182, Ouagadougou, Burkina Faso
[email protected]
BONAVENTURE SOME
Centre Régional Agrhymet, BP 11011, Niamey, Niger
JEAN PIERRE TRIBOULET
Direction de l'Hydraulique, BP 4931, Nouakchott, République Islamique de Mauritanie
Abstract Since 1970 the West African Sahel has experienced a significant drought
which, according to some authors, corresponds to a discontinuity (abrupt change) in
the rainfall series. An annual rainfall index was calculated over the period from 1896
to 2000, with a selection of 21 synoptic stations updated regularly by the Agrhymet
Regional Centre in Niamey. Several statistical analyses of the index confirmed the
previous descriptions, particularly the significance of the drought since 1970 and a
nonstationarity, with abrupt change in the series between 1969 and 1970. Although the
two recent wet years, 1994 and 1999, gave some hope of an end to the drought, the
statistical results and the temporal distribution of the dry and wet years showed that
the drought was not over at the end of 2000.
Key words Sahel; rainfall; drought index; nonstationarity; discontinuity
Analyse d'un indice des précipitations annuelles au Sahel sur la
période 1896-2000; la sécheresse n'est pas terminée
Résumé Depuis 1970 le Sahel ouest africain connaît une sécheresse importante
correspondant, selon certains auteurs, à une discontinuité dans les séries
pluviométriques. Un indice des précipitations annuelles a été calculé sur la période
1896-2000 à partir d'une sélection de 21 stations synoptiques suivies en permanence
par le Centre Régional Agrhymet de Niamey. Les analyses statistiques des valeurs
annuelles de l'indice ont confirmé les schémas déjà décrits, en particulier l'importance
de la sécheresse depuis 1970 et la non-stationnarité de la série avec une rupture entre
1969 et 1970. Malgré les deux années humides récentes 1994 et 1999 qui ont apporté
un espoir de rémission de la sécheresse, les différents tests statistiques et la répartition
dans le temps des années sèches et humides ont permis de conclure que la sécheresse
n'était pas encore terminée en fin 2000.
Mots clefs Sahel; précipitations; indice de sécheresse; rupture
INTRODUCTION
In the West African Sahel, knowledge about the long-term variability and trend of
rainfall is important because of the extreme vulnerability of the environment where the
climate can be roughly defined by annual rainfall within 300 to 700 mm (L'Hôte et al.,
Open for discussion until 1 February 2003
553
564
Yann L'Hôte et al
1996). The main results of rainfall studies in this region can help decision makers to
manage, e.g. water resources, agriculture, environmental protection, water projects.
The Sahel has experienced a fairly continuous, severe drought since 1970,
described and analysed by numerous authors (e.g. Sircoulon, 1976; Lamb, 1982, 1985;
Olivry, 1983; Nicholson, 1985; NOAA, 1994; Le Barbé & Lebel, 1997; Nicholson et
al, 2000), showing diagrams of annual rainfall index series commonly used by
decision makers.
According to Hubert et al. (1989, 1998) numerous stations in the Sahel have
shown discontinuities in annual rainfall series around 1970. As a consequence, the
averages calculated with data before and after 1970 are not statistically similar. The
latest 30-year WMO standard over the years 1971-2000 takes into account only the
three driest decades of the 20th century. Therefore, with regard to the 1971-2000
average, many of the wet years observed after 1970 are actually dry years in terms of
longer averages. On the basis of the latest WMO average, some experts thought that
the drought in the Sahel might be over, or on the way to being so (personal
communications). Numerous contacts with decision makers in Africa show that such a
raised hope may result in decisions being made that are too optimistic.
In this paper, a new annual rainfall index series, updated from 1896 to 2000, is
analysed, calculated on the basis of previous studies (Lamb, 1985; Nicholson, 1985;
NOAA, 1994).
METHOD AND DATA
The values of an annual rainfall index from 1896 to 2000 are provided, as well as other
related values thought to be particularly relevant, as follows:
(a) 21 synoptic stations, updated regularly by the Agrhymet Regional Centre of
Niamey, were selected, enabling regular updating of the index.
(b) A longest (being also a most recent) period, compared to those of the previous
studies, was selected. Historical data were found in old reports that are not readily
available, all published values were checked and, finally, a critical analysis of
these values was performed.
(c) A common 80-year reference period was selected (1921-2000), because of the low
level of missing data (2.6%) and good representativeness for the climate in the
Sahel.
For each station, s, and year, y, the partial station index, Ssy is its standardized
rainfall departure:
Ssy^(Rxr-Rs)/as
(1)
where Rsv, Rs and as are the annual rainfall of station s for year y, and the mean annual
rainfall and standard deviation of station s in the reference period 1921-2000.
Finally, the regional annual index, RIV for each year, v, is the average of partial
indexes of the n stations observed during this year:
i
RI
»
= -Ysn.
" .! = !
Analysis of a Sahelian annual rainfall index from 1896 to 2000
565
In previous studies, with about one hundred stations available in the sub-region of
interest, Nicholson et al. (1985, 2000) calculated an index based on the periods 1901—
1984 and 1901-1998, taking the observation times of each station as the periods of
reference. Lamb (1985) published an index, known by his name, calculated on the
period 1941-1983, taking into account 20 stations well spread west of the meridian
9°E, between 11° and 18°N. Finally, NOAA (1994) published an index based on the
rainy season (May-September) of 1921-1994, calculated using all the available
stations having at least 30 years of historical records, and located in Africa, between 8°
and 18°N, and west of 20°E.
In this study, the 21 stations chosen are those used in Lamb's (1985) study, with
three suppressions and four additions. The list is given in Table 1 and the stations are
indicated in Fig. 1. These stations are homogeneously located west of the meridian
20°E, having between 100 and 1000 mm of annual average rainfall, according to the
map of isohyetal lines for the period 1951-1989 for West and Central Africa (L'Hôte
& Mahé, 1996). The area receiving precipitation in this range can be estimated at about
2.5 x 106 km", and this corresponds approximately to density of one station for each
120 000 km 2 .
Table 1 List of stations, number of observed years, averages and standard deviations (in mm) of the
reference period 1921-2000.
Station
Country
No. of Average Std dev.
years
Saint Louis
Dakar
Tambacounda
Podor
Banjul
Néma
Boutilimit
Bamako
Gao
Kaves
Mopti
Senegal
Senegal
Senega]
Senega]
Gambia
Mauritania
Mauritania
Mali
Mali
Mali
Mali
78
80
80
80
80
77
79
80
80
76
78
302.7
472.6
830.4
263.5
1103.0
256.7
160.4
1009.7
232.8
675.0
504.2
119.1
183.6
196.4
99.7
305.1
96.1
82.2
193.4
86.7
159.9
121.7
Station
Country
No. of
years
Average Std dev.
Tombouctou
Niamey
Agadez
Nguigmi
Tahoua
Zinder
Ouagadougou
Bobo Dioulasso
N'Djamena
Sarh
Mali
Niger
Niger
Niaer
Niger
Niger
Burkina Faso
Burkina Faso
Chad
Chad
79
80
78
79
79
80
80
80
69
64
191.8
576.6
145.5
211.5
383.4
463.9
802.1
1105.0
577.7
1037.3
Fig. 1 Location of stations and mean annual rainfall network (1951-1989).
65.4
142.1
59.9
94.0
101.2
129.6
141.5
196.8
154.7
172.2
566
Yann L'Hôte et al.
Fig. 2 Sahelian annual rainfall index variation (top) and number of stations (bottom)
by years.
The period chosen for the calculation of the index starts in 1896 because this is the
year in which the observations of two continental stations (Kayes and Tombouctou in
Mali) began. The data balance the high interannual variability observed at the coast
and particularly at the three oldest stations of Saint Louis and Dakar in Senegal
(recording since 1848 and 1853, respectively), and Banjul in Gambia (since 1885). In
addition, the data collected before 1896 from the two Senegalese stations above, on
different hand-written or typed archives, are not of good quality and are sometimes
unusable. This is also the case for a more recent period (1919-1929) for the Saint
Louis station: the data used in this study are those published by Asecna (1968) for the
years identified as defective. Figure 2 (bottom), represents changes in the number of
stations, which increases with time: four to six stations from 1896 to 1903, seven to 12
stations from 1904 to 1920 and 15-21 from 1921 to 2000.
The data were chosen according to elements published in 26 reference books
(CIEH et al., 1973-1979, 1989-1990) which give the daily rainfall in 13 African
countries, published in two parts "from the origin to 1965", and "from 1966 to 1980".
For the oldest data, it was necessary to refer to the available publications, mainly those
of Asecna (1968), Sircoulon (1976) and Olivry (1983), as well as different archives,
copies of which are held in the "Fort de Saint Cyr" in France. The years after 1980
were documented from different sources, mainly the Meteorological Services of each
country (manual notes, annals). Finally, records for the most recent years (1995-2000)
come from the monthly reports of the Agrhymet Regional Centre, with some additions
also coming from National Services and from the Monthly Climatic Data for the
World (NOAA, 1995-2000). For all these series, a few missing monthly values only
were infilled for the dry season (no rain), and a highly critical analysis was performed.
RESULTS AND INTERPRETATION
Table 1 gives the average and standard deviation for each of the 21 stations, calculated
over the reference period of 80 years (1921-2000). These values would normally allow
the calculation of a future partial index for each station in each year. Table 2 gives the
Analysis of a Sahelian annual rainfall index from 1896 to 2000
567
Table 2 Sahelian annual rainfall index from 1896 updated to 2000.
Decade
1890
1900
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
YearO
-0.028
-0.228
-0.112
0.381
0.123
1.104
0.118
-0.404
-0.415
-0.998
-0.363
Year 1
0.380
-0.566
-0.534
-0.164
-0.474
0.589
0.524
-0.598
-0.604
-0.422
Year 2
-0.369
-0.668
0.169
0.646
-0.275
1.279
0.131
-0.931
-0.827
-0.637
Year 3
0.071
-1.479
-0.062
0.810
1.289
0.823
0.132
-1.056
-1.382
-0.830
Year 4
0.280
-0.553
0.324
-0.129
-0.075
1.053
0.673
-0.198
-1.363
0.438
Year 5
0.510
-0.094
-0.020
0.533
0.472
0.890
0.207
0.254
-0.669
-0.548
Year 6
-0.210
1.605
-0.043
-0.468
1.167
0.704
0.405
0.186
-0.193
-0.685
-0.676
Year 7
-0.232
-0.588
0.297
1.046
-0.151
0.028
0.689
0.430
-0.974
-1.093
-0.856
Year 8
0.115
0.298
1.336
0.367
0.148
0.030
0.728
-0.230
-0.045
-0.170
-0.031
Year 9
0.254
1.104
-0.344
0.264
0.442
-0.350
0.440
0.197
-0.400
-0.215
0.563
series of annual regional index calculated over the period 1896-2000, whereas Fig. 2
(top) represents the variation of this index with time. As a first interpretation of Fig. 2,
one may refer to the descriptions of several authors (e.g. Lamb, 1982; Nicholson et al.,
2000) of three series of characteristic periods: (a) from 1896 to 1949 (54 years) an
apparently random succession of dry periods, "normal" periods and wet periods, but
with seven successive years showing a deficit centred on the well-known drought year
1913; (b) from 1950 to 1969—a series of 20 wet years, except for 1968; and (c) from
1970 to 2000—a series of 31 years that were overall dry or very dry, with only three
wet years, among which two (1994 and 1999) are recent.
It is worth noting in Table 3 that there are high coefficients of correlation with
Sahelian indexes developed in the literature, e.g. 0.924 with that of Nicholson (1985)
and 0.939 with that of NOAA (1994).
Table 3 Comparison between IRD-Agrhymet (2002), Nicholson (1985) and NOAA (1994) indexes.
Characteristics
Similar period to IRD-Agrhymet index
Number of similar years
Number of years with indexes of opposite signs
Correlation coefficient with IRD-Agrhymet index
(IRD-Agrhymet)
(1896-2000)
(105)
-
Nicholson
1901-1984
84
6
0.924
NOAA
1921-1994
74
4
0.939
Nonstationarity and discontinuity in the series
Discontinuities (abrupt changes) in a chronological time series correspond to significant differences in its parameters (average and/or standard deviation). Thus, after
Hubert et al. (1989, 1998), one looks for the presence of a discontinuity year (or
several years), characterized by a sudden change in value of one of these parameters,
while the moment of change is usually unknown.
After Lubès-Niel et al. (1998), the software KhronoStat (IRD-Orstom, 1998)
allows the study of the stationarity by the use of four statistical tests. First, the test of
Pettitt (1979) and the U-statistic of Buishand (1984) clearly show that the annual
rainfall index over the period 1896-2000 cannot be considered as a stationary series,
since, in the two tests, the zero hypothesis of absence of discontinuity is rejected at the
99% confidence level. The Bayesian estimation of Lee & Heghinian (1977) allows one
568
Yann L'Hôte et al.
to detect a clear discontinuity in the record between 1969 and 1970 with a very high
posterior probability density of 0.383. Finally, the procedure of segmentation of
Hubert et al. (1989) with a significance level of the test of Scheffé at 1%, points out
only one abrupt change (between 1969 and 1970). The test of Hubert with this
significance level can be considered as adequate. Applying this test to the series
referenced over all the years before and after 1921 (instead of the common period
1921-2000), two other discontinuity points were detected, before and after the drought
of 1913, but no discontinuity has been noted since 1970. Hence no sign of an end of
the drought has been observed until 2000.
Furthermore, applying these four tests on each of the 21 stations one often finds
the same dates of discontinuity. For instance, using Hubert's segmentation, 12 stations
show one discontinuity between 1966 and 1970 (modal value in 1969) spread all over
the area.
It is worth mentioning that Mahé & Olivry (1999, 2001) found a discontinuity
point between 1969 and 1970 for the series of regional rainfall over the period 1951—
1989 for the whole of West and Central Africa (tropical and equatorial areas). In
contrast, according to Aka et al. (1996), Servat et al. (1997) and Mahé & Olivry
(1999), the series of discharges of the large regional rivers (e.g. Niger, Congo,
Ogooué) show a later discontinuity point (between 1970 and 1971) for the majority of
them. Paradoxically, this discontinuity appears earlier (1967-1968) in the northwest of
the area (Senegal and Fouta-Djalon Mountains).
Geographical extent of drought
The values of partial indexes, Ssv, for the 21 stations are represented in Fig. 3,
corresponding to seven dry and very dry years. The years considered are: 1913
(regional index: -1.479), four very dry years after 1970 (indexes <—1.05) as well as
1913
--0--1973
*
1983
— - X - - 1984
3K
1987
--O--1990
•
Fig. 3 Station indexes Ssv of the 21 stations in seven dry years or very dry years.
1997
Analysis of a Sahelian annual rainfall index from 1896 to 2000
569
1990 (-0.998) and 1997 (-0.856). One can see that the majority of the 137 values are
under zero with only five positive indexes, among which two occurred in 1997 (at
Agadez and Sarh). The majority of the indexes (130 out of 137) have values lower than
-0.35. Out of 21 stations, 11 stations during the exceptional year 1983 and seven in
1984 experienced a serious drought with indexes below -1.50. These observations
show that drought of concern was not only very strong but also largely spread over the
area. According to Sircoulon (1976) this criterion of extent characterizes the periods of
drought in the Sahel.
Among the four recent dry years (1995, 1996, 1997 and 2000), i.e. out of 84
station-years, there are 11 indexes close to the average (from -0.25 to 0) and 12 wet
years (>0) among which five are greater than 0.5, and one year was very wet (>1). This
shows that the criterion of wide extent, which characterizes the droughts in the Sahel,
does not seem to be as marked since 1994.
Persistence of dry or wet spells over several years, autocorrelation
One can see (Fig. 2) three dry episodes, centred on 1913, 1973 and 1983-1984, and a
long series of humid years in the 1950s and 1960s. By restricting the analysis in an
arbitrary way to dry or wet periods of at least five years duration, the following
persistences are clearly shown: a deficit from 1910 to 1916 (7 years), an excess from
1950 to 1967 (18 years), a deficit from 1970 to 1974 (5 years) and a deficit from 1976
to 1993 (18 years).
A significant autocorrelation coefficient of an annual chronological series shows
that the value in one year depends on the value(s) in the previous year(s). A confidence
interval of the autocorrelation coefficient can be estimated via a random series
approach. The software KhronoStat (IRD-Orstom, 1998), which allows such analysis,
is applied to the series of annual rainfall index. Significantly high autocorrelation
coefficients are found for the temporal argument between one and three years (Fig. 4).
The coefficients decrease for more than three years but remain significant on the 99%
confidence level for temporal argument going up to 12 years.
0.500
0.400
0-300
|
V ^
a = = a =
^
0.200 —
<D
8
0.100
J
o.ooo
"m
£ -0.100
§ -0.200 | ~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-'•
j
| -0.300 I
j
-0.400
-0.500 •
• —
1
2
3
4
5
6
7
8
9
—
|
. _ _ _ _ _ !
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
Delay (years)
Fig. 4 Autocorrelation diagram for argument varying from 1 to 26 years. Levels of
confidence: 99% (solid line), 95% ( ) and 90% (
).
5 70
Yann L'Hôte et al.
Furthermore, applying this method to each of the 21 stations, one finds several
significant coefficients of autocorrelation, especially for one and two years (13 and
nine stations, respectively), and until five and even ten years (four and two stations).
Nevertheless these stations do not show any spatial organization.
The drought was still going on at the end of 2000
Several symptoms have been noted which were interpreted by some experts as a
possible indicator of the tendency of termination of drought. Indeed, three wet years
(1975, 1994 and 1999) have occurred since 1970, but they were always separated from
1.00
-
—-,
0.80
j
0.60
|
0.40
0.20
0.00
j
i
i
ii
'
i
-0.20
-0.40
-0.60
-0.80
-1.00 J
i
19001909
19101919
19201929
19301939
19401949
19501959
19601969
19701979
19801989
19901999
Fig. 5 Decadal averages of annual index.
-0.50
I
)
I
-1.00
-1.50
L n o i n o i n o i o O L O O u ^ O L f i o t n o m o m o i o o
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^
T
-
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N
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-
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l
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n
c
û
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t
^
E
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T
C
n
O
c o a > a > o > a > c 7 > c 7 > C T > C T > a > a s c n a > a 3 c n c B a i m a i c n O T O
Fig. 6 Annual index calculated with WMO last period of reference 1971-2000, for
comparison with Fig. 2.
Analysis of a Sahelian annual rainfall index from 1896 to 2000
571
each other by at least four non-wet years. Dry years, or very dry years, during the last
31 years have been arranged in periods of at least two successive years. The 1990s
were indeed less dry, on average, than the 1970s and 1980s, as shown in Fig. 5, but
still it was the third driest decade in the century.
Using an average over the last official reference period of 30 years of WMO
standard (1971-2000), one can clearly see that the three driest decades of the century
were the 1970s, the 1980s and the 1990s. Figure 6 shows a simulation of the annual
index of the whole period 1896-2000 with reference to the last standard 1971-2000. It
may be noted that this choice of a reference period has grave effects on the
classification of earlier years. Almost all (72 of the 75) years from the 1896-1970
period are quite uniformly recognized as wet (index >0), except for 1907 and 1912
(close to the average) and 1913 (clearly dry). Clearly, another reference period should
be used containing wet decade(s) such as the 1950s and/or 1960s.
These observations confirm that drought was still going on at the end of the year
2000.
CONCLUSIONS
The main results of the present study are: the confirmation of the significance of the
drought since 1970 with an abrupt change (downwards) in the series between 1969 and
1970, which is the main climatic event of the 20th century in the Sahel, and lack of
detection of a sustained upward change since 1970, despite two recent dry years (1994
and 1999). Even if the areal extents of drought areas during the recent dry years are
smaller than during the earliest ones, the wet years are still very isolated from each
other since 1970 and have not led to recovery from drought. Both these last results
show that the drought was not over as at the end of the year 2000.
The analysis presented is limited to the statistics and the distribution of the dry and
wet years, without taking into consideration ecological or agricultural matters, but the
impacts of climatic drought in these areas are very important.
Acknowledgements The daily work done by observers and meteorologists in each
country, who collected data used in this study for more than a century, is gratefully
acknowledged. Pioneers in the data collection: doctors and chemists of the French
Navy, meteorologists of Colonial Services and missionaries of Christian churches are
particularly acknowledged. More recently this work was continued by National
Meteorological Services as well as Asecna (Agence pour la Sécurité de la Navigation
Aérienne), Orstom Hydrological Service, CIEH (Comité Interafricain d'Études
Hydrauliques), Agrhymet and several state or private companies, whose staff have
conscientiously collected daily data.
REFERENCES
Aka, A. A., Serval. E., Paturel, J. Il, Kouainé, B„ Lubes, H., Masson, J. M. (1996) Analysis of the temporal variability of
runoff in Ivory Coast: statistical approach and phenomena characterisation. Hydrol Sci. J. 41(6), 959-970.
Asecna (Agence pour la Sécurité de la Navigation Aérienne) (1968) Normales climatologiques du Sénégal. Service
Météorologique, Dakar, République du Sénégal.
Buishand, T. A. ( 1984) Tests for detecting a shift in the mean of hydrological time series. J. Hydro!. 73, 51 -69.
572
Yann L'Hôte et al.
Centre Régional Agrhymet (1995-2000) Bulletins mensuels Agrhymet, d'avril à septembre. Niamey.
CIEH, Orstom Hydrologie (1973-1979) Précipitations journalières de l'origine des stations à 1965. Républiques du
Dahomey, Tchad, Côte d'Ivoire, Mali, Sénégal, Niger, Haute-Volta, Mauritanie, Togo, Gabon, Cameroun (début à
1972)—12 volumes. Orstom, Paris.
CIEH, Orstom Elydrologie, Asecna, Services Météorologiques Nationaux (1989-1990) Précipitations journalières de 1966
à 1980. Républiques du Dahomey, Tchad, Côte d'Ivoire, Mali, Sénégal, Niger, Haute-Volta, Mauritanie, Togo,
Gabon, Cameroun—14 volumes. Orstom, Montpellier.
Hubert, P., Carbonnel, J. P. & Chaouche, A. (1989) Segmentation des séries hydrométéorologiques. Application à des
séries de précipitations et de débits de l'Afrique de l'ouest. J. Hydro]. 1 10, 349-367.
Hubert, P., Servat, E., Paturel, J. E., Kouamé, B., Bendjoudi, J. P., Carbonnel, J. P., Eubès-Niel, H. (1998) La procédure de
segmentation dix ans après. In: Water Resources Variability in Africa during the XXih Century (Proe. Abidjan
Symp., November 1998), 267-273. IAHS Publ. no. 252.
IRD-Orslom (1998) KhronoSlat version 1.0. Logiciel, Orstom, Montpellier.
Lamb, P. J. (1982) Persistence of Subsaharan drought. Nature 299, 46^17.
Lamb, P. .1. (1985) Rainfall in Subsaharan West Africa during 1941-83. Z Gletscherk. Glazialgeoi. 21, 131-139.
Le Barbé, L. & Lebel, T. (1997) Rainfall climatology of the Hapex-Sahel region during the years 1950-1990. J. Hydrol.
188-189, 43-73.
Lee, A. F. S. & lleghinian, S. M. (1977) A shift of the mean level in a sequence of independent normal random variables:
a Bayesian approach. Teehnometrics 19(4), 503-506.
L'Hôte, Y., Dubreuil, P. & Lerique, J. (1996) Carte des types de climats "en Afrique Noire à l'ouest du Congo". Rappels et
extension aux régimes hydrologiques. In: L 'hydrologie Tropicale: Géoscience et Outil Pour le Développement
(Mélanges à la mémoire de Jean Rodier, Paris, mai 1995), 55-65. IAHS Publ. no. 238.
L'Hôte, Y. & Mahé, G. (1996) Afrique de l'ouest et centrale, précipitations moyennes annuelles (période 1951-1989).
Carte à l'échelle 1/6 000 000. Orstom, Paris.
Lubès-Niel, H., Masson, J. M., Paturel, S. E. & Servat, E. (1998) Variabilité climatique et statistiques. Etude par simulation
de la puissance et de la robustesse de quelques tests utilisés pour vérifier l'homogénéité de chroniques. Rev. Sei. Eau
11(3), 383-408.
Mahé, G. & Olivry, J. C. (1999) Assessment of freshwater yields to the ocean along the intertropical Atlantic coast of
Africa. C. R. Acad. 5c/., Paris série II328, 621-626.
Mahé, G., L'Hôte, Y., Olivrv, J. C. & Wotliniî, G. (2001) Trends and discontinuities in regional rainfall of West and
Central Africa: 1951-1989. Hydro!. Sei. J. 46(2), 211-226.
Nicholson, S. E. (1985) Sub-Saharan rainfall 1981-84.7. dim. Appl. Met. 24, 1388-1391.
Nicholson, S. E., Some, B. & Kone, B. (2000) An analysis of recent rainfall conditions in West Africa, including the rainv
seasons of the 1997 El Nino and the 1998 La Nina years. J. Clim. 13(14), 2628 -2640.
NOAA (1994) Special climate summary: wettest rainv season (May-September) since 1964 recorded across the western
half of the Sahel. Weekly Climate Bull. 94(42), 7-8.
NOAA (1995-2000) Monthly Climatic Data for the World. National Climatic Data Center, Asheville, North Carolina,
USA.
Olivry, J. C. (1983) Le point en 1982 sur l'évolution de la sécheresse en Sénégambic et aux Iles du Cap Vert. Examen de
quelques séries de longue durée (débits et précipitations). Call Orstom, sér Hydrol. XX( 1 ), 47-69.
Paturel, J. E., Servat, H., Deiattre, M. O. & Lubes-Niel, H. (1998) Analyse de séries pluviométriques de longue durée en
Afrique de l'Ouest et Centrale non sahélienne dans un contexte de variabilité climatique. Hydrol. Sei. J. 43(6), 937946.
Peltitt, A. N. (1979) A non-parametric approach to the change-point problem. Appl. Statist. 28(2), 126-135.
Servat, E„ Paturel, J. E., Lubès-Niel, IL, Kouamé, B., Travaglio, M. & Marieu, B. (1997) De la diminution des
écoulements en Afrique de l'Ouest et centrale. C. R. Acad. Sei.. Paris série II325, 679-682.
Sircoulon, J. (1976) Les données hvdropluviomélriques de la sécheresse récente en Afrique intertropicale. Comparaison
avec 18
lesJune
sécheresses
1913 et 1940.
Cah 2002
Orstom. sér Hydrol. XIU(2), 75-174.
Received
2001; accepted
S March