INVESTIGATION ONTHE 1970S AND 1980S DROUGHTS IN FOUR TIRBUTARIES OF THE NIGER RIVER BASIN (WEST AFRICA)

Abstract

West Africa has experienced severe droughts during 1970s and 1980s. On the other hand, the region is characterized by high inter-annual rainfall variability and there seems to be a recent recovery. But has the drought stopped? To answer this question, we evaluated spatio-temporal pattern of rainfall and runoff in four tributaries (Sota, Alibori, Mekrou and Kompa-gorou) of the Niger River basin, covering a total area of 40,000km2 for the period 1971 to 2010. First, decadal rainfall variability was investigated using Kriging-based isohyets. Cross entropy method was then applied to detect breakpoints in rainfall and runoff series. Additionally, the rainfall-runoff relationship was assessed via Spearman’s rank correlation coefficients. Yet the drought started in 1970s peaked in 1980s, but the wetness of the last two decades led to an overall increase of both rainfall and runoff over the study area. Though a moderate to strong (0.57-0.66) rainfall- runoff correlation was obtained for three of the four investigated catchments, the breakpoints in rainfall and runoff series were not per se consistent probably due to gaps in discharge data. Rainfall depicted a shift around 1992 but runoff around 1983. The wetness of the decades, 1990s and 2000s and the manifold floods records of this first half of 2010s over West Africa are evidences that the droughts of 1970s and 1980s have stopped.

Full text

E-proceedings of the 36th IAHR World Congress
28 June – 3 July, 2015, The Hague, the
Netherlands
1
INVESTIGATION ONTHE 1970S AND 1980S DROUGHTS IN FOUR TIRBUTARIES OF THE NIGER
RIVER BASIN (WEST AFRICA)
DJIGBO F. BADOU (1), ABEL AFOUDA (2) , BERND DIEKKRÜGER (3) & EVISON KAPANGAZIWIRI (4)
(1) Graduate Research Program on Climate Change and Water Resources, University of Abomey-Calavi, Abomey-Calavi, Benin,
fdbadougmail.com
(2) Graduate Research Program on Climate Change and Water Resources, University of Abomey-Calavi, Abomey-Calavi, Benin
aafouda@yahoo.fr
(3) Department of Geography, University of Bonn, Bonn, Germany
b.diekkruger@uni-bonn.de
(4) Hydrosciences Research Group, CSIR Natural Resources and the Environment, Pretoria, South Africa
evisonk@gmail.com
ABSTRACT
West Africa has experienced severe droughts during 1970s and 1980s. On the other hand, the region is characterized by
high inter-annual rainfall variability and there seems to be a recent recovery. But has the drought stopped? To answer
this question, we evaluated spatio-temporal pattern of rainfall and runoff in four tributaries (Sota, Alibori, Mekrou and
Kompa-gorou) of the Niger River basin, covering a total area of 40,000km2 for the period 1971 to 2010. First, decadal
rainfall variability was investigated using Kriging-based isohyets. Cross entropy method was then applied to detect
breakpoints in rainfall and runoff series. Additionally, the rainfall-runoff relationship was assessed via Spearman's rank
correlation coefficients. Yet the drought started in 1970s peaked in 1980s, but the wetness of the last two decades led to
an overall increase of both rainfall and runoff over the study area. Though a moderate to strong (0.57-0.66) rainfall-
runoff correlation was obtained for three of the four investigated catchments, the breakpoints in rainfall and runoff series
were not per se consistent probably due to gaps in discharge data. Rainfall depicted a shift around 1992 but runoff
around 1983. The wetness of the decades, 1990s and 2000s and the manifold floods records of this first half of 2010s
over West Africa are evidences that the droughts of 1970s and 1980s have stopped.
Keywords: Drought, Recovery, Breakpoint analysis, Cross-entropy method, Niger River basin
1. INTRODUCTION
There is a consensus regarding the break of early 1970s in hydro-climatic series and the correlated drought and famine
of 1980s in West Africa, (Goula et al., 2007; Mahé et al., 2005; Nicholson., 2001; Omotosho, 2008; L’Hote et al., 2002).
However, while certain authors demonstrated that the drought is still continuing, others proved that it has stopped.
Omotosho (2008) reported a shift to recovery at the station of Kano (Sahelian zone of Nigeria) in recent years with the
period1996-2000 being the wettest 5-years period since 1931, whereas L’Hote et al. (2002; 2003) demonstrated that the
drought of 1980s continues. Paturel et al. (1998) pointed out the succession of wet and dry decades during both halves
of the twentieth century along with a heterogeneous manifestation of the drought in space within 16 non-Sahelian West
and Central African countries. Lebel and Ali (2009) specified that the recovery was complete in eastern Sahel, almost
complete in central Sahel but not yet actual in western Sahel. In this debate, certain zones get only little attention. It is
for instance the case of upper Benin Republic which is located in the Sudanian zone of West Africa and the study area
for this work. The research area (40,000 km2) is an active part of the transboundary Niger River basin. Home for more
than 1.5 million people (RGPH4, 2013); it constitutes the largest cotton and vegetable production, as well as cattle
breeding zones of the country. It also contains the W Park, one of the most important West African wildlife parks.
Contrary to the ecosystem services this area provides, there are very few studies undertaken in that area (Vissin, 2007),
and unfortunately, these studies are limited to the period 1955-1992 only. The aim of the present study is to investigate
whether the great drought is still continuing in the research area.
2. MATERIALS AND METHODS
2.1 Study area
The study area is made up of 4 tributaries of the Niger River basin: Mekrou with a catchment area of 10,552 km2, Alibori
13,684 km2, Sota 13,449 km2 and Kompa-Gorou 2,041 km2. 95% of the area of these basins is located in Benin
Republic and situated between 1°50' and 3°75' W longitude and 10°0' and 12°30' N latitude. Characterized by a

E-proceedings of the 36th IAHR World Congress,
28 June – 3 July, 2015, The Hague, the Netherlands
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unimodal rainfall regime, the mean annual rainfall of the area for the period 1971-2010 is about 936 mm, with mean
minimum and maximum temperatures of 21.5°C and 34.6°C respectively.
2.2 Data sources
Data were collected from different sources as shown in Table1.
Table 1 : Data sources. DMN stands for Direction Météorologique Nationale, CeRPA for Centre Regional de Production Agricole, DGEau for
Direction Générale de l’Eau
Data Number of
stations &
period covered
Data type Sources
Climate 27 stations
1970-2010
Rainfall DMN Benin,
CeRPa Benin,
DMN Burkina Faso
and Niger Met.
Office
Discharge
05 stations
1970-2010
-
Discharge
DGEau
2.3 Inter-decadal rainfall variability
Using annual rainfall data, decadal rainfall variability was investigated via Kriging-based isohyets.
2.4 Breakpoints analyses of rainfall and runoff
Cross entropy method (Priyadarshana & Sofronov, 2014) embedded in R software was applied to detect the breakpoints
in rainfall and runoff series. By providing multiple breaks and by offering the possibility for the user to choose the
number of breaks desired, the cross entropy method seems appropriate to the context of high inter-annual rainfall and
runoff variability of West Africa. The determination of the breakpoint in rainfall and runoff data was done first for
individual rain gauge and hydrometric station using cross entropy technique and then for the entire research area by
applying the t-test at 5% level of significance.
2.5 Correlation of rainfall-runoff relationship
Mean annual catchment precipitation was computed using Thiessen polygon method. The correlation rainfall-runoff was
investigated by assessing Spearman’s rank coefficients (Spearman, 1904).
3. RESULTS AND DISCUSSION
3.1 Inter-decadal rainfall variability
A visual inspection of Figure 1 reveals that for the decade 1970s, the isohyets varied between 1150mm and 700mm.
They dropped down in between 1050mm and 650mm during the decade 1980s. However, in contrast to the two previous
decades, they jumped up in between 1300mm and 750mm and in between 1250mm and 750mm respectively for the
decades 1990s and 2000s. An overall increase of rainfall was hence observed for the research area.

E-proceedings of the 36th IAHR World Congress
28 June – 3 July, 2015, The Hague, the Netherlands
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Figure1: Mean decadal rainfall variability
3.2 Breakpoints analyses of rainfall and runoff
The result of breakpoint analysis at individual rain gauge is given in Table 2 below. The application of the t-test resulted
for the entire area in a break (hereafter referred as Break-1) around 1992 ±2.5 years.
Table 2: Year of break in rainfall data. Rain gauges are ordered from the South to the North of the study area
Station Longitude Latitude Year of break
South
Parakou 2.60 9.35 1988
Djougou 1.67 9.70 2003
Nikki 3.20 9.93 1988
Ina 2.73 9.97 1988
Birni 1.50 10.00 1997
Boukoumbe 1.10 10.17 1991
Bembereke 2.67 10.20 1988
Kalale 3.38 10.30 1989
Natitingou 1.38 10.32 2003
Kouande 1.68 10.33 1991
Tanguieta 1.27 10.62 1991
Centre
Kerou 2.10 10.83 1979
Segbana 3.70 10.93 1996
Kandi 2.93 11.13 1998
Banikoara 2.43 11.30 1994
Alfakoara 3.07 11.45 1997
North
Mahadaga 1.75 11.70 1986
Malanville 3.40 11.87 2003
Namounou 1.70 11.87 1988
Gaya 3.45 11.88 2003
Karimama 3.18 12.07 No break
Diapaga 1.78 12.07 1994
Fada 0.35 12.07 1991
Ouna 3.15 12.17 1989
Tapoa 2.40 12.47 1994
Tamou 2.17 12.75 1987
Niamey 2.17 13.48 1989
Table 3 displays, for each hydrometric station, the year of breakpoint in runoff data. For the entire area, the application
of the t-test resulted in a break (hereafter referred as Break-2) in 1983 ±6.8years.
At first glance, there is a mismatch between Break-1 (1992 ±2.5 years) and Break-2 (1983 ±6.8years). Gaps in runoff
data within the period 1993-2003 might be the cause of such a difference. However, the upper limit of Break-2 (1989-
1990) is in line with the lower limit of Break-1 (1989-1990). Thus, one might be tempted to establish a consistency
between the two breakpoints. These results and those of the section 3.1 show that the shift to wet condition in the
research area as a whole occurred around 1990. Similar conclusions were drawn by Lebel and Ali (2009) and Omotosho
(2008) who reported the cessation of the drought in eastern Sahel and in northern Nigeria (station of Kano) respectively.

E-proceedings of the 36th IAHR World Congress,
28 June – 3 July, 2015, The Hague, the Netherlands
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Table 3: Year of break in runoff series.
Station Longitude Latitude Year of
break
Kompongou 2.195
11.399
1986
Yankin 2.661
11.247
1975
Malanville 3.396
11.88
1982
Gbasse 3.25
10.978
1988
Couberi 3.326
11.737
1988
.
3.3 Correlation of rainfall-runoff relationship
As can be seen in the Table 4, the rainfall-runoff correlation was moderate to strong for all catchments with the
exception of Gbasse. Data quality (49% of gap) of the station of Gbasse might be questionable and responsible for the
poor rainfall-runoff relationship.
Table 4: Spearman rank coefficients of correlation rainfall-runoff and the corresponding p-values
Station ρ p-value
Kompongou 0.571 0.01851
Yankin 0.664 0.0001661
Couberi 0.660 7.96e-05
Gbasse 0.332 0.141
4. CONCLUSIONS
An update of recent rainfall and runoff variability in 4 non-Sahelian tributaries of the Niger River basin has been done
through this study. Despite moderate to strong (0.57-0.66) rainfall-runoff correlations for three of the four investigated
catchments and probably due to gaps in discharge data, the break in rainfall and runoff data were not per see consistent.
Runoff depicted a jump in 1983 ±6.8 years whereas the shift in rainfall occurred in 1992 ±2.5 years. We demonstrated
that the cessation of the great drought of the 1970s and 1980s occurred around 1990 in the research area.
ACKNOWLEDGMENTS
The authors are grateful to the German Ministry of Education and Research (BMBF), the West African Science Service
Center on Climate Change and Adapted Land Use (WASCAL), the Graduate Research Program Climate Change and
Water Resources of the University of Abomey-Calavi and the Council for Scientific and Industrial Research (CSIR). The
institutes which provided climate and runoff date are also highly acknowledged. These are the Agence pour la Securite
de la Navigation Aerienne en Afrique et a Madagascar (ASECNA) of Benin, Burkina Faso and Niger, the Centre Regional
pour la Promotion Agricol (CeRPA) and the Direction Generale de l’Eau (DGEau).
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