3.1. Temporal Analysis
In this section, the trends (1981–2015) of each ETCDDI extreme climate index were analyzed and then averaged over each country. For each country, the trends of the six indices and their significances are shown in Table 3
Both Senegal and Niger (Figure 4
) show significant increases in PRCPTOT (+3.7 and +2.9 mm per decade respectively, Table 3
), RR1 (+5 and +4 days per decade respectively, Table 3
), and CWD (+3 and +3 days per decade respectively, Table 3
). However, Senegal and Niger present significant decreases in CDD (−5 days per decade each, Table 3
), R95P (−1.5 and −3 mm per decade respectively, Table 3
) and SDII (−1.5 and −2.9 mm/day per decade, respectively, Table 3
). Hence, over Senegal and Niger, the analysis reveals a recent increase in precipitation that results from an increase in precipitation frequency (increase in RR1) as well as the maximum length of wet spells (increase in CWD).
In the adjacent area of the Sahelian zone in Burkina Faso (Figure 5
), there is a significant increasing trend in PRCPTOT, RR1, and SDII (+3.8 mm, +3 days, and +2.5 mm/day per decade, respectively, Table 3
), while the CDD (−5 days per decade) exhibits a significant decreasing trend. Decreasing trends of CWD and R95P (−0.5 days and −0.1mm per decade) were also observed over Burkina Faso but are not significant. It should, however, be noted that R95P increases after 2000. Hence, over Burkina Faso, the analysis shows a recent augmentation of precipitation that results from an increase in precipitation frequency (increase in RR1 but not in CWD) and intensity (increase in SDII), including very wet days after 2000 (increase in R95P after 2000).
Over Côte d’Ivoire and Benin (Figure 6
), the results show an increase in PRCPTOT (+1 and +1.3 mm per decade), a decrease in CWD (−0.2 and −1.8 days per decade, respectively, Table 3
) and CDD (0 and −1 days per decade, respectively, Table 3
), an increase in RR1 (+1 and +1day per decade, respectively, Table 3
) and SDII (+0.1 and +2.7 mm/day per decade, only significant in Benin respectively, Table 3
), and no clear change in R95P (Figure 6
). Hence, over Côte d’Ivoire and Benin, the analysis shows an increase in precipitation as a result of a weak increase in precipitation frequency (increase in RR1 but not in CWD) and intensity (increase in SDII but not in R95P).
3.2. Spatial Variability of the Changes over the Last Two Decades
To highlight the spatial distribution of these variations over time, Figure 7
, Figure 8
, Figure 9
, Figure 10
and Figure 11
show the changes in PRCPTOT, RR1, CWD, CDD, R95P and SDII over of the periods 1996–2005 and 2006–2015, using 1986–1995 as reference period in all five countries. The first column of these Figures represents the mean value of each index (according to the row) over reference decade 1986–1995 while the second and third columns show the anomaly between reference decade and the periods 1996–2005 and 2006–2015, respectively.
Over Senegal, the analysis of the annual mean over the reference decade (first column of Figure 7
exhibits that the southern part (south of 14° N) recorded the highest PRCPTOT (750–1500 mm/year), RR1 (60–100 days during JJAS period) and CWD (6–10 days), and the lowest CDD (0–10 days during JJAS period). In addition, the highest values of SDII and R95P are observed over the central and southern part of the country during the reference decade while the lowest mean values are recorded over the northern and eastern parts. During the first (1996–2005) and the last (2006–2015) decades in Senegal (second and third columns Figure 7
), PRCPTOT and RR1 have increased over the entire country except for some areas in the southern part during the last decade. The increase in PRCPTOT is significant in the western, northern and southeastern parts of the country, whereas the increase in RR1 is statistically significant over the entire country for the two considered decades relative to reference decade. However, the CDD presents a significant decreasing trend over the entire country for the two decades while the CWD presents an increase, which is only significant in some areas. Large parts of the country present significant decreasing trends for R95P and SDII but present slightly increasing trends over parts of the northern and southeastern parts of the country. Hence, during the first period, the increase in precipitation observed over Senegal is mostly as a result of an increase in precipitation frequency (including shorter dry spells). In addition, an increase in precipitation intensity (including very wet days) over the northern and southeastern parts of the country contributed to the total increase in precipitation amount. Furthermore, during the second decade (2006–2015; third column Figure 7
), PRCPTOT, R95P, and SDII increased over the entire country except over the central-southern region, where they clearly decrease. In addition, RR1 and CWD increased over the entire country, while CDD slightly decreased over the entire country. Indeed, during the second decade, the rise in precipitation observed over Senegal results from a clear increase in precipitation frequency (including longer wet spells and shorter dry spells) and to some extent in precipitation intensity (including very wet days). The strong decrease in precipitation observed over the central-southern part of the country is due to the weakening of the rain intensity (including a reduction in the number of the very wet days).
The agricultural system in Senegal is essentially rainfed and relies on both cash crops (groundnuts, cotton, horticultural products) and food crops (mainly cereals). Senegal suffered the worst droughts in the 1970s, which caused a major famine requiring international food aid [50
]. However, during the last twenty years, the significant increase in cumulative annual rainfall (PRCPTOT), in the number of rainy days (RR1) and in consecutive wet days (CWD) have been favorable to agriculture [52
]. This has led to an increase in productivity and helped to reduce the national acute malnutrition rate from 10.6% in 2010 to 8.8% in 2012 and 5.9% in 2014 [54
]. Indeed, according to the National Agency of Statistics and Demography [55
] in recent decades, groundnut production with a high concentration in the central-western regions (Kaolack, Kaffrine, Fatick, Diourbel, Thiès, Louga) has amounted to 1,050,042 tons, i.e., a 125% achievement rate relative to the annual target of the Programme d’Accélération de la Cadence de l’Agriculture Sénégalaise (PRACAS; 838,728 tons) and an increase of 57% compared to previous years. Similarly, cotton production, especially in the southern zone (production area), experienced a 16% increase in 2015. Generally, in the last decade, there has been an increase in the production of other food crops, particularly rice (62.1%), sorghum (84.2%), cassava (70.9%), cowpea (33.6%), fonio (48.1%), millet (83.3%) and maize (70.3%) [55
]. Despite this good performance in agricultural production, the report of [56
] mentions that the country suffered from locust attacks during 1987 and 1998, which devastated crops, and that a rainfall deficit caused by an early cessation of rains during the 2003 dry season had a severe impact on water resources and crops, particularly millet, which had not reached maturity. In 2012, excess rainfall was noted throughout the country and caused damage to rainfed crops [56
Over Niger, the first column of the Figure 8
presents the spatial mean indices over the reference period. The PRCPTOT, RR1 and CWD increased in the south during the reference decade, whereas the CDD decreased. Nevertheless, the R95P show higher values (20–40 mm) in the southern and northern parts of the country while the SDII presents lower values over the entire country (0–20 mm/day) except in central and north-western parts where the values reach 20–30mm/day. The second and third columns of Figure 8
present the anomaly of computed indices for first (1996–2005) and last (2006–2015) decades, respectively. The RR1 increased significantly over entire country for both decades relative to the reference decade. PRCPTOT also increased over the entire country but the increase is only significant in some parts (especially the north-western and southern-central parts for the first decade and between 10° E–12° E longitude for last decade). North of 14° N latitude, the CWD slightly increased significantly during the two decades relative to the reference decade but the change is more pronounced during the last decade. The CDD decreased significantly over the entire country but this change is more pronounced during the last decade. R95P and SDII slightly increased during the first decade (significant in the western and southern parts) and decreased slightly during the last decade in major parts of the country, especially in the central and northern parts. Hence, during the first period, the increase in precipitation observed over the entire country was mostly due to an increase in precipitation frequency, including longer wet spells (over the north-east) and shorter dry spells. The anomalies observed over the second period (Figure 8
) are very similar to the ones observed over the first period, albeit a slight decrease in precipitation observed over the western part of the country that results from a clear decrease in precipitation intensity (including very wet days).
Strongly dependent on climatic conditions characterized by high aridity accentuated by high temperatures, very high spatio-temporal variability of rainfall as well as a southward displacement of isohyets, Niger’s agricultural sector has experienced an overall increase since 1993 according to the Programme Détaillé du Développement de l’Agriculture Africaine [57
]. Figure 8
shows that, over the first decade, there was a significant increase in all indices in the south and a significant decrease in the number of consecutive dry days (CDD). This increase in rainfall coupled with changes in surface conditions (increase in cultivated areas) has modified runoff and infiltration and this has contributed to a 10% increase in groundwater reservoirs since 1990 [58
]. Recent reports of [59
] on the evolution of rainfall confirm this increasing trend of rainfall and suggest that isohyets are moving northward again. Figure 8
shows that this trend is mainly explained by an increase in the number of rainy days in the last two decades. Despite the overall performance recorded, the agricultural sector in Niger has never managed to grow more than two consecutive years according to [57
]. The main reason is the high inter-annual variability of rainfall. The World Bank report [60
] shows that, despite a generally increasing trend, the years 1984, 1997, 1993, 1997, 2000, 2004, and 2009 rainfall deficits were very closely linked to the poor harvests and a reduction in seasonal lakes that provide drinking water to humans and animals during the rainy season [59
]. Although the increase in average annual rainfall has generally been favorable to crop production, since the 1990s it has led to an increased frequency of flooding. Most floods occur during the rainy season (July to September) when the main crops are sown. They are usually the result of heavy rainfall for a short period of time, leading to flash floods or localized flooding along the banks of the Niger River, leaving little time for preparation. Damage and losses are usually localized, but floods have devastating effects where and when they occur. Homes and buildings are destroyed, fields are flooded and large numbers of livestock are washed away. The number of displaced people can also be high, especially when these floods affect urban areas.
In Burkina Faso (Figure 9
), PRCPTOT, RR1, R95P and SDII increased in the southern part of the country, while CWD and CDD decreased in the same areas. The latitude 13° N is seen as the transitional zone. During the first decade (second column of Figure 9
), PRCPTOT and RR1 decreased in major parts of the country, especially in the southern part, while they increased significantly in the central and northern parts over. During the last decade (3rd column), PRCPTOT increased significantly northward of 11° N latitude, while RR1 increased significantly over entire country. The CWD present a decrease below 14° N for both decades but the change is more pronounced and significant in large areas during the first decade. CDD decreased over entire country for both decades relative to the reference but is more pronounced and significant during the last decade. For the first decade, R95P and SDII decreased in major parts of the country, especially in the central region where both indices were significant while a significant increase is observed in the northern parts. Nevertheless, the decreasing trend of SDII in the central part of the country declined and even disappeared during the second decade, while the increased observed in the northern part became largely significant. Generally, in Burkina Faso, PRCPTOT, RR1, and SDII increased (decreased) over the eastern (western) part of the country during the first period, while CWD and R95P (CDD) clearly decreased (increased) over the entire country. During the second period, PRCPTOT, RR1, and SDII generally increased (decreased) over the western (eastern) part of the country, CWD slightly decreased, CDD increased (decreased) over the eastern (western) part of the country, and R95P increased over the entire country. Hence, trends in annual precipitation show an opposite east–west dipole in the two periods. Changes in both decades are due to a change in precipitation frequency and intensity (including very wet days in the second period). Note that the increase in precipitation frequency occurs despite the shortening of wet spells and the lengthening of dry spells.
Agricultural production in Burkina Faso is dominated by cereals (sorghum, millet, maize and rice), the main food crops, and cotton, the main cash crop. The agricultural production has increased overall in recent years at the national level [61
]. This performance was not only due to an increase in the area under cultivation [63
], but also due to favorable climate and a good adaptation to climatic conditions [64
]. As shown in Figure 9
, the country experienced significant increases in cumulative rainfall (PRCPTOT) and in the number of rainy days (RR1) over almost the entire country during the last two decades. Despite the progress made in recent years, Burkina Faso’s agricultural sector is experiencing disruptions due to several unfavorable factors, the most important of which is based on climatic hazards [65
]. Indeed, the country regularly undergoes meteorological disturbances that cause droughts and floods. Since 1994, Burkina Faso has experienced more than ten major disasters with significant consequences [64
]. Figure 9
on heavy rains (R95P) confirms that northern Burkina Faso was more prone to floods during the first two decades but much more so in the second decade. This result is in line with those obtained by [66
], explaining that the country recorded violent floods, especially in the northern zone during the years 1994, 2007, 2008, 2009, 2010, 2011. Moreover, droughts have had many consequences on food security. In 2004 and 2007, severe droughts caused a significant drop in agricultural production (−16% in 2007), accentuating the food insecurity of rural populations and leading to a slowdown in the growth of the agricultural sector [67
] as well as a deficit in the filling of rivers and the scarcity of water resources for various uses [68
]. During the years 1995, 1996, 1997, 2001, 2011 and 2012, droughts affected 170 of the country’s 352 districts in 10 regions of the country and caused a significant cereal deficit affecting 3.5 million people [64
]. This situation is consistent with the length of consecutive wet days (CWD) which showed a significant decrease during the first decade in most parts of the country and in a few places during the second decade and could have had an influence on the production of cereal crops [56
]. At the same time, the results on intense days (SDII) show a significant decrease, mainly in the center and west during the first decade and only in the south during the second decade. These decreasing trends could explain the slowdown in agricultural growth, thus justifying the impact on food security during the last two decades. The recurrence of climatic shocks in Burkina Faso often affects populations that are already vulnerable due to poverty and the difficult agro-climatic environment. Among the most vulnerable in Burkina Faso, subsistence farmers and herders see their capacities eroded over time under the combined action of disasters and a lack of financial resources.
During the reference decade in Cote d’Ivoire (1st column of Figure 10
), the mean PRCPTOT, RR1, R95P and SDII decreased gradually from west to east and from south to north gradients. The greatest value of PRCPTOT recorded in the western part is due to the orographic effects caused by the hills and mountains. However, the CDD increased gradually according to a north-south gradient. Northward of 9° N and southward of 6° N, the country recorded the largest value (4–6 days) of CWD. The PRCPTOT increased significantly along the coast and in the northwestern part of the country during the first decade while this increase was only significant in the western and in the northwestern parts during the last decade. The RR1 decreased significantly in entire country during the first decade, while it decreased significantly along the coast and in north-central part of the county during the last decade. The CWD decreased during both decades in the entire country and significantly during the first decade. The CDD significantly increased in the entire country except the central part during the first decade. It is important to highlight that this increase was maintained during the last decade except along the coast (below 7° N), where a reduction in CDD was observed. The R95P index increased considerably in all regions south of 8° N, while it decreased significantly in the region north of 8° N for both decades. The SDII index generally increased significantly in the regions south of 9° N, except in the northeastern part during the first decade. During the last decade, the SDII increased significantly in the south and western parts while in the northeastern part decreased significantly. Thus, during the first decade over Côte d’Ivoire, PRCPTOT increased (decreased) over the western and southern (eastern) part of the country, RR1 and CWD decreased over the entire country, CDD increased (decreased) over the southern (western) part of the country, and R95P and SDII increased over almost the entire country. During the second decade, PRCPTOT increased over the central part of the country but decreased in the other parts, RR1 and CWD increased over the entire country, and CDD, R95P, and SDII decreased over the entire country. Hence, the results show a switch from less frequent (including longer dry spells over the south and shorter wet spells over the whole country) and more intense (including very wet days) rainfall before 2005 to more frequent (including longer wet spells and shorter dry spells) and less intense (including very wet days) rainfall after 2005.
The small increase in rainy days and the reduction in the number of consecutive dry days recorded during the last two decades in the southern, south-western, western and north-western zones are consistent with the increase in cashew nut and cotton production in the north and coffee, cocoa and rubber production in the south and south-west [69
]. The climate has been favorable to agriculture in Côte d’Ivoire in recent decades, but this has also had significant impacts on the environment and society. Indeed, since the 1990s, many lives and properties are lost every year due to deadly floods and landslides [70
]. This is in line with Figure 10
, which shows that the southern and western parts of the country have been marked by a significant increase in extreme rainfall and rainy intensities (R95P and SDII, respectively) since the first decade.
In Benin, PRCPTOT and CWD during the reference period (1st column of Figure 11
) are highest in the northwestern part, especially in the Atacora channel (channel of mountains in Benin). The largest values of RR1 are observed between 7–11° N, while those of R95P and SDII are seen in the central, the north and northeastern parts of the country. During the reference period, the CDD decreases with the latitude (North-south direction). The PRCPTOT increased (significant only during the first decade) in the central and northern parts (north of 10° N) during both decades but was more pronounced during the first decade. During the first decade, RR1 increased significantly north of 10° N latitude except in some areas of the western part and below 10° N latitude. Moreover, this increase was significantly intensified in the last decade, during which RR1 for the areas south of 10° N latitude also increased. Furthermore, the CWD decreased in the entire country for both decades but was statistically significant in the south and central parts for the first decade and in the central–west for the last decade. In addition, the CDD increased (significantly decreased) in the areas south of latitude 10° N (north of 10° N latitude) but was significant only near the coast. The R95P decreased in both decades in large parts of the country (significant only in the central-east part). Nevertheless, the SDII increased (not significant) in the entire country between the first and the second decade, while in the last decade, the major part, especially the northern part, showed a decrease (significant from the center to the north-east region). Generally, over Benin, PRCPTOT and RR1 increased (decreased) over the northern (southern) part of the country during the first period, while CWD decreased over the entire country, and CDD, R95P and SDII increased over the entire country. During the second period, PRCPTOT decreased (increased) over the northern (southern) part of the country, RR1 increased over the entire country, CDD, R95P, and SDII decreased over the entire country, and CWD show no clear change. Hence, the precipitation increase observed over the northern part of the country during the first period was due to an increase in both precipitation frequency (despite longer dry spells and shorter wet spells) and intensity (including very wet days) while the precipitation increase observed over the southern part of the country during the second period was mainly as a result of an increase in precipitation frequency (including shorter dry spells). The reduction in rainfall amount at the southern part was due to a strong decrease in precipitation intensity (including very wet days).
Over the last thirty years, the climate in Benin has been favorable to agriculture and mainly to maize (the most consumed cereal in the country) with relatively stable production in almost all growing areas until 2015 [72
]. Usually grown in the southern and central parts of the country (Ouémé, Mono, Atlantique, and Zou), maize cultivation started to develop in the northern regions (especially in Borgou) around the 1990s [73
]. This is coherent with the significant increase in rainy days (RR1) as well as with a significant decrease in consecutive dry days (CDD) in this northern zone during the first and second decades, as shown in Figure 11
. This improvement in production has also been observed for cotton, the country’s main export commodity, accounting for 45% of total national exports by value [74
]. Cotton production is concentrated in the northern zone, in the Alibori department, and, to a lesser extent, in the departments of Borgou and Atacora (northwest). It has declined in the central zone, where the production was important in the 1990s. After reaching a record in the 2004/2005 season [75
], the production fell sharply in 2010/2011. The production recovered in 2011/2012 and has continued to increase until 2014/2015. The drop of cotton yield is coherent with the increase in the number of consecutive dry days (CDD) during the rainy season in the production zones.