1
S1. Droughts and drought parameters
2
Any objective definition of droughts would take into consideration the duration, deficit and threshold at which
3
droughts are said to occur (Dracup et al. 1980). These are important because any specific drought must have a given
4
duration in time ranging from a few days to several weeks and years; a given moisture deficit that warrants the
5
appellation drought and a specific moisture threshold below which droughts are said to occur. However, a major
6
challenge in drought studies and synthesis has been the difficulty for the scientific community to arrive at specific
7
globally acceptable moisture deficits, moisture shortage durations and thresholds at which droughts are said to
8
occur. These polemics are fueled by the fact that environmental conditions and perceptions of what constitutes a
9
water deficit, drought duration and threshold varies from place to place (Lamb 1982; Glantz 1994). Whatever the
10
case, if scientists seem to agree on one thing, it is the fact that droughts are unpredictable as they remain recurrent.
11
While several types of droughts have been reported in the literature, the three main types are: agricultural droughts
12
(declines in agricultural yields), hydrological drought (declines in discharge in rivers and streams) and
13
climatological droughts (declines in rainfall) (Dracup et al. 1980; Glantz 1994).
14
In terms of drought parameters, the duration is marked by the period from the start to the end of a drought and
15
represented by D1, D2, D3, and D4 on fig. S1. The deficit on the other hand is represented on fig. S1 by S1, S2, S3
16
and S4 and it denotes the moisture level below the threshold (QO). The threshold (QO) is the minimum moisture
17
level below which droughts occurs. The inter-event time represented on fig. S1 by T1, T2 and T3 is the period of
18
moisture deficit below the minimum moisture threshold (QO) between two droughts. V1, V2 and V3 on fig.S1
19
represent the inter-event volume which is the period of moisture surplus above the minimum threshold (QO)
20
between two droughts. Normally, when there is a drought the moisture is said to be below the threshold (QO). This
21
is followed by the inter-event time which is a period of recovering moisture which however is still below the deficit.
22
Once there is a recovery, the cycle gets into the inter-event volume which is the moisture surplus above the
23
threshold. If the inter-even volume is not properly handled, the moisture level falls below the threshold and a new
24
drought begins. All in all, the critical issue is the subjective water content below which there is a deficit at which
25
crops, plant, forests and rivers are unable to have sufficient water for survival and discharge (Fleuret 1986;
26
Tallaksen et al. 1997).
27
28
Fig. S1. Sketch of the different drought parameters. Source: Inspired from Tallaksen et al.
(1997).
29
S2.Theoretical framework
30
Theoretically, the principal causes of droughts in the Sahel are changes in SST, surface albedo; dust feedbacks and
31
human induced climate change (CC) (fig. S2). One of the reasons why it is difficult to subscribe to a single factor as
32
the main cause of droughts is linked to the high degree of interdependence and feedbacks between these variables
33
(Hosseini et al. 2009). Lower SST for example would lead to drier conditions because of limited convection;
34
increase albedo through reduced vegetation cover while increased albedo will reinforce drier conditions (Hulme,
35
2001). Increase albedo would be accompanied by increase atmospheric dust transportation (Zeng 2003) in the
36
atmosphere due to reduced vegetative cover (Zeng 2003). The feedbacks from dust accumulations are, higher
37
temperatures due to reduced rainfall, higher albedo (Nicholson et al 1998) due to reduced vegetation cover and
38
increase effects of human induced CC (Vecchi and Soden 2007). The latter would generally increase dust feedback,
39
albedo and temperatures (Shanahan, et al. 2009).
40
41
Fig. S2. Spatially aggregated rainfall anomaly in standard deviations against years for the Sahel between 1900 and
42
2010. Source: Inspired from New et al. (2000) and After Anyamba and Tucker (2005).
43
In general, droughts would have effects on forests, biodiversity, crop yields and aquatic systems (Hosseini et al.
44
2009). Generally, when droughts occur, reduced water availability leads to increase tree mortality in many parts of
45
the world such as in the boreal forests of Canada and in the Sahel (Peng et al. 2011; Zeng 2003), reduced biomass
46
carbon stock, increased atmospheric CO2 (Ma et al. 2012; Molen et al. 2011) and reduced crop yields (Dilley et al.
47
2005). Increase tree mortality may also lead to reductions in the water balance at the land phase of the hydrological
48
cycle). The feedbacks of reduced crop yields on the other components have not been sufficiently researched. In the
49
Sahel, health and national security problems linked to HIV/AIDS and Malaria (Muller 2004;UNAIDS 2002) and
50
civil wars (resource wars) such as the rush for diamonds in Angola are making it difficult for the people to
51
effectively handle the effects of droughts especially on food systems. These diseases and wars erode human capital
52
and provide unstable conditions which reduce the population’s ability to work on their farms (Patz and Olson 2006;
53
U.S Centers for Disease Control 1973).
54
S3. Principal Drought Types, Indices and Quantification methods
55
The three principal types of droughts are meteorological, hydrological and agricultural (Mishra and Singh 2010;
56
Hosseini et al. 2009; Tallaksen et al. 1997; Glantz 1994). Meteorological droughts are concerned with the rainfall
57
levels at which droughts are said to occur. This has been quantified with the use of the Standardized Precipitation
58
Index (SPI) that relies essentially on rainfall (McKee et al. 1993), the Palmer Drought Severity Index (PDSI) and the
59
Standardized Precipitation Evapotranspiration Index (SPEI) which both use precipitation, evapotranspiration and
60
soil moisture (Palmer 1965; Vicente-Serrano et al. 2010). In the case of hydrological droughts, reference is made to
61
changes in the amount of water discharged by rivers or through overland flow. This is often measured using the
62
Palmer Hydrological Drought Index (PHDI) (Palmer 1965) and the Surface Water Supply Index (Palmer 1968).
63
Agricultural droughts represent moisture shortage that affects crop growth. This has also been quantified using the
64
Crop Moisture Index (CMI) (Palmer 1968), Crop Specific Drought Index (CSDI) (Meyer et al. 1993) and Crop
65
Water Stress Index (CWSI) (Idso et al. 1981; Zarger et al. 2011; Zelenhasic and Salvai 1987).
66
In the case of drought modeling and quantification, several methods have been used (see fig. S3). However, greater
67
emphasis will be given to drought forecasting in this section. This method is critical in drought management in the
68
Sahel as it plays a vital role in drought early warning signals and mitigation (Mishra and Singh, 2011; Zhao et al.
69
2011). Drought forecasting has several techniques such as regression models (Leilah and Khateeb 2005; Li et al.
70
1996; Liu and Negron-Juarez 2001; Mishra and Singh 2011), time series models (Mishra and Desai 2005). Other
71
forecasting methods are achieved through probability models, artificial neural network, hybrid models, long-lead
72
forecasting and data mining (Morid et al. 2007; Kim and Valdes 2003; Dhanya and Nagesh Kumar 2009). Other
73
non-drought forecasting methods include drought probability, time series analysis and global (regional) climate
74
models (Dupuis 2010; Kim et al. 2003; Chowdhary and Singh 2010; Santos 1983; Shin and Salas 2000; Kim et al.
75
2002; Soule 1992; Wang 2005; Rind et al. 1990; Mishra and Singh 2010; Westphal et al. 2007; Zagar et al. 2011).
76
77
Fig. S3. Schematic representation of the various techniques of drought simulation and quantification. Source:
78
Inspired from Mishra and Singh (2011).
79
S4. Sahelian Rainfall Trends prior to and during the 20th Century
80
Prior to the 1960s, data on droughts could mainly be obtained from palaeoclimatic sources which indicated
81
enhanced aridity in the Sahel during the glacial era (Sweezy 2001; Goudie and Middleton 2001; Jolly et al. 1998).
82
About 21 thousand years ago, the Sahara desert occupied a much larger area than at present as revealed by the dating
83
of fossil dunes around 5 °S of the present extent of the mobile dunes (Kukla and Gavin 2004; Tucker and Dregne
84
1991). Measurements at the 200 mm/year rainfall isohyet reveal that in 1984 the size of the Sahara at this point was
85
about 9 million Km2 while in 1994 it measured about 8 million Km2 due mainly to orbital forcings (Tucker and
86
Nicholson 1999).
87
The Sahel has experienced alternating sub-millennial scale humid and arid blips against a backdrop of greater aridity
88
than existed before the Saharan desiccation which started some 5 thousand years ago (Stokes et al. 2004). Brooks
89
(1998) makes mention of a sequence of arid and humid eras when he describes an arid period from 300 BC-300 AD
90
when the Sahara was said to be drier than at any other time during the past 2000 years; this was followed by a wetter
91
period from 300 AD to 1100 AD and a progressive arid period from 1100 AD-1500 AD (Nicholson 1998; Fleitmann
92
et al. 2003).
93
It is suggested that the lack of reliable data between 1900 and 1940 on rainfall variability has been a major hurdle
94
for prognostics (Grove 1973; Lamb 1982). As such, historical records of fire, tree rings and water levels from rivers
95
Senegal and Niger and Lake Chad suggest that among the years 1921, 1926 and 1931 that had evidence of droughts
96
(Bunting et al. 1976; Tanaka et al. 1975); the droughts of 1931 were as severe as those of 1982/1984 (Tippett 2006).
97
A critical analysis of some pre 1960 rainfall anomalies contradicts this view and shows that among the most
98
remarkable periods of droughts in the 20th century Sahel which were (1910-1915, 1926-1927, 1940-1950, 1972-
99
1973, 1982-1985); only the droughts of 1910-1915 can be compared to those of 1982-1985 based on the intensity of
100
the deficits in moisture (fig. S4) (Dai et al. 2004; New et al. 2000; Anyamba and Tucker 2005; Olsson 1993; Glantz
101
1994; Nicholson 1998; Nicholson et al. 1996; New et al. 2000; Bell et al. 2000).
102
2
1.5
1
0.5
2010
2005
2000
1995
1990
1985
1980
1975
1970
1965
1960
1955
1950
1945
1940
1935
1930
1925
1920
1915
1910
-0.5
1905
0
1900
Annual rainfall anomaly in the Sahel
2.5
-1
-1.5
-2
-2.5
Years
Rainfall
103
104
Fig. S3. Spatially aggregated rainfall anomaly in standard deviations against years for the Sahel between 1900 and
105
2010. Source: Inspired from New et al. (2000) and After Anyamba and Tucker (2005).
106
107
108
109
110
111
112
113
114
115
116
117
Table S1. Synthesis of the causes of Droughts
Key Findings
References
Sea surface temperatures
Giannini et al. 2008; Zeng 2003; Wolff et al. 2011
Vegetation and land degradation
Wang et al. 2005;Charney et al.1977
Dust feedbacks
Hui et al. 2008;Prospero and Lamb 2003
Human induced climate change
Giannini et al.2003;Hulme 2001;Gonzalez et al. 2012
118
119
120
Table S2. Synthesis of the effects of Droughts
Key Findings
References
Increase forests dieback
Gonzalez et al. 2012; Nicholson 2001
Reduced tree species diversity
Gonzalez 2001; Allen et al. 2009
Reduced carbon sequestration
Woomer et al. 2004
Forest Retraction
Gonzalez 2001
Reduced crop yields
Haile 2005; Clover 2010 Olsson 1993
121
122
123
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