4 Climate Change and Water Resources in West Africa …
59
Therefore, in terms of recorded rainfall and rainy seasons, there are three climatic
zones in Senegal and Burkina Faso: the Sahelian zone in the north, the NorthSudanian zone in the center and the South-Sudanian zone in the south. For Ivory
Coast and Benin, the South-Sudanian zone spans the northern and central areas
while the southern area is located in the Guinean zone. In those countries, interannual variability of rainfall was used to determine the evolution of water resources
and especially their scarcity because of drought that recurs in the area.
2.2 Data
In each of the four countries, ten stations were selected based on their geographic
distribution, time-series lengths, meteorological significance, and data reliability.
There were a total of forty stations (Fig. 1 and Table 1).
At most stations, observations began in the 1920s. For consistency, the study
applied a starting year of 1921 and ending year as 2016 for all four countries, a
time series of 96 years. The data came from various national meteorological services
in the countries and the Inter-African Committee of Hydraulic Studies through the
Overseas Office of Scientific and Technical Research (ORSTOM). For each country,
the Standardized Precipitation Index (SPI) was calculated to determine if a given
year is in excess (SPI > 0) or in deficit (SPI < 0) of the baseline standard. Although
the SPI has been designed to detect droughts and is still widely used to characterize
dry phases (Agnew 2000; WMO 2016), it allows an estimation of both dry and wet
periods (McKee et al. 1993; WMO 2012), thus enabling the determination of breaks
and phases in rainfall. The SPI is calculated as follows:
SPI =
P − ¯
P
σ
(1)
• where P is the average rainfall of the year in the country;
• ¯
P is the average rainfall in the 1921–2016 time series in the country; and
• σ is the standard deviation of the annual rainfall series in the country.
Rainfall-break detection remains important in any analysis of climate change.
Rainfall series were subjected to Pettitt’s (1979) and Lee and Heghinian’s (Lee and
Heghinian 1977) break-detecting tests. Pettitt’s approach is nonparametric because
it makes no assumptions about the underlying distribution of data derived from the
Mann–Whitney test (Pettitt, 1979). Pettitt’s procedure is applied here to certify the
research findings of our study. As a result, the absence of a break in the series (xi) of
size T constitutes the null hypothesis (no break in the time series). When conducting
this test, we assumed that for any moment t between 1 and T, the time series (xi) i =
1 at t and t + 1 at T belong to the same population. The variable to be tested is the
maximum in absolute value of the variable Ut, with T defined by
59
Therefore, in terms of recorded rainfall and rainy seasons, there are three climatic
zones in Senegal and Burkina Faso: the Sahelian zone in the north, the NorthSudanian zone in the center and the South-Sudanian zone in the south. For Ivory
Coast and Benin, the South-Sudanian zone spans the northern and central areas
while the southern area is located in the Guinean zone. In those countries, interannual variability of rainfall was used to determine the evolution of water resources
and especially their scarcity because of drought that recurs in the area.
2.2 Data
In each of the four countries, ten stations were selected based on their geographic
distribution, time-series lengths, meteorological significance, and data reliability.
There were a total of forty stations (Fig. 1 and Table 1).
At most stations, observations began in the 1920s. For consistency, the study
applied a starting year of 1921 and ending year as 2016 for all four countries, a
time series of 96 years. The data came from various national meteorological services
in the countries and the Inter-African Committee of Hydraulic Studies through the
Overseas Office of Scientific and Technical Research (ORSTOM). For each country,
the Standardized Precipitation Index (SPI) was calculated to determine if a given
year is in excess (SPI > 0) or in deficit (SPI < 0) of the baseline standard. Although
the SPI has been designed to detect droughts and is still widely used to characterize
dry phases (Agnew 2000; WMO 2016), it allows an estimation of both dry and wet
periods (McKee et al. 1993; WMO 2012), thus enabling the determination of breaks
and phases in rainfall. The SPI is calculated as follows:
SPI =
P − ¯
P
σ
(1)
• where P is the average rainfall of the year in the country;
• ¯
P is the average rainfall in the 1921–2016 time series in the country; and
• σ is the standard deviation of the annual rainfall series in the country.
Rainfall-break detection remains important in any analysis of climate change.
Rainfall series were subjected to Pettitt’s (1979) and Lee and Heghinian’s (Lee and
Heghinian 1977) break-detecting tests. Pettitt’s approach is nonparametric because
it makes no assumptions about the underlying distribution of data derived from the
Mann–Whitney test (Pettitt, 1979). Pettitt’s procedure is applied here to certify the
research findings of our study. As a result, the absence of a break in the series (xi) of
size T constitutes the null hypothesis (no break in the time series). When conducting
this test, we assumed that for any moment t between 1 and T, the time series (xi) i =
1 at t and t + 1 at T belong to the same population. The variable to be tested is the
maximum in absolute value of the variable Ut, with T defined by