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G. A. Manful and Y. Opoku-Ankomah
2.2 The River System
The Volta River has three main tributaries: the Black Volta, which originates from
Burkina Faso as Mouhoun; the White Volta, also from Burkina Faso where it is
called the Nakambe, and the Oti River, originating in Benin, where it is known as
the Pendjari (Lemoalle and de Condappa 2009). The Oti passes through Togo before
it joins the main Volta in Ghana. All these tributaries drain into Lake Volta, which,
together with the Lower Volta downstream, also receives flows from other small
tributaries, including the Pru, Sene and Afram. Therefore, the basin is considered to
have four sub-basins: the lower Volta, the black Volta, the white Volta and the Oti
(see Fig. 1).
2.3 Climate Conditions
The climate of the basin is determined by the north-south migration of the InterTropical Convergence Zone (ITCZ), an interface between the hot, dry and dusty
North-East Trade Winds blowing from the Sahara Desert, and the warm, moist tropical maritime climate from the Atlantic Ocean known as the Monsoon. In addition,
squall Lines (SLs) originate over West Africa and move in a westerly direction. They
are mesoscale disturbances and are the most important convective rainfall system
of the West Africa Sahel region, yielding precipitation in the summer months from
June to September (Peters and Tetzlaff 1988). SLs, which join larger-scale features
such as the African Easterly Jet, the Easterly Waves and the Monsoon Front, produce
fields of flow and moisture (Peters and Tetzlaff 1988) and generate about 80% of
annual precipitation of the Sahel (Dhonneur 1981). In southern West Africa, SLs
contribute to annual rainfall, but its contribution to total rainfall decreases from 71
to 56% in the coastal zone (Maranan et al. 2018).
In general, there are three types of climatic zones: the humid south, with two
distinct rainy seasons (March/April-May/June, September-October), the tropical
transition zone, also with two rainy seasons which (May/June, September-October)
are narrowly separated, and the semi-arid north with only one season of rainfall
(May-September), with a peak around August. The south-north gradient of rainfall
is used to identify the main agro-climatic zones of the basin according to the FAO
classification for West Africa (FAO/GIEWS 1998). They are the Sudano-Sahelian
zone, the Sudanian zone and the Guinean zone. The Sudano-Sahelian zone receives
rainfall between 500 and 900 mm, the Sudanian zone 900 to 1100 mm and the
Guinean zone more than 1100 mm (Lemoalle and Condappa 2009).
The mean annual temperatures range from 24 °C in the South to about 36 °C in the
North (Oguntunde 2004; Hayward and Oguntoyinbo 1987). The high temperatures
in the basin amplify the impact of evapotranspiration, which is mainly driven by heat
G. A. Manful and Y. Opoku-Ankomah
2.2 The River System
The Volta River has three main tributaries: the Black Volta, which originates from
Burkina Faso as Mouhoun; the White Volta, also from Burkina Faso where it is
called the Nakambe, and the Oti River, originating in Benin, where it is known as
the Pendjari (Lemoalle and de Condappa 2009). The Oti passes through Togo before
it joins the main Volta in Ghana. All these tributaries drain into Lake Volta, which,
together with the Lower Volta downstream, also receives flows from other small
tributaries, including the Pru, Sene and Afram. Therefore, the basin is considered to
have four sub-basins: the lower Volta, the black Volta, the white Volta and the Oti
(see Fig. 1).
2.3 Climate Conditions
The climate of the basin is determined by the north-south migration of the InterTropical Convergence Zone (ITCZ), an interface between the hot, dry and dusty
North-East Trade Winds blowing from the Sahara Desert, and the warm, moist tropical maritime climate from the Atlantic Ocean known as the Monsoon. In addition,
squall Lines (SLs) originate over West Africa and move in a westerly direction. They
are mesoscale disturbances and are the most important convective rainfall system
of the West Africa Sahel region, yielding precipitation in the summer months from
June to September (Peters and Tetzlaff 1988). SLs, which join larger-scale features
such as the African Easterly Jet, the Easterly Waves and the Monsoon Front, produce
fields of flow and moisture (Peters and Tetzlaff 1988) and generate about 80% of
annual precipitation of the Sahel (Dhonneur 1981). In southern West Africa, SLs
contribute to annual rainfall, but its contribution to total rainfall decreases from 71
to 56% in the coastal zone (Maranan et al. 2018).
In general, there are three types of climatic zones: the humid south, with two
distinct rainy seasons (March/April-May/June, September-October), the tropical
transition zone, also with two rainy seasons which (May/June, September-October)
are narrowly separated, and the semi-arid north with only one season of rainfall
(May-September), with a peak around August. The south-north gradient of rainfall
is used to identify the main agro-climatic zones of the basin according to the FAO
classification for West Africa (FAO/GIEWS 1998). They are the Sudano-Sahelian
zone, the Sudanian zone and the Guinean zone. The Sudano-Sahelian zone receives
rainfall between 500 and 900 mm, the Sudanian zone 900 to 1100 mm and the
Guinean zone more than 1100 mm (Lemoalle and Condappa 2009).
The mean annual temperatures range from 24 °C in the South to about 36 °C in the
North (Oguntunde 2004; Hayward and Oguntoyinbo 1987). The high temperatures
in the basin amplify the impact of evapotranspiration, which is mainly driven by heat