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Three main river systems drain the country:
- The Volta river system consists of the Oti and Daka rivers, the White and Black Volta rivers, and the Pru, Sene and Afram rivers. The basin covers 70 percent of the country area.
- The southwestern river system comprises the Bia, Tano, Ankobra and Pra rivers and covers 22 percent of the country area.
- The coastal river system comprises the Ochi-Nakwa, Ochi Amissah, Ayensu, Densu and the Tordzie rivers, covering 8 percent of the country area.
Groundwater occurs mainly in the following formations:
- The Voltaian formation has little or no primary porosity and thus groundwater occurrence is associated with the development of secondary porosity because of jointing, shearing, fracturing and weathering. In the wet forested southwestern part of the country, the weathered zone has an average thickness of 60 m while it is thinnest in the semi-arid area in the extreme northeast where the mean thickness is about 10 m. Yields rarely exceed 6 m3/hr.
- The Cenozoic and Mesozoic sediments occur mainly in the extreme southeastern and western part of the country. Three aquifers occur in this formation. The first aquifer is unconfined and occurs in the Recent Sand very close to the coast. It is between 2 and 4 m deep and contains meteoric water. The intermediate aquifer is either semi-confined or confined and occurs mainly in the Red Continental Deposits of sand clay and gravel. The depth of this aquifer varies from 6 m to 120 m. The third aquifer occurs in the limestone and varies in depth between 120 and 300 m. Groundwater in this aquifer occurs under artesian conditions and is fresh. The average yield in this limestone aquifer is about 184 m3/hr.
Falling groundwater levels have been observed in the Upper Regions where over 2 000 boreholes have been drilled since the mid-1970s in the rural areas to provide potable water to communities.
Wetlands constitute about 10 percent of Ghana’s total land area. The three main types of wetlands are: i) marine/coastal wetlands; iii) inland wetlands; iii) human-made wetlands. Wetlands in Ghana are very productive and their resources have been traditionally used by local populations as a source of the basic necessities of life, ranging from building materials, hunting and fishing areas, to sources of water for humans and livestock. Local populations have developed traditional knowledge systems and practices which govern the management of wetlands. Ghana is a signatory to the Ramsar Convention and there are five Ramsar sites of international importance in the country: i) Densu Delta; ii) Songor; iii) Keta Complex; iv) Muni-Pomadze; v) Sakumo Lagoons. All these are protected areas and they have been gazetted as such. Other wetlands located in the forest and wildlife reserves of the Mole National Park, Black Volta, Sene, Bia and Owabi Wildlife Sanctuaries are protected too. Some wetlands, which fall outside the conserved wetland areas, are subject to traditional conservation practices such as the rivers Ankobra and Pra. The two most important lakes in the country are Lake Volta and Lake Bosomtwi in the Ashanti region.
Ghana’s total actual renewable water resources are estimated to be 53.2 km3/yr, of which 30.3 km3/yr are internally produced (Table 3). Internally produced surface water amounts to 29 km3/yr, while groundwater is estimated at 26.3 km3/yr. The overlap between surface water and groundwater is estimated at 25 km3/yr. About 22.9 km3 of surface water enter the country annually, of which 8.7 km3 come from Burkina Faso, 6.2 km3 from Côte d’Ivoire and 8 km3 from Togo.
The Akosombo Dam was completed in the mid-1960s and impounds the Volta River to form Lake Volta, one of the largest artificial lakes. The hydropower capacity of the dam is 912 MW. Lake Volta has a surface area of 8 502 km2, a maximum depth of 91 m and a capacity of 147.96 km3. The total dam capacity of the country is 148.5 km3.