10.2.1.2. Major River Basin Systems
Box 10-3. Impact of Drought
in the Akasompo Dam (Graham, 1995)
Multiple droughts in recent decades have forced Ghana to reduce the generation
of hydroelectricity, provoking a national debate about power supply.
The Akosombo and Kpong generating stations—commissioned in 1965
and 1982, respectively—provide the overwhelming bulk of Ghana's
electricity. The two stations account for 1,072 MW of a total national
power-generating capacity of 1,160 MW; Akosombo alone provides 833 MW.
Until recently, most of the minority of Ghanaians who use electricity
tended to regard the Volta hydroelectric dams as sources of uninterruptible
power. The unprecedented drought of 1982-1983, which compelled rationing
of electricity until 1986, shattered that illusion. And if that drought's
power cuts jolted the nation's complacency about hydroelectricity,
the 1994 incident concentrated minds forcefully on the impermanence of
power from Akosombo and Kpong and the need for alternative sources.
According to the Volta River Authority (VRA), the statutory power generating
body, "cumulative inflow" into Volta Lake by the middle of August
1994 was "the worst…in the 50-year record of Volta river flows—worse
than the same period in 1983." At its lowest, in early August, the
level of Volta Lake was 73 m. This was well below the 75.6 m the VRA claims
is the minimum level for generating power without risk of damaging the
turbines.
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The heterogeneity of ground records in Africa imposes
serious limitations in constructing future scenarios of water resources. Where
consistent long-term climatic data are available, they indicate a trend toward
reduced precipitation in current semi-arid to arid parts of Africa. Figure
10.3 of the Hulme et al. (2001) publication shows possible scenarios
for different climatic regions of Africa. Although there is ambiguity in the
Sahel, the simulations appear to indicate possible increases in precipitation
in east Africa, whereas most simulations in southern Africa indicate reduced
precipitation in the next 100 years.
Table 10-1 shows estimates of ranges of percentage
changes in precipitation, potential evaporation, and runoff in African river
basins as reconstructed from Arnell (1999, Figure 3). In some basins, estimates
given by the HadCM3 simulation have been excluded where they appear to be outliers.
A change in the hydrographs of large basins (Niger, Lake Chad, and Senegal)
has been observed. Between the mean annual discharge of the humid and drought
periods, the percentage of reduction varies from 40 to 60% (Olivry, 1993). Figure
10-6 shows the change in the hydrograph of the Niger River at the Niamey
station. This illustrates a clear modification of the Niger River regime at
Niamey. Similar situations are observed at the N'djamena station on the Chari
at the entrance to Lake Chad. In the Nile basin, Sircoulon (1999) cites a reduction
in runoff of 20% between 1972 and 1987, corresponding to a general decrease
in precipitation in the tributary basins calculated by Conway and Hulme (1993).
In recent years there have been significant interruptions in hydropower generation
as a result of severe droughts (see Box 10-3).
Instrumental data and climate model simulations cited above indicate imminent
water crisis in large parts of Africa. Several seminal works have appeared in
the literature that analyze water for Africa, including Falkenmark (1989) and
Gleick (1992, 1998).
Large basin-scale analyses often give the wrong impression that many areas
of Africa are rich in water reserves, in which case local water problems could
be solved easily by technology that would transfer water from the source to
areas under stress, assuming that financial resources are available for such
enterprises. Although in theory this may be a practical solution to many water
problems in most of Africa, the very high costs associated with such projects
make them impractical. Political goals such as self-sufficiency in food production
and general socioeconomic development cannot be achieved under severe water
scarcity (Falkenmark, 1989). Drought-prone zones of Africa already are water-limited,
further increasing their vulnerability to water problems.
About 63% of the total land in Africa lies within transboundary river basins.
Five major river basins—the Congo, Nile, Niger, Chad, and Zambezi—occupy
about 42% of the geographical area and sustain more than 44% of the African
population. Other shared basins in the continent are the Senegal, Gambia, Limpopo,
Orange/Senqu, and Cunene Basins.
In west Africa, the dependency ratio—defined as the ratio between renewable
water produced out of a country and the total renewable water of the same country—is
more than 40% for seven of the nine countries comprising the Permanent Interstate
Committee for Drought Control in the Sahel (CILSS). This ratio is nearly 90%
for the Niger and the Mauritania. Similar transboundary dependencies are evident
in southern Africa and on the Nile basin. The Congo basin is shared by the most
countries (13), followed by the Niger and Nile basins (11 countries each) and
the Zambezi and Chad basins (9 and 8 countries, respectively).
Figure 10-6: Decadal changes in hydrograph of the Niger River at Niamey
Station between 1961 and 1999. |
The impact of changes in precipitation and enhanced
evaporation could have profound effects in some lakes and reservoirs. Conway
and Hulme (1993) and Calder et al. (1995) have discussed the hydrology
and paleohydrology of various African Lakes. Magadza (1996) has examined the
impact of drought on reservoirs in Zimbabwe. Reports fron Ghana (Graham, 1995)
indicate severe drought impacts on this large reservoir. Studies show that,
in the paleoclimate of Africa and in the present climate, lakes and reservoirs
respond to climate variability via pronounced changes in storage, leading to
complete drying up in many cases. Furthermore, these studies also show that
under the present climate regime several large lakes and wetlands show a delicate
balance between inflow and outflow, such that evaporative increases of 40%,
for example, could result in much reduced outflow. In the case of Lake Malawi,
it has been reported that the lake had no outflow for more than a decade in
the earlier part of this century (Calder et al., 1995).
Predictions of response by the Nile to global warming are confounded by the
fact that different simulations give conflicting results (Smith et al.,
1995), varying from 77% flow reduction in the Geophysical Fluid Dynamics Laboratory
(GFDL) simulation to a 30% increase in the Goddard Institute for Space Studies
(GISS) model. Arnell's (1999) model results suggest increased precipitation
in the Nile basin, but such gains are offset by evapotranspiration. Gleick (1992)
projects that future climatic changes in the Nile basin would be significant
and possibly severe. The response of the Nile basin to precipitation change
is not linear, though it is symmetric for increased and decreased precipitation.
Hulme (1992) shows a decline in total precipitation and overall warming of about
0.5°C over the last half on the 20th century. Conway and Hulme (1993) conclude
that the effects of future climate change on Nile discharge would further increase
uncertainties in Nile water planning and management, especially in Egypt. Nile
precipitation responds more to changes in equatorial circulation, with little
influence by the north African monsoon (Sestini, 1993).
Arnell (1999) shows that the greatest reduction in runoff by the year 2050
will be in the southern Africa region, also indicating that as the water use-to-resource
ratio changes countries such as Zimbabwe and the Magreb region will shift into
the high water-stress category. The Zambezi River has the worst scenario of
decreased precipitation (about 15%), increased potential evaporative losses
(about 15-25%), and diminished runoff (about 30-40%).
Lake Chad varies in extent between the rainy and dry seasons, from 50,000 to
20,000 km2. Precise boundaries have been established between Chad,
Nigeria, Cameroon, and Niger. Sectors of the boundaries that are located in
the rivers that drain into Lake Chad have never been determined, and several
complications are caused by flooding and the appearance or submergence of islands.
A similar process on the Kovango River between Botswana and Namibia led to a
military confrontation beteen the two states.
Vorosmarty and Moore (1991) have documented the potential impacts of impoundment,
land-use change, and climatic change on the Zambezi and found that they can
be substantial. Cambula (1999) has shown a decrease in surface and subsurface
runoff of five streams in Mozambique, including the Zambezi, under various climate
change scenarios. For the Zambezi basin, simulated runoff under climate change
is projected to decrease by about 40% or more.
Growing water scarcity, increasing population, degradation of shared freshwater
ecosystems, and competing demands for shrinking natural resources distributed
over such a huge area involving so many countries have the potential for creating
bilateral and multilateral conflicts (Gleick, 1992). Feddema (1998, 1999) has
evaluated the impacts of soil degradation and global warming on water resources
for Africa. All major watersheds are affected by global warming; although the
trend is toward drying in most locations, there are significant differences
in watershed-level responses, depending on timing and distribution of rainfall,
as well as soil water-holding capacity. Soil water-holding capacity is modified
by the degree of soil degradation.
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