10.2.1.1 Overview of Regional Water Resources

Figure 10-4 shows that the hydrological performance of Africa results in much less runoff yield than in other regions. Apart from the Zambezi/Congo Rivers, the major African rivers (Nile, Niger, Senegal, Senqu/Orange, Rufiji) traverse semi-arid to arid lands on their way to the coast. Of the world's major rivers, the Nile has the lowest specific discharge (i.e., flow per unit catchment area), even if only the part of the catchment that receives precipitation is considered (Reibsame et al., 1995). Furthermore, because these major rivers originate within the tropics, where temperatures are high, evaporative losses also are high in comparison to rivers in temperate regions. Elevated temperatures will enhance evaporative losses; unless they are compensated by increased precipitation, runoff is likely to be further reduced.

In Morocco, the northward displacement of the Azores high-pressure cell is a subject of study because of its asscociation with a drought cycle that is related to the dipole between the positioning of the Azores high-pressure cell and the Iceland low-pressure cell. These severe droughts seem to manifest themselves in Morocco in periodicities varying between 2 and 13 years (Stockton and Allali, 1992).

The Magreb region is characterized by erratic and variable rainfall, with a high rate of evapotranspiration (almost 80%). In addition, the Magreb region will have water scarcity by 2025, especially in Tunisia and Libya.

Associated with the poor hydrological performance of African river basins is the fact that most of the lakes in Africa have a delicate balance between precipitation and runoff; all of the large lakes show less than 10% runoff-to-precipitation ratio (Talling and Lemoalle, 1998), and important water basins like Lake Chad and the Okavango Delta have no outflow because evaporation and permiation balance runoff (see Figure 10-5).

In the savanna regions, the incidence of seasonal flow cessation may be on the increase, as shown by some streams in Zimbabwe (Magadza, 2000). Drought periods now translate into critical water shortages for industrial and urban domestic supplies (Magadza, 1996).

Figure 10-3: Observed annual rainfall anomalies for three African regions, 1900-1998, and model-simulated anomalies for 2000-2099. Model anomalies are for 10 model simulations derived from seven DDC GCM experiments; the four HadCM2 simulations are the dashed curves. All anomalies are expressed with respect to observed or model-simulated 1961-1990 average rainfall. Model curves are extracted directly from GCM experiments, and results are not scaled to SRES scenarios used in Chapter 3. Smooth curves result from applying a 20-year Gaussian filter (Hulme et al., 2001).

Figure 10-4: Comparative hydrology in world regions—total runoff as percentage of precipitation (GEMS, 1995).

Figure 10-5: Evaporative losses as percentage of total hydrological income in selected African lakes (constructed from Talling and Lemoalle, 1998).

Table 10-1: Estimates of ranges of percentage changes in precipitation, potential evaporation, and runoff in African river basins, constructed from Figure 3 of Arnell (1999). In some basins, estimates given by HadCM3 simulation have been excluded where they appear to be outliers.
Basin	Change in Precipitation (%)	Change in Potential Evaporation(%)	Change in Runoff (%)
Nile	10	10	0
Niger	10	10	10
Volta	0	4 to -5	0 to -15
Schebeli	-5 to 18	10 to 15	-10 to 40
Zaire	10	10 to 18	10 to 15
Ogooue	-2 to 20	10	-20 to 25
Rufiji	-10 to 10	20	-10 to 10
Zambezi	-10 to -20	10 to 25	-26 to -40
Ruvuma	-10 to 5	25	-30 to -40
Limpopo	-5 to -15	5 to 20	-25 to -35
Orange	-5 to 5	4 to 10	-10 to 10

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