Rainfall

The Zambezi River Basin receives about 960 mm rainfall per year, mostly concentrated in the wet season. Considerable variability in rainfall occurs across the basin, from arid/semi-arid regions in the south and southwest to high rainfall regions in the north. Inter-annual variability in rainfall is also high (coefficient of variation = 0.35). Long-term rainfall patterns are difficult to discern from this spatial and temporal variability. However, three significant trends in rainfall for the Zambezi region are apparent from direct observations over the past 40 years (IPCC 2007a):

  • A slight reduction in annual precipitation;
  • Increased inter-annual variability with more intense and widespread droughts;
  • A significant increase in heavy rainfall events in many Zambezi Basin countries (including Angola, Namibia, Mozambique, Malawi, and Zambia), including evidence for changes in seasonality and extreme weather events.

Climate change predictions, based on the average (not extreme) of diverse climate models, have profound implications for future hydropower production and development in the Zambezi River Basin.

Over the next century, multiple studies cited in IPCC (2007a) estimate that rainfall across the Zambezi Basin will decrease by 10-15%. The predicted decrease in rainfall is associated with a reduction in the number of rainy days and in the average intensity of rainfall. Based on the average of six GCMs, Shongwe et al. (2009) project a decreasing rainfall trend with more extreme droughts in northern Botswana, western Zimbabwe, and southern Zambia; generally drier conditions in Zambia and Malawi; and less clear precipitation trends in eastern Zimbabwe and Central Mozambique during the 21st century.

Significant changes in the seasonal pattern of rainfall over the Zambezi River Basin are also predicted, although the magnitude of change is less certain. Shongwe et al. (2009) indicate a 10-16% reduction in rainfall during autumn (March-May), 31-35% reduction during winter (June-August) and spring (September-November), and a slight 1% reduction in summer (December-February) (Figure 11). The simulated annual climatic cycles suggest that the rainfall season may begin one month later than the recorded norm, effectively shortening the duration of the rainy season in the northern parts of the Zambezi Basin.

Tadross et al. (2005) and New et al. (2006) noted evidence of increasing weather extremes in several Zambezi Basin countries, including Mozambique, Malawi, and Zambia. Usman and Reason (2004, cited in IPCC 2007a) predicted a significant increase in heavy rainfall events over Southern Africa (including Angola, Namibia, Mozambique, Malawi, and Zambia). According to the IPCC models, the frequency of extremely dry austral winters and springs will increase to roughly 20%, while the frequency of extremely wet austral summers will double in Southern Africa. There is an emerging consensus that the intensity of tropical cyclones will increase, with less certainly about whether the frequency of these events will increase.3

3. Cyclonic events are not explicitly modeled by existing GCMs.