Climate change and Zambezi hydropower production

By 2050, the Zambezi River Basin is expected to become hotter and drier, with a 0.3-0.6° C increase in temperatures per decade (0.8° C in the summer months), and a 10-25% increase in evaporation and 10-15% reduction in rainfall across the basin, relative to the 1961-1990 baseline. Runoff is projected to decrease by 26-40% on average over this time period. A shift in the timing (a delayed onset) of the rainy season is expected, as are more amplified seasonal variations (increasing high flows and reducing low flows). The intensity of rainfall will increase, compounded by a high likelihood of more frequent and intense tropical cyclones. Overall, the Zambezi will both be drier and more variable, experiencing more prolonged drought periods and more extreme floods.

These staggering 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. According to the World Commission on Dams (WCD 2000), climate change has the potential to affect hydropower installations in at least five important ways:

  • Reduced reservoir inflows on a seasonal and annual basis, due to decreased basin runoff and more frequent and prolonged drought conditions, reducing energy generation capacity;
  • Increased surface water evaporation, especially from upstream reservoirs and floodplains, further reducing energy generation capacity;
  • Increased extreme flooding (inflow) events, due to higher rainfall intensity and more frequent and intense tropical cyclones, affecting dam safety and operational rule curves designed to prevent over-topping;
  • Altered timing of the wet season flows, especially delayed onset of the rainy season, affecting dam operations as well as downstream release patterns;
  • Increased sediment load to reservoirs, resulting from higher rainfall intensity and corresponding erosion, resulting in reduced reservoir capacity (lifespan) and water quality.

Table 4. The effect of a selected number of combinations of temperature, precipitation, and evapotranspiration on inflows to Kariba reservoir, for the period 2030-2050 (SWRSD 2010).

Table 5. The effect of a selected number of combinations of temperature, precipitation, and evapotranspiration on inflows to Cahora Bassa reservoir, for the period 2030-2050 (SWRSD 2010).

Numerous studies have assessed the impact of climate change on hydropower development in the Zambezi River Basin. Some of earliest studies of Zambezi Basin climate change, using first generation climate change models, suggested the potential for significant reductions in hydropower generation (Salewicz 1996), with one study suggesting that Kariba would fail to meet its generation capacity due to low water levels, even in tandem with the proposed Batoka Gorge (Urbiztondo 1992). IPCC (2001) found that hydro-power production at Kariba Dam decreased under different two climate change scenarios due to the reduction in river flows caused by higher surface temperatures and associated increase in evapotrans-piration.

World Bank (2010) assessed the potential impact of climate change on multi-sector development scenarios for the Zambezi River Basin. They simulated modest basin development with a system of new hydropower production plants as envisaged under the Southern African Power Pool, using moderate climate change scenarios. The projected impact on energy productivity is substantial. Compared to baseline, firm energy6 falls by 32% from 30,013 to 20,270 GWh per year. Similarly, a significant reduction is seen in the average annual energy production, falling by 21% from 55,857 to 44,189 GWh per year. With less optimistic climate change assumptions, more substantial reductions in firm power (43%) and average energy (25%) are predicted.

When climate considerations are incorporated, the financial risks may significantly undermine the feasibility of existing and future hyropower projects.

The SADC-GTZ (SWRSD 2010) study generated a series of simple models to test the sensitivity of hydropower production to climate change and hydrological variability at Kariba and Cahora Bassa Dams, for the period 2030-2050. The model simulated more extreme variability in the predicted flow series, by taking the long-term historic inflow series for each dam, and multiplying the deviation of the historical flow series from the long-term mean by a constant factor for years drier than the mean and another constant factor for years wetter than the mean (effectively making the dry years drier, and wet years wetter). The model results suggest that very substantial reductions in inflows to Kariba (Table 4) and Cahora Bassa (Table 5), would occur under widely accepted climate forecasts, resulting in significant reductions in generating capacity.

The African Dams Project (Beck and Bernauer 2010) examined the effects of three different localized climate change scenarios7, coupled with different levels of water demand for agriculture, municipalities, and other uses, for the Zambezi River Basin, 2000-2050, including effects on hydropower. Current consumptive water use is about 15-20% of total Zambezi runoff. The research aimed to test how sensitive the basin is to different types and degrees of changes in water demand and supply. The scenarios suggest a reduction in average basinwide runoff ranging from 5% for the best-case scenario to 70% for the worst-case scenario. Flows reaching the Indian Ocean (Zambezi Delta) are reduced by 5-43%. These basinwide effects are even stronger during the dry season, with 10%, 70%, and 93% reductions in mean annual flow, respectively. Correspondingly, the scenarios reflect significant reductions in hydropower generation for Kariba and Cahora Bassa Dams on the Zambezi mainstem. For the worst-case scenario, hydr opower is reduced by 60% at Cahora Bassa and by 98% at Kariba. Kafue Gorge Dam, a run-of-river operation, is only minimally affected.

Finally, Beilfuss (2010) developed a simulation model using a 97-year historical flow series, aimed at assessing trade-offs between environmental flow scenarios and firm power reliability8 and total power generation from Cahora Bassa Dam. The flow series captures the full range of natural variability observed over the past century. The sensitivity of model output to a reduction in mean monthly inflows was also tested. The impact of a 10% reduction in mean monthly flows was moderate; firm power reliability remained at an industry-acceptable 95% level, with a 3.9% reduction in total power generation. More substantial reductions in runoff resulted in unacceptable levels of firm power reliability, however. For a 20% flow reduction, for example, firm power reliability fell to 91.8% with a 13.7% reduction in total power production. These results indicate that firm power contracts and other energy commitments will require renegotiation for modest reductions in future Zambezi River runoff, with corresponding reduction in revenue generation.

Collectively, these diverse studies suggest that future hydropower development in the Zambezi Basin could be very risky from a hydrological perspective. However, misperceptions about this risk are commonplace. A scoping study conducted for the World Bank by Vattenfall Power Consultant (Rydgren et al. 2007), for example, notes:

"Most hydropower/reservoir operators do not see climate change as a particularly serious threat. The existing hydrological variability is more of a concern, and the financially relevant planning horizons are short enough that with variability being much larger than predicted changes, the latter do not seem decisive for planning."

It is hard to understand this attitude, given the long life of dams, the scale of these investments compared to the size of many African energy sector budgets, and the hydrological uncertainty that climate change is surely bringing. Substantial economic risks are associated with reduced mean annual flows, more extreme flood and drought cycles, and increased evaporative water loss – including risk of structural failure if the design flood is underestimated, and financial risk associated with overestimated firm power generation, reduced revenue from total energy production, and other uncertainties. Water-dependent ecosystem services affected by over-designed hydropower development also are at risk. The financial implications of these risks are discussed in the next chapter.

6. Firm Energy is contractual, non-interruptible power guaranteed by the supplier to be available at all times, except for uncontrollable circumstances.

7 . The three scenarios reflect a range of assumptions about changes in population, urbanization, irrigated agriculture, industrial activity/mining, and water storage/hydropower production for the Zambezi Basin, ranging from status quo to strong growth in each sector.

8. Firm power reliability reflects the dependability with which contractual obligations for firm energy supply are satisfied. Firm power reliability can be event-based (number of months during which the target firm output could be met relative to total months of generation) or quantity-based (number of megawatts generated relative to target production) criteria. A 95% firm power reliability indicates that firm energy obligations are met on average in 95 of 100 months.