Falling Water Levels Hurt Sub-Sahara’s Biggest Hydro Plant

  • Drought that cut Kariba’s levels now affecting Cahora Bassa
  • Forecasts for above-normal rainfall might mitigate impact

The Cahora Bassa dam in 1987.

Photographer: Patrick Durand/Sygma via Getty Images

Sharply falling water levels at Mozambique’s Cahora Bassa dam, sub-Saharan Africa’s biggest hydropower plant, are threatening electricity supplies to countries including South Africa, which buys about a third of its output.

Levels are the lowest Phil Bezuidenhout, a lodge owner at the dam, has seen in the 24 years he’s operated on its shores in the country’s northwest.

“This is definitely the lowest it’s been in my time,” he said by phone. “These levels are ridiculously low.”

Southern Africa’s worst drought in 35 years has cut inflows from rivers including the Zambezi and prompted Zambia and Zimbabwe to last year more than halve power production at the Kariba dam, upstream from Cahora Bassa. Levels at the Mozambican reservoir have plunged by more than 8.8 meters (29 feet) since the start of the year, according to data from its operator, Hidroelectrica de Cahora Bassa. Both dams are counting on forecasts of above-normal rainfall in the wet season currently under way to keep their turbines turning.

Kariba Drop

Kariba, which provides about half of Cahora Bassa’s water, is at 16 percent of capacity and falling. That’s lower than 12 months ago, before levels bottomed at 11 percent in early 2016. Cahora Bassa was 34 percent full at the start of the month, Maputo-based news website Zitamar reported Dec. 2, citing a National Water Directorate document.

“Both Kariba and Cahora Bassa have quite low water levels and they will continue to gradually decrease over the next months until the seasonal flood arrives at Kariba and Cahora Bassa,” Harald Kling, a hydrologist at Poyry Hydropower in Vienna who’s co-authored studies on the dam, said by e-mail. “The water levels in June 2017 will very much depend on how high the 2016-17 seasonal flood will be.”

HCB, as the utility is also known, has already cut electricity supplies to South Africa’s Eskom Holdings SOC Ltd. by about 13 percent because of the drop. Eskom, Africa’s biggest power producer, bought more than two-thirds of the power Cahora Bassa generated in 2015, according to HCB’s annual report. The plant has capacity to produce 2,075 megawatts. This week, the dam’s operator reduced generation a further 18 percent because of a technical fault, after some insulating material ruptured, it said in an e-mailed statement.

Kariba, the world’s largest manmade freshwater reservoir, according to the International Commission on Large Dams, has three times the storage capacity of Cahora Bassa, at 180 cubic kilometers (43 cubic miles). While the flow of the Zambezi, that feeds both dams, has started to increase upstream from Kariba, it’s still more than 10 percent lower than 12 months ago, according to data posted to the website of the Zambezi River Authority, which regulates the dam that straddles Zambia and Zimbabwe’s border. Kariba has the capacity to produce 1,830 megawatts.

In 2015, only 4.1 percent of Cahora Bassa’s generation went to Mozambican power utility EDM, according to HCB’s annual report. Zimbabwe’s state-owned electricity company, ZESA Holdings Ltd., bought about 27 percent of Cahora’s supply in the period. Zambia is buying as much as 120 megawatts of power from EDM. About 20 percent of Mozambique’s citizens had access to power in 2012, World Bank data show.

HCB’s revenue climbed 23 percent to a record 4.3 billion rand ($317 million) in 2015 from a year earlier. The utility invoices using the South African currency. HCB didn’t respond to e-mailed requests for comment.

Increased Volume

Despite Kariba’s low and falling levels, the authority has increased the volume of water Zambia and Zimbabwe can use to generate power at the dam in 2017 by 50 percent over this year, basing the decision on regional weather forecasts projecting normal to above-normal rainfall in most parts of southern Africa.

Cahora Bassa also gets its water from the Luangwa and Kafue rivers, which both receive most water from rains in Zambia. The dam is 270 long and as wide as 30 kilometers. NASA in July flagged the reservoir’s falling water levels through satellite images.

“The logical thing for Cahora Bassa to do is to keep the present power production and monitor carefully what is happening in the basin,” Alvaro Vaz, a professor in hydrology and water resources, said in reply to e-mailed questions from Maputo, Mozambique’s capital.

The dam’s water level of 312 meters above sea level as at Dec. 11 is still 17 meters higher than the minimum operating level for the hydropower turbines of 295 meters, according to HCB’s website. If the above-normal rains forecast for this season don’t materialize, this could cause problems.

“For Cahora Bassa, I do not think there is much risk during the 2016-17 rainy season,” that the dam will reach the minimum level, Poyry’s Kling said. “However, if the upcoming 2016-17 flood is again as low as in 2014-15 -- which was extremely low -- then a real problematic situation might develop later on.”

— With assistance by Borges Nhamire

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