The perils of electricity in the water-supply mix
While the Vaal Dam is at 77% from 2017’s 79.4%, it is down a full percentage point from a week ago
It might be difficult to imagine a water-supply crisis in Gauteng midway through the rainy season in the Southern African interior. The dams, after all, are more or less full. But when electricity is added to water, the picture changes.
Where it matters most to the economy — in the integrated Vaal river system (IVRS) — the dam levels are at an average of 71.3%, almost exactly the same level this time in 2017 (71.2%). This number should be reassuring to those alarmed by the relatively low rainfall for the first half of the season, and by the SA Weather Service’s forecasts of even lower figures and higher temperatures in the second half.
But averages can be misleading, and not all dams are equal. While the Vaal Dam, which supplies about 15-million people with drinking water, is at 77% from 2017’s 79.4%, it is down a full percentage point from a week ago. Still, it would seem adequate if the balance between supply and demand was maintained.
To assess this ratio, a closer look at the state of the IVRS dams is required. Among these, the dam perhaps most unequal in the system is the Mohale Dam, a relatively unknown impoundment on the Senqunyane River in Lesotho and, under treaty, part of the Lesotho Highlands Water Project (LHWP). This dam is at 16.9% full, from 27.7% at the same time in 2017, according to the water and sanitation department’s data.
Mohale qualifies as a large dam, but it is not that big. Its full storage capacity is 846.3-million cubic metres, compared with the Gariep Dam’s 5.2-billion cubic metres and the Vaal and Sterkfontein Dams each with a capacity of more than 2.6-billion cubic metres. At prevailing abstraction rates, Mohale has about 180 days in storage.
But capacity is not what makes Mohale important. The dam is arguably the most critical asset in the IVRS because it is the uppermost in the IVRS which, logically, should be the fullest in the system. Instead, it is the closest to empty. The next dam downstream in the LHWP is the Katse Dam, the next lowest at 43.7%.
Pieter van Niekerk, a water-supply specialist (now retired from the water department) foresaw the risk to the system as long ago as in April when Mohale was at 36% and Katse at 66%. Since April, at a decant rate of about 24 cubic metres per second, Mohale’s level has dropped nearly 19 percentage points over eight months.
Van Niekerk writes that withdrawal of water should be from lower dams for as long as possible to reduce evaporation losses (the Vaal Dam evaporates at 10 times the rate of the Lesotho dams). It also ensures flood control.
The trouble is that rainfall in the Mohale and Katse catchments has been poor, as it has been for the whole region, yet the rate of abstraction is maintained. This is so because under the treaty establishing the LHWP, abstraction from Mohale and Katse must be maintained to allow a hydro-electric power stations in Lesotho to supply that country with about 80MW of power, or about half of what it needs.
If the flow drops below the minimum, Eskom would have to make up the difference from its already diminished capacity.
About 3% of Lesotho’s GDP is derived from the LHWP in royalties and associated earnings.
The South African delegation to the treaty has noted the need to compensate for Lesotho’s electricity supply and royalty losses, as expressed in negotiations about the second phase of LHWP. The second phase, which would include the construction of the Polihale Dam, was expected to bring relief possibly by 2020, but the project was now expected to come onstream sometime in 2025 after delays in the construction tender process under former water and sanitation minister Nomvula Mokonyane.
Department spokesperson Sputnik Ratau says there has been no delay to the negotiations to improve the treaty. “Over time, the treaty has had six periods of improvements, including looking at issues of best practice, and matters of project governance have also become pertinent. Each side will take time to prepare its own input.”
Ratau says also that SA’s electricity supply (or lack thereof) would have “no impact” on the level of the LHWP dam levels.
However, the IVRS’s other main storage dam at altitude is the Sterkfontein Dam in the eastern Free State. This dam, now at 94.3%, is a pumped-storage dam designed to deliver 1,000MW of power to Eskom at peak demand. Its great virtue is that it can come online within two and a half minutes, but only if there is adequate water supply, pumped up over the escarpment from the Tugela River system at off-peak electricity demand.
This means the link between high-altitude water impoundments and the demand on SA’s electricity supply is critical to the IVRS and thus to the national economy. It means that as the heat and dry weather persist over the interior of the country, demand for water will rise in SA and in Lesotho, and water supplies will diminish in the IVRS and in the Tugela valley, driving the economy ever closer to the edge.
There is very little SA and Lesotho can do about the weather and the climate, but the one thing that is supposed to be well within the capacity of the two countries’ politicians is to accelerate the negotiations to improve the treaty and keep the water and electricity flowing.
They must do it now while there is still time.