As the water level of the Vaal dam nears the critical 18% mark, preparations are under way to release water from the back-up Sterkfontein dam to augment water supply for Gauteng and surrounding regions.

This formed part of the measures put in place as the Lesotho Highlands tunnel, which transfers 780-million cubic metres of water a year into the Integrated Vaal River System (IVRS), was shut down in October for a six-month planned maintenance programme.

Ahead of the maintenance, 700-million cubic metres of water was transferred, with the shortfall of 80-million cubic metres of water due to be transferred after the reopening of the tunnel in March 2025.

A Department of Water and Sanitation (DWS) analysis in May indicated that the impact of the outage on the overall IVRS would be insignificant, considering that dams in the system, including the Sterkfontein dam, were relatively full.

At the time, the DWS found that there was a low 5% probability for release from Sterkfontein dam to augment Vaal dam; however, with below normal rainfall and high evaporation rates, this risk has now materialised.

The standard operating rule for releases of water from the Sterkfontein dam into the Vaal dam is when the latter reaches 18% of its full supply capacity.

The Vaal dam is currently at 28.2%, with a 1.5% to 2% weekly decrease owing to climatic factors, including low inflows owing to the lack of rainfall in the Vaal catchment and elevated temperatures which have led to increased evaporation losses.

Accordingly, the DWS has issued a notice to water users and the general public along the Nuwejaarspruit and Wilge rivers of the possible release of water from Sterkfontein dam, advising that movable equipment such as irrigation pumps be moved by December 15 and that low lying bridges should be avoided.

It takes about three to four days for water released from Sterkfontein dam to reach Vaal dam.

The department is also monitoring water supply to communities along the Liebenbergsvlei and Wilge rivers in the Dihlabeng, Nketoane and Mafube local municipalities, which directly benefit from the Lesotho Highlands Water Project (LHWP) and have been adversely affected since the tunnel closure.

Some towns in Setsoto and Mantsopa municipalities in the Free State normally abstract water from the Caledon river, while others abstract from the Vaal and Rhenoster rivers. These municipalities benefit indirectly from the LHWP and the department is also monitoring water supply to communities within these municipalities.

Relief intervention programmes, including the upgrade of water treatment works, raw water pumps and abstraction works, as well as the construction of new reservoirs and the development of groundwater resources by drilling and equipping boreholes, are underway in the municipalities to ensure sustainable water supply to communities during the tunnel system shutdown.

Further, municipalities have implemented water restrictions to regulate irrigation in their area of operation for the duration of the closure.

The DWS has also gazetted water use restrictions for farmers in the area, stipulating that water users on the left bank of the Ash and the Liebenbergsvlei rivers may abstract water for agricultural purposes from 06:00 to 06:00 on Monday, Tuesday and Wednesday, and water users on the right bank may abstract water from 06:00 to 06:00 on Wednesday, Thursday and Friday.

These restrictions are necessary to ensure that there is enough water during the closure of the LHWP tunnel.

The ongoing maintenance, repair and refurbishment of the tunnel is on schedule.

The DWS is monitoring the progress of the maintenance repair work and is satisfied that it will be completed within the scheduled time of six months.