The severe El Niño drought of 2015/16, which culminated in the Vaal dam reaching an alarming low water level (~25%), prompted scientists to try and predict whether climate change could bring a drought so severe and long lasting that Gauteng could run out of water. 

Prof Francois Engelbrecht, a Professor of Climatology at the Global Change Institute (GCI), University of the Witwatersrand, is one of the scientists working on this project and says though they cannot predict a Day Zero drought with certainty, he thinks it is possible that Gauteng might run out of water in the 2030s or 2040s.

 “This is the biggest climate change risk South Africa faces”, he said.  

Prof Engelbrecht and Dr Sophie Biskop from the Institute of Geography at the Friedrich Schiller University Jena, Germany, together with other scientists are working on a project involving hydrological modelling to predict and prevent a Day Zero from happening. Dr Biskop presented their research paper titled ‘Projected hydrological futures of South Africa's mega-dam region’ at the second Southern African Mountain Conference (SAMC2025) in March, indicating there is a high risk that the water demand in Gauteng will exceed available water resources within the Integrated Vaal River System (IVRS) under future climate change.

Gauteng may be severely compromised

The IVRS, a large, complex water system comprising water resources of different river basins, and several mega-dams within, has been constructed to secure the water supply of the Gauteng province, the economic hub in South Africa. 

According to the researchers, Southern Africa is a water-stress hot spot and is projected to become significantly warmer and likely also drier under global climate change, increasing the risk of devastating hydrological droughts. The IVRS, Dr Biskop told the attendees, is vulnerable to the occurrence of multi-year droughts as experienced between 2012 and in 2016. The alarming low water level of the Vaal dam after a period of drought of 2015/16 provided early warning that water security of Gauteng may be directly and severely compromised in a changing climate. Potential evapotranspiration will increase as a consequence of strong regional warming.

Answering questions

“There is consequently a high risk that the water demand in the Gauteng province will exceed available water resources within the IVRS under future climate change. This raises the question if under ongoing climate change the natural hydrological system (without considering water transfers between dam catchments) can maintain dam levels in South Africa’s eastern mega-dam region, and particularly within the Lesotho Highlands,” explained Dr Biskop. 

 “To answer this question, the aim of our study is to quantify future water balance changes of several dams under changing climate conditions using the Jena Adaptable Modelling System (JAMS), a software framework for component-based development of environmental models. For this purpose, we build process-based hydrological models for several dam catchments.”

She said an ensemble of high-resolution regional climate change projections is subsequently used as forcing, to generate future hydrological projections. The analysis of projected changes in hydrological system components (precipitation, evapotranspiration, run-off) provides probabilistic estimates of the occurrence of a regional climate change tipping point - when the natural water supply can no longer achieve the full storage capacity of the mega-dams which supply the Gauteng region.

Working to prevent Day Zero 

According to Prof Engelbrecht, they are working with the City of Johannesburg, the National Department of Water and Sanitation and Rand Water on this project. Their hope for this research is to create awareness in order to try and prevent Day Zero from happening. They also hope to assist these role players in building resilience and help them prepare for Day Zero. Their work with the City of Johannesburg also includes helping the city to reduce water wastage and change water users’ behaviour as well as formulating a disaster management plan should Day Zero happen.