Hydrological projections for water risk assessment in South Africa’s eastern mega-dam region

Southern Africa is a water-stress hot spot, and is projected to become significantly warmer and likely also drier under low mitigation futures, increasing the risk of devastating droughts. There is increasing concern about water and food security in southern Africa, due to potentially unprecedented climate change impacts on water resources and ecosystems, and limited adaptation options in this water-stressed region. South Africa’s Gauteng Province with more than fifteen million inhabitants is the economic hub of the country and highly vulnerable to the occurrence of multi-year droughts, one of the biggest disasters risks South Africa needs to prepare for in a warmer world.

The Integrated Vaal River System (IVRS), west of the Lesotho Drakensberg in the South African interior, connects several mega-dams to secure the water supply of the Gauteng Province. The alarming low water level (~25%) of the Vaal dam after a period of drought culminating in the El Niño drought of 2015/16 provided early warning that water security of the Gauteng Province may be directly and severely compromised in a changing climate. Potential evapotranspiration will increase as consequence of a strong regional warming, and in the presence of unprecedented future multi-year droughts the risk exists 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. To answer this question, the aim of this 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 built process-based hydrological models for several dam catchments. An ensemble of high-resolution regional climate change projections is subsequently used as forcing, to generate future hydrological projections. The applied regional climate projections will include the CORDEX-CORE Africa ensemble and newly generated projections from regional climate models (CCAM and REMO-NH) forced with CMIP6 global climate projections. The analysis of projected changes in hydrological system components (precipitation, evapotranspiration, runoff) provides probabilistic estimates of the occurrence of a regional climate change tipping point - when the natural water supply can’t longer achieve the critical threshold of storage capacity of the mega-dams which supply South Africa’s Gauteng Region.

The research is part of the “Water security in Africa – WASA” programme, project WaRisCo, which deals with water risks and resilience in urban-rural areas in southern Africa and the co-production of hydro-climate services for an adaptive and sustainable disaster risk management.