Critical role of hydrological extreme events in future water security and management of the Integrated Vaal River System

Abstract

The southern African region is heavily impacted by climate change, which significantly alters water availability. The intensity and frequency of hydrological extremes, such as droughts and floods, have greatly increased in the past decades and will likely persist into the future due to projected rises in extreme precipitation and rising temperatures. In this regard, water resource management remains a major challenge in this region. Climate change impacts make it even more critical by transforming water security risks into substantial water insecurity and management challenges, especially for one of the key river systems in South Africa, the highly complex Integrated Vaal River System (IVRS). The IVRS involves inter-basin and transboundary water transfers (i.e., Lesotho Highlands) and is considered a lifeline for Gauteng Province’s water supply. The system faces the risk of a day-zero drought when water levels drop to around 20% or lower in the Vaal Dam, causing taps to run dry.This study offers insights and prospects on how integrating advanced hydrological models with km-scale (i.e., 4km) high-resolution projected climate change data can help better understand and quantify the role of hydrological extremes in the IVRS.Initial calibration at different gauging stations shows Kling-Gupta Efficiency (KGE) ranges between 0.60 and 0.70, and the Talsim hydrological model effectively captured seasonal flow and storage dynamics in the Vaal Dam. The storage volumes within the Vaal dam show approximately 8% deviation from observations when operational rules are excluded. The absence of operational rules is identified as the main limitation in current simulation runs. The future work will focus on integrating operational rules and long-term storage changes within the IVRS.This research is part of the WaRisCo (Water Risks and Resilience in Urban-Rural Areas in Southern Africa - Co-Production of Hydro-Climate Services for Adaptive and Sustainable Disaster Risk Management) project, which is funded within the “Water Security in Africa – WASA” programme.