Globally, water resources are being over-utilised; a situation exacerbated by degenerating water quality of rivers. To achieve sustainable management of water resources, uncertainty under climate change and development must be considered. A companion study was the first to incorporate uncertainty within water resources development scenario modelling for a catchment in South Africa using the Water Evaluation and Planning (WEAP) model. That study is extended in the current study by considering water quality in the form of nutrients and salinity. The WEAP model was calibrated against available observed data for the period 1999–2005. Using the calibrated WEAP model, driven by flow predicted using downscaled climate change models and projected future development, water quality was simulated for the years 2046–2065. Future simulations indicated marginally increased dilution capacity as well as increased nutrient inputs. It is evident that WEAP suffers major limitations in its water quality simulation capacity. Adaptive management along with continual monitoring as a strategy to cope with uncertainty associated with climate change and development is recommended. The shortcomings identified within WEAP in the current study were the motivation for the development of a new water quality decision support system specific to the requirements of water management in southern Africa.

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