Blog // Revisiting transboundary water allocation in the Incomati: emerging evidence from a CGIAR modelling effort
This article presents a transboundary water modeling initiative in the Incomati River Basin, engaging stakeholders from South Africa, Eswatini, and Mozambique to explore alternative water allocation scenarios in light of climate change and growing demand.
Figure 1: Maguga Dam, Eswatini Photo by Sansha van der Merwe
This article was submitted to the Nexus Resource Platform on 04.06.2025. It was originally written by Sansha van der Merwe, Buyani Fakudze & Jonathan Lautze.
From the highlands of Mpumalanga in South Africa, through the rolling landscapes of Eswatini, and into the estuarine plains of Mozambique, the Incomati River Basin remains a lifeline for communities, agriculture, energy, industry, and ecosystems in southeastern Africa (Figure 1). In recent years, the basin has faced increasing pressure from population growth, expanding irrigation, and climate change. Climate projections for the basin point to rising temperatures and more unpredictable rainfall in the coming decades, making preparedness and adaptation essential. These trends are already becoming evident. For example, El Niño-induced droughts in 2015/2016 caused crop losses of up to 30% in Eswatini, dried up boreholes, and left reservoirs at critically low levels. And in 2018, over 3 million people in the Greater Maputo area faced severe water rationing — a region expected to double in population by 2035.
Enhancing basin wide management constitutes a powerful tool to respond to these realities. In the Incomati, discussion on enhancing basin-wide management often centers on revisiting the allocation rules contained in the 2002 Interim IncoMaputo Agreement (IIMA). To support this process, CGIAR Nexus programs developed a Pywr-based model and used it to discuss alternative water management scenarios with stakeholders from Eswatini, Mozambique and South Africa. Model results were used to weigh the benefits and risks of the current approach to basin management against alternative ones—supporting constructive dialogue toward improved ways forward. Key prerequisites supporting effective engagement on model results were iteration with stakeholders on scenario formulation and model capacitation, which allowed stakeholders to directly engage with the model.
Structuring effective iteration with stakeholders
To facilitate stakeholder input, a Steering Committee (SC) was established with guidance from Incomati and Maputo Watercourse Commission (INMACOM). Throughout 2024, the SC — with broad representation from all three countries and key water authorities — played a central role in shaping the model and its application (Figure 2). Engagement took place through a mix of regular meetings with the full committee, bilateral sessions with key institutions, and in-person discussions focused on pressing issues with smaller groups. This layered approach ensured that both upstream and downstream perspectives were considered, fostering collaboration across national and institutional boundaries.
Stakeholders contributed directly to refining the model’s inputs and scenarios, enhancing its accuracy and relevance. Updates included more detailed abstractions, demand patterns, and operational data for planned reservoirs. Results were presented in an iterative format, enabling validation and shared learning. These interactions helped shift the model from a purely technical tool to a platform for dialogue — supporting decision-making around allocation, equity, and future planning. Stakeholder input also clarified key priorities for future development, including flood management and climate adaptation.
Building Capacity to engage with the model
Alongside model refinement, we launched a hands-on training initiative designed to build long-term capacity within the basin. A 50-hour training program was rolled out earlier this year, with SC members nominating participants from their institutions. The goal? Not just one-off capacity building, but a steady process of practice, reflection, and applied learning.
Participants explored how to use the Pywr-based model, the WaterStrategy interface, input new data, run scenarios, and interpret outputs. In doing so, they’ve begun to move from passive users to empowered analysts and decision-makers. The training also helped surface local needs and practical questions, which informed ongoing model improvements.
Emerging results
Early model results point to three important findings. First, there is a need for greater focus on environmental flows throughout the basin. While Mozambican stakeholders have raised concerns about maintaining ecological conditions in the Maputo estuary, there is limited basin-wide data and attention on balancing environmental flow requirements with water demands. This highlights a gap that needs to be addressed to protect ecosystems and biodiversity across the Incomati basin, including in the Kruger Park (Figure 3).
Second, while the minimum transboundary flow is generally being met, challenges tend to arise during dry months in drought years. These shortfalls are difficult to detect through routine monitoring or modelling averages, making proactive planning a challenge. It would be beneficial to have more disaggregated and detailed data to better understand where and when these tensions emerge, especially during critical low-flow periods.
Third, tapping the basin’s full potential may require increased water storage. Emerging evidence points to more fruitful discussion on allocation when the topic is coupled with increased storage. Nonetheless, while additional reservoirs bring benefits such as flood management, drought resilience, and irrigation expansion, they also involve major trade-offs — including high financial costs, ecological disruption, and risk of displacing communities – and as such require careful examination.
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Sansha van der Merwe is a Water Researcher at Wits University, Johannesburg, South Africa and a consultant for the International Water Management Institute (IWMI). Buyani Fakudze is a Data Manager at INMACOM. Jonathan Lautze is Research Group Leader – Integrated Management of Basins and Aquifers, IWMI.
This work was carried out as part of the CGIAR program Nexus Gains between 2022 and 2024. From 2025, it is being pursued through the CGIAR Policy Innovations program Area of Work 4 Nexus Policy Gains. More information is available here.