Climate change and water scarcity in the Great Ruaha River Basin, Tanzania: Implications for ecosystems and adaptive water management

Drought and water scarcity are global challenges threatening water security, ecosystems, and community livelihoods across Tanzania’s Great Ruaha River Basin (GRRB). These conditions have worsened in recent decades with increasing hydrological stress manifested through recurrent droughts, declining river flows, and growing pressure on water resources. As a result, critical economic sectors vital to both local communities and national economic stability such as agriculture, water supply, and energy production have been severely impacted. At the same time, climate change introduces additional uncertainty regarding future water availability in the basin.

Despite these growing challenges, drought monitoring systems in the basin remain weak, and the impacts of climate change on the hydrological regime are still insufficiently understood. This persistent knowledge gap limits evidence-based decision-making and constrains the design of effective climate adaptation strategies. To address these scientific and policy shortcomings, this study combines drought analysis, climate impact attribution and modelling, and participatory risk-informed approaches to evaluate current and future water security risks and co-develop resilient adaptive strategies for effective water resources management in the basin. Beyond technical analysis, the study further bridges the gap between science and local practice by integrating model-based evidence with stakeholder knowledge using a structured decision-support process. By linking quantitative analysis with inclusive planning processes, this approach strengthens water resources decision-making under conditions of deep uncertainty.

With drought identified as a major challenge to water security and ecosystem resilience, this research initially focuses on evaluating five key drought indices for historical hydrological drought detection. These included the Standardized Precipitation Index (SPI), Standardized Precipitation Evapotranspiration Index (SPEI), Standardized Precipitation Actual Evapotranspiration Index (SPAEI), Standardized Streamflow Index (SSI), and Water Scarcity Index (WSI). The indices were computed using a combination of remote sensing and in-situ data. Drought events computed from the indices were systematically compared with observed drought events sourced from scientific literature and government reports. Results show that combining multiple drought indices improves drought identification and characterization. Indices such as SPI, SPEI, SSI, and WSI were effective in capturing drought events and low-flow periods. This approach further enabled the identification of non-climatic drivers, such as increased water abstraction, affecting the basin’s hydrological system through a comparative analysis of drought trends. These findings underscore the need of utilizing appropriate drought indices for informed decision-making and adaptive planning. A careful selection of drought indices will enhance reliability of early warning systems and support effective drought mitigation in the basin.

Next, this research investigates whether the observed decline in river discharge (1963-2017) in the GRRB is attributable to climate change. A climate impact attribution framework from the Inter-Sectoral Impact Model Intercomparison Project Phase 3a (ISIMIP3a) was integrated with the Soil and Water Assessment Tool Plus (SWAT+) model to simulate water balance components and river discharge. Results indicate a clear tendency toward wetter conditions, with increases in annual precipitation and surface runoff contributing to an increasing trend in simulated annual river discharge. These changes are likely driven by climate change. In contrast, observed data analysis shows consistent decline in river flows, a trend linked to increased water use. In other words, the increase in precipitation is overcompensated by rising water use leading to declining river flows. This finding underscores the importance of integrating both climatic and non-climatic factors into basin’s water management frameworks to effectively address emerging water and ecosystem challenges. Greater emphasis should be placed on water management reforms rather than relying solely on climate adaptation measures.

Thereafter, projected impacts of climate change on future water availability were assessed by coupling the SWAT+ model with outputs from five bias-adjusted Global Climate Models (GCMs). Two climate scenarios, the SSP3-7.0 and SSP5-8.5 were used to examine projected changes in water balance components, river discharge, and hydrological extremes at both seasonal (wet/ dry) and annual timescales. The historical (1961-1990), mid-future (2036-2070), and far-future (2071-2099) periods were used to examine the differences in model simulations. Results show consistent rise in temperature (1.8-4.2°C) and increased rainfall variability. This climatic shift will alter basin’s hydrological balance by reducing percolation, baseflow, and river flows.

Concurrently, high-flows rise substantially by over 200%, while low-flows decrease by more than 25%. These findings highlight increased hydrological variability characterized by escalated risks to both flooding and droughts. These changes might compromise aquatic ecosystems, prolong drought conditions, and intensify intersectoral water conflicts. Our findings underscore the urgent need for adaptive water management strategies that account for both intensified extremes and altered seasonal water availability.

Finally, this research applied the Climate Risk Informed Decision Analysis (CRIDA) framework to identify critical water system vulnerabilities (risks) and co-develop robust adaptation strategies through a participatory and inclusive approach. CRIDA enabled engagement of diverse stakeholders, ranging from grassroots representatives to national-level institutions. Hydroclimatic projections for the basin developed using SWAT+ model informed the process. Through guided group discussions, stakeholders defined unacceptable outcomes, identified critical vulnerabilities, proposed robust actions, and developed adaptive pathways for future water management. Findings indicate that key risks in the basin arise from the interaction between hydroclimatic stressors (i.e., droughts, floods and heat stress) and non-climatic stressors like population growth, increased water demand, and governance constraints (i.e., poor monitoring, weak law enforcement, and coordination gaps). Proposed actions emphasized strengthening monitoring systems, law enforcement, improving coordination and data sharing, and enhancing water use efficiency. These actions were organized into adaptive pathways that outline sequenced responses to climatic and non-climatic stressors, enabling progressive implementation and future adjustments based on observable system indicators under deep uncertainty. The study demonstrates the value of CRIDA as a decision- and risk-focused framework, highlighting that integrating scientific insights with inclusive governance is essential for safeguarding water security under conditions of variability and uncertainty.

Overall, this PhD research addresses critical research gaps in drought assessment, climate impact attribution, and future water security in a basin already facing the compounded effects of climate change and anthropogenic pressures. By combining robust scientific evidence with local knowledge, the study bridges critical information gaps needed to support evidence-based policy formulation and decision-making, while contributing to the achievement of multiple Sustainable Development Goals (SDGs) related to water, food, climate, and ecosystem sustainability. The significance of this work extends beyond the GRRB offering integrative drought monitoring approaches, climate impact attribution and climate impact modelling methods, and participatory frameworks that are applicable to tackle drought and water scarcity in other semi-arid regions. In doing so, it provides the broader scientific community and policymakers with scientific tools and knowledge to develop climate adaptation strategies, water governance, and sustainable water resources management under growing anthropogenic and hydroclimatic pressures.