Efficiency and equity of water transfers to megacities: reconfiguring urban water security in Istanbul.

Background: Rapid urbanization, population growth, and climate-induced water stress have made interbasin water transfers (IBTs) a central strategy for augmenting urban water supplies worldwide. Istanbul, Türkiye's largest metropolitan area, relies heavily on water transfers to meet its rising demand. However, concerns around the long-term sustainability, efficiency, and equity of these transfers, particularly their impact on donor regions, remain underexamined. This study investigates the hydrological and social performance of Istanbul's major water transfers, with a focus on balancing urban water security with environmental and regional justice considerations. Methods: A socio-hydrological case study approach was used to assess three major water transfers supplying Istanbul from Düzce, Tekirdağ, and Kırklareli provinces between 2000 and 2023. Two quantitative indicators were applied: the Natural Efficiency Index, which compares transfer volumes to renewable freshwater availability in both donor and recipient basins; and the Stress Relief Index, which analyzes social efficiency by evaluating the water demand- and population-weighted change in water stress resulting from each transfer. In addition, a document-based qualitative analysis grounded in hydrosocial theory was conducted to explore governance narratives, trade-offs, and regional impacts. Results: Findings reveal substantial variation in both hydrological and social efficiency across the three IBTs. The Melen system transfer from Düzce demonstrates a relatively high efficiency in relieving Istanbul's water stress with moderate ecological cost. In contrast, transfers from the Istranca system in Tekirdağ, and Kırklareli exhibit lower natural and social efficiency, suggesting a disproportionate burden on already water-stressed source regions. The qualitative assessment highlights that these transfers, while effective in meeting Istanbul's supply needs, often reinforce centralized, supply-driven governance models and overlook the socio-environmental impacts on donor regions. Recurring droughts, reduced streamflow, and competition with local agricultural needs further exacerbate these tensions. Conclusions: The study demonstrates that water transfers can create uneven outcomes between donor and recipient regions, particularly when hydrological limitations and social vulnerabilities are not explicitly addressed in planning. While large-scale transfers may appear effective in securing urban water supply, they may also deepen regional inequalities and environmental risks. The findings call for a shift toward integrated and adaptive water governance models that consider long-term hydrological sustainability, ecosystem health, and inter-regional equity. For cities like Istanbul, this means rethinking reliance on external water sources and investing in demand management, local resilience, and participatory planning frameworks.