Abstract

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 long-distance IBTs 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 IBTs, 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 IBTs 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 Social Efficiency or Stress Relief Index, which evaluates the 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 transfer from Düzce demonstrates a relatively high efficiency in relieving Istanbul’s water stress with moderate ecological cost. In contrast, transfers from the Istranca Streams 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 IBTs 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.