How Does Climate Change Alter the Partitioning of Rainfall into Blue and Green Water and Drive Water Scarcity?

Water scarcity is emerging as a critical challenge under changing climatic conditions, especially in semi-arid river basins, where rainfall variability governs the balance between blue and green water resources. Achieving the United Nations Sustainable Development Goals - 6 (Clean Water and Sanitation) and 13 (Climate Action) requires a clear understanding of how climate change alters hydrological processes and drives water scarcity. This study presents an integrated, model-based framework to evaluate the influence of climate change on the partitioning of precipitation into blue water (surface runoff and sub-surface runoff) and green water (soil moisture) and the resulting implications for water availability in the Upper–Middle Godavari (UG–MG) basin, India. A physically based SWAT hydrological model was developed, calibrated, and validated for 1979–2020 and forced with climate projections from SSP245 and SSP585 scenarios from CMIP6. The framework quantified blue water resources (BWR), green water resources (GWR), and associated scarcity indices by integrating irrigation, domestic, and industrial demands with environmental flow requirements. The basin receives an average annual precipitation of 770 mm, which is partitioned into 137 mm (17.8 %) of BWR and 629 mm (81.7 %) of GWR. Future projections indicate a 10–40 % increase in rainfall, accompanied by consecutive high-intensity rainfall events that are projected to enhance BWR by 50–100 % in some of the sub-basins. In contrast, GWR declines by up to 30 %, particularly in agricultural regions where rising temperatures (+3.5 °C under SSP585) intensify evapotranspiration and reduce soil-moisture retention. The BWR/PCP ratio is projected to exceed 30 %, signifying a shift toward runoff-dominated hydrological regimes. The results suggest that climate change is transforming the UG–MG basin from a green-water-dominated to a runoff-driven system, simultaneously heightening downstream flood risks and agricultural water deficits. The findings emphasize the urgency of adaptive and integrated water management strategies to restore the blue–green water balance and advance progress toward SDGs 6 and 13.

Keywords: Blue water resources; Green water resources; Climate change; SWAT model; Rainfall partitioning; water scarcity