This study presents a new method for assessing global monthly water scarcity by comparing blue water footprints (consumptive use of surface and groundwater) with blue water availability, considering ecological needs and seasonal variations. The research analyzed 405 river basins from 1996 to 2005, revealing that 201 basins with 2.67 billion inhabitants experienced severe water scarcity during at least one month of the year. The study highlights the ecological and economic consequences of water scarcity, including river desiccation, biodiversity loss, and economic disruption. It defines blue water scarcity as the ratio of blue water footprint to blue water availability, accounting for environmental flow requirements. The study provides a more accurate assessment by using monthly data, considering water consumption rather than withdrawals, and incorporating ecological flow needs. It identifies four levels of water scarcity: low, moderate, significant, and severe. The results show that 223 basins (55% of the studied basins) had blue water footprints exceeding availability during at least one month, with 201 of these basins experiencing severe scarcity. The study emphasizes the importance of considering seasonal variations in water availability and scarcity, as well as the need for improved water management to prevent ecological and economic damage. The findings underscore the critical nature of water shortages globally and the importance of using accurate indicators for water management and policy-making.This study presents a new method for assessing global monthly water scarcity by comparing blue water footprints (consumptive use of surface and groundwater) with blue water availability, considering ecological needs and seasonal variations. The research analyzed 405 river basins from 1996 to 2005, revealing that 201 basins with 2.67 billion inhabitants experienced severe water scarcity during at least one month of the year. The study highlights the ecological and economic consequences of water scarcity, including river desiccation, biodiversity loss, and economic disruption. It defines blue water scarcity as the ratio of blue water footprint to blue water availability, accounting for environmental flow requirements. The study provides a more accurate assessment by using monthly data, considering water consumption rather than withdrawals, and incorporating ecological flow needs. It identifies four levels of water scarcity: low, moderate, significant, and severe. The results show that 223 basins (55% of the studied basins) had blue water footprints exceeding availability during at least one month, with 201 of these basins experiencing severe scarcity. The study emphasizes the importance of considering seasonal variations in water availability and scarcity, as well as the need for improved water management to prevent ecological and economic damage. The findings underscore the critical nature of water shortages globally and the importance of using accurate indicators for water management and policy-making.