Freshwater scarcity is a global issue of environmental concern that threatens agricultural production and human health.In this study,we established freshwater stress indices(WSIs) for the nine water basins of Tanzania by using the quantity of freshwater available and various water uses.The relationship between water availability and different water uses,including environmental water requirements,was analyzed,with uncertainty and sensitivity analysis performed by a Monte Carlo simulation technique.Extreme WSI values close to 1.00 were obtained in the Rufiji,Pangani,and Wami-ruvu basins,Internal drainage,and Lake Rukwa,while low and moderate WSI values ranging from 0.03 to 0.84 were found in Lake Victoria and the Ruvuma,Tanganyika,and Nyasa basins.This study adds further knowledge on the level of freshwater scarcity,relationships between water availability and different water uses,and suggests policy options to reduce freshwater scarcity at the basin level for sustainable water supply. Freshwater scarcity is a global issue of environmental concern that threatens agricultural production and human health. In this study, we established freshwater stress indices (WSIs) for the nine water basins of Tanzania by using the quantity of freshwater available and various water uses. The relationship between water availability and different water uses, including environmental water requirements, was analyzed, with uncertainty and sensitivity analysis performed by a Monte Carlo simulation technique. Extremes WSI values ​​close to 1.00 were obtained in the Rufiji, Pangani, and Wami-Ruvu basins, Internal drainage, and Lake Rukwa, while low and moderate WSI values ​​ranging from 0.03 to 0.84 were found in Lake Victoria and the Ruvuma, Tanganyika, and Nyasa basins. This study adds further knowledge on the level of freshwater scarcity, relationships between water availability and different water uses, and suggests policy options to reduce freshwater scarcity at the basin level for sustainable water supply.