This chapter examines spatial and temporal variations in water supply and consumption across 103 river basins in Sri Lanka. Water accounting of the surface runoff, a crucial initial step in evaluating current water resource use, shows that several basins in the dry zone are already experiencing seasonal or year-round water stress. In these areas, 12 total process water depletion exceeds 60% of the utilizable water resources. Process water depletion encompasses the evaporation and transpiration (or consumptive water use) from the irrigation, domestic, and industrial sectors, as well as non-process evaporation from water bodies. Many river basins in the northern regions face severe seasonal water stress during the dry season. This stress intensifies when environmental flows are allocated from the utilizable water resources. The increasing water shortages underscore the necessity of providing accurate weather predictions, enhancing rainfall and irrigation water use efficiency, improving storage management, promoting crop diversification, and other measures to address the challenges posed by population growth, economic development, environmental flow allocations, and climate change.