Global monthly water stress: 2. Water demand and severity of water stress

This paper assesses global water stress at a finer temporal scale compared to conventional assessments. To calculate time series of global water stress at a monthly time scale, global water availability, as obtained from simulations of monthly river discharge from the companion paper, is confronted with global monthly water demand. Water demand is defined here as the volume of water required by users to satisfy their needs. Water demand is calculated for the benchmark year of 2000 and contrasted against blue water availability, reflecting climatic variability over the period 1958-2001. Despite the use of the single benchmark year with monthly variations in water demand, simulated water stress agrees well with long-term records of observed water shortage in temperate, (sub) tropical, and (semi)arid countries, indicating that on shorter (i.e., decadal) time scales, climatic variability is often the main determinant of water stress. With the monthly resolution the number of people experiencing water scarcity increases by more than 40% compared to conventional annual assessments that do not account for seasonality and interannual variability. The results show that blue water stress is often intense and frequent in densely populated regions (e. g., India, United States, Spain, and northeastern China). By this method, regions vulnerable to infrequent but detrimental water stress could be equally identified (e. g., southeastern United Kingdom and northwestern Russia).