Given increasing water scarcity and the associated deterioration of the quantity and quality of water sources in many parts of the world, many “tools” have been developed to map water scarcity risk or water risk. Typically, these tools are based on estimates of the average water supply and demand in each administrative jurisdiction. Often, water risk is basin-wide, while decisions are made at the city or county level. Therefore, the analyses on which such tools are based understate the potential water risk. In most places, even if the resource is not over-appropriated on average, persistent shortages induced by climate variation can lead to stress. A clear understanding of shortages, in terms of the magnitude, duration and recurrence frequency will better inform localities.