Stress vs. Scarcity

Source: Global physical and economic water scarcity (2007) 

In order to determine those areas which are most vulnerable to water insecurity, it is important to distinguish between some key terms. Water stress is a measure of availability and is referred to when annual water supplies drops below 1,700m³ per person for any given area. When this figure reaches below 1,000m³, the issue becomes one of access and the region is then said to experience water scarcity (WWAP, 2012). 

This map from the UN splits water scarcity up into physical scarcity and economic scarcity. Physical water scarcity refers to when water resources have exceeded their sustainable limits; this does not necessarily mean that dry areas are therefore physically water scarce in this sense. Economic water scarcity refers to the financial capability to extract water from a particular area; therefore some areas may be abundant in water supplies but may not have the means to use this water.  

It may be easier to determine which regions are experiencing water stress due to the quantified definition, however when discussing scarcity it becomes somewhat more difficult. Although there is a threshold of 1,000m³,  the extent of which regions experience physical and economic scarcity are changing all the time, and so defining those regions experiencing this can sometimes be difficult. At present, one third of the world's population is experiencing some kind of physical or economic water scarcity.