2.3. How can water stress be calculated?

Much like the issue of defining sustainability, which we discussed in Session 1 of this module, a formal scientific definition of how to calculate water stress has not fully been agreed. There are, however, two methods that are applied commonly, to calculate water stress; the “water resources vulnerability index” and the “water stress indicator”.

2.3.1. The water resources vulnerability index

An index of water stress that is often calculated is the “water resources vulnerability index”. The index is a measure of pressure on water resources and it can be readily applied to multiple watersheds. The index calculates the ratio of annual water withdrawals to annual runoff (renewable freshwater supply). Watersheds where withdrawals are less than 20% of supply (i.e. a ratio of less than 0.2) have low or no water stress, watersheds with a ratio between 0.2-0.4 have medium stress, and watersheds where  withdrawals are greater than 40% of supply (i.e. a ratio of greater than 0.4) have high stress (Alcamo et al., 2007; Hanasaki et al., 2008).

The threshold of withdrawals being greater than 40% of supply, has been arbitrarily chosen amongst the scientific water resources modelling community as an indicator of high water stress, since the larger the amount of water that is withdrawn from the environment and used and discharged back into rivers, the more degraded and/or depleted the resource becomes, and the higher the water stress (Alcamo et al., 2007). In this sense, 100% of the resource does not need to be withdrawn for a watershed to be classed as water stressed. 

The water resources vulnerability index tends to highlight pressures in watersheds with large amounts of irrigation, because the index is largely based upon withdrawals. Projections of future withdrawals are contingent upon future population change and assumptions about future changes in domestic, industrial and agricultural water use intensity. Note, however, that this metric does not account for water quality.

2.3.2. The water stress indicator

The “water stress indicator” is based on water availability per person. A threshold of 1000 m3/person/year is generally used to indicate exposed to water resources stress; i.e. if there is less than  1000 m3 of water available for each person in a year, then that region is classed as water stressed. The measure is simple to calculate but it assumes that water resources pressures are a function of the numbers of people only, not the amount of water that those people actually use.