'Water Terms': Definitions and Metrics

A crucial part of any discussion regarding the theme of water is establishing a metric for the myriad of ‘water terms’ that appear in literature; these include ‘water stress’, ‘water scarcity’, ‘water [in]security’ and so on. ‘Water stress’ is a sweeping term that generally applies to the inability to meet demand for water and can lead to ‘water scarcity’ where there is a lack of sufficient available water (Naik 2017). ‘Water [in]security’ is the ‘capacity to safeguard sustainable access to adequate quantities of acceptable quality water for sustaining livelihoods, human well-being, and socioeconomic development …’ (UN Water 2013).

Falkenmark's Water Stress Index 

The Water Stress Index (WSI) (Falkenmark et al. 1989, cited in Damkjaer and Taylor 2017) (Table 1) is perhaps the most notable metric, linking food security to freshwater availability, quantified by mean annual river runoff (MARR). Utilised on a national scale, the WSI has been promoted due to easily accessible data and its adoption by the FAO (Nepomilueva 2017). However, MARR also assumes fluctuations in freshwater storage are insignificant (Taylor 2009) and consequently fails to acknowledge the fluctuations in seasonal rainfall that characterises Sub-Saharan Africa. Further to this, MARR doesn’t account for ‘green water’ i.e. precipitation stored in the soil root zone (Rockstrom and Falkenmark 2015). The role of ‘green water’ in influencing agricultural uses of water makes its omission from estimates of MARR, and therefore WSI, particularly important; under the WSI, domestic water demand is 100L/capita/day whereas agricultural and industrial demand is 20 times this. 

Alternative Metrics: WTA and Social WSI

The emergence of different metrics like the Withdrawal-To-Availability (WTA) ratio is unsurprising given these limitations. The WTA is a measure of annual withdrawals across the domestic-agricultural-industrial (DAI) sectors, based on demand, relative to availability, gauged through MARR calculations; a country is categorised ‘water stressed’ if this ratio is between 20% and 40% and ‘severely stressed’ above 40% (Alcamo et al. 2003). Despite offering an advantage over the WSI by defining stress based on water demand : availability, it suffers from many of the same weaknesses, primarily because of its use of MARR, but also because it neglects return flows and recycled water in withdrawal data. Estimates of withdrawals may also be a problem – it is far easier to assume per capita demand, but potentially far more accurate to calculate freshwater withdrawals.

An intriguing alternative to these metrics is the social WSI. Variables like political participation and the Human Development Index (HDI) which is comprised of measures of life expectancy, literacy rate and GDP/capita are used as proxies to ascertain the ‘adaptive capacity’ of a country in response to water stress. This method helps to distinguish between ‘physically’ and ‘economically’ stressed countries (Molden et al. 2007); the former defines a country where >75% of river flows are withdrawn for DAI use and demands beyond this cannot be met, even after considering adaptive capacity, whilst the latter defines a country where renewable water resources are sufficient due to reduced withdrawals, but the absence of effective investment in water infrastructure limits availability. I find this metric to be especially useful as it encourages me to explore the significance of water access and infrastructural developments in the context of food.

Definitions of ‘food security’ have evolved to reflect an appreciation for the complexities of policy issues. An initial focus was on volume and stability of supplies before the concept of ‘security’ was expanded to one of physical and economical access, especially for vulnerable groups. A World Bank report in 1986 titled ‘Poverty and Hunger’ emphasised the importance of temporal dynamics, creating a distinction between chronic insecurity because of continuing poverty and transitory insecurity because of a natural disaster or economic collapse. Further revisions led to the FAO (1996) definition of food security where ‘all people, at all times, have physical and economic access to sufficient, safe and nutritious food to meet their dietary needs and food preferences for an active and healthy life’.  

I want to take you back to my earlier mention of different ‘water terms’ before concluding this post. Whilst writing this brief review of the different metrics available, I have stumbled several times by interchanging ‘stress’ with ‘scarcity’ and ‘insecurity’. This is a common theme in literature and is one of the main reasons that definitions vary so much – a quick search on the Internet will prove this. As such, I want to emphasise that the goal of this post was to draw attention to the different methods of measuring metrics of water stress and scarcity, and to recognise that whilst distinctions between definitions can be made, these shouldn’t be set in stone. Assigning any one of these labels to a country then should be carefully considered so as to appreciate the complexity and differences between countries and their level of [in]security.