Water Stress and Urbanisation

Variability in the Water Stress Indicator across the globe.
See for details of how the Water Stress Indicator (also known as relative water demand) is derived. Briefly, the Water Stress Indicator is a measure of total annual water withdrawals (total amount of water removed from freshwater sources by various users, e.g. local, municipal, industrial, agriculture) expressed as a percentage of the total average annual available freshwater surface and groundwater (“blue water”). The Water Stress Indicator shown here is shown as the ratio of water withdrawals in 2010 to the mean available blue water over the period 1950–2008. Available blue water is the total amount of water available to a catchment before any use is satisfied, calculated as all water flowing into the catchment from upstream catchments plus any imports of water to the catchment, minus upstream consumptive use, plus runoff in the catchment. Higher stress values indicate an increasing inability of existing water supplies to meet water demands. The Water Stress Indicator is thus a measure of chronic, rather than drought, stress. Global water stress and Big Cities: the 1 692 Big Cities in the world (as of 2015, with a population over 300 000, are shown in relation to the Water Stress Indicator.
Source: Aqueduct Global Maps.

Increasing vulnerabilities of Big Cities to water shortages

Water is a critical natural resource for both natural ecosystems and human subsistence, including human needs related to consumption, agricultural, municipal, industrial, and recreational uses. Freshwater resources are limited yet global water use has grown at twice the rate of population growth over the past 100 years. Humans affect the global water cycle in a variety of ways. There are (i) direct effects, including the construction of dams, diversion of major rivers, and the withdrawal of surface and groundwater reserves for industrial, agricultural, and domestic usages; and (ii) indirect effects, mainly via climate change, which has the potential to significantly modify the water cycle and thus influence global water availability and alter demand.
Access to, and sustainability of, freshwater resources sufficient to meet human demands is one of the most critical challenges of the near future. The World Economic Forum asserted that the global economy failure to meet human demands for water would represent the greatest shared risk to the global economy. The only way to avoid a global water crisis is to limit water consumption, increase water-use efficiencies, and develop efficient, cooperative, international strategies to share limited freshwater resources. However, the magnitude of this challenge is illustrated by the fact that currently about two-thirds of the global population – half of whom reside in India and China – subsist under conditions of severe water scarcity at least one month of the year; over 120 million people in the European region do not have access to safe drinking water; and half a billion people in the world face severe water scarcity year round.
To prepare, manage, and adapt to water shortages, there is a need for different sectors of human society (private, nation states, their institutions and organisations, etc.) to better understand the scope of the problem. However, quantifying water dynamics at the scale of the globe is a difficult task due to the inherent complexity of the hydrologic cycle, which includes many nonlinear interactions and feedbacks between climate, water availability, water quality, human interventions, and water demand. In part to aid in this understanding, the World Resources Institute developed the Aqueduct water risk mapping tool. Generally, Aqueduct provides insight into the complex topic of water risk by using available data, indicators, and modelling. It combines data on 12 different indicators of water risk – from water stress to access to clean drinking water. Here, we present the Water Stress Indicator, which is from the Aqueduct Global Maps.
Big Cities, climate change, water resources and land degradation
Climate change, e.g. processes that lead to increased global aridity, will affect Big Cities. Since urban areas are particularly vulnerable to water shortages, this is one of the greatest risks of urbanisation. The graphs suggest that under current climate conditions, 44 % of the Big Cities face High- to Very High water stress. In a study of 71 Cities with more than 750 000 inhabitants, it was estimated that 35 % of the Megacities are presently vulnerable to water shortages.
How might these trends change in the future? To explore this, the Water Stress Indicator was recalculated for the Imminent Future scenario (2011-2040) using both the RCP 4.5 and RCP 8.5 scenarios. Overall, water stress becomes more severe with a 24 % increase in the number of Big Cities with Very High stress (from 463 to 572). For the Megacities, it was estimated that 29 % more of them would be vulnerable to water shortages by 2040 if no action is taken.
While climate change is an important driver of global water shortages, increases in population growth and rapid economic development will have a much stronger effect on water stress than climate change per se. This is especially the case for many developing nations. The rate of social and economic development associated with urbanisation poses serious threats to the environment and often leads to serious land degradation. Some of the most immediate pressures on land that lead to degradation include diversion of surface waters and the removal of groundwater reserves to meet human, industrial and agricultural demands, construction of domestic and industrial landfills, air pollution, the conversion of adjacent grasslands, forests and wetlands to agriculture, building of infrastructure to support urban needs and various industrial activities, e.g. thermoelectric production and mining. In addition, urban areas are major local sources of pollutants into waterways and groundwater reservoirs. Paradoxically, another cost is that as urban expansion inescapably spreads over existing agricultural lands, the increasing land values and markets around cities often result in the abandonment of productive agricultural land as owners anticipate profits for development.
The interactions and feedbacks between urbanisation, climate change, population growth, land use, land cover, land degradation and water resources are highly complex. With regard to water resources and water security, activities such as reducing water consumption via greening cities and water recycling and the development of resilient infrastructures are needed to help reduce the vulnerability of these cities to climate change and projected water shortages. Although there is no single best policy that can be universally applied, water conservation is increasingly being singled out as the most important strategy for water planning and management for the future decades.

Water stress indicator calculated for the Imminent Future scenario (2011-2040) (using RCP 4.5 and RCP 8.5 scenarios - differences in RCP scenarios were slight).
Source: Reynolds, J. et al.

Trends in Water Stress Indicator: Dryland vs. Non-Dryland Cities:
The number and percentage of dryland and non-dryland cities in each of the Water Stress Indicator categories under current climate conditions. A total of 74 % of the dryland cities are found in High to Very High stress regions whereas 55% of non-dryland cities are located in None- to Low-stress regions. (Note: water stress data missing for some cities hence, total percentage with negative change is not equal to percentage with positive change).
Source: Reynolds, J. et al.

Trends in all Big Cities
Based on current climate, Big Cities are roughly split evenly in terms of their water stress ratings: 44 % of them experience High- to Very High-water stress, compared 42 % that are in None- to Low-water stress regions. The remaining 16 % are in Mid-water stress.

Source: Reynolds, J. et al.