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Freshwater vulnerable cities due to rapid urbanization

March 25, 2015
Manhattan skyline in the view ©RohithRoy

Manhattan skyline in the view ©RohithRoy

Introduction:

According to a new research, more than 40% of the world’s great cities supplied by surface water could become freshwater vulnerable to shortages and drought by 2040. More than 3 out of 10 were already vulnerable in 2010, the study states.

Factors responsible:

For the first time in history, more than half the world’s population is now concentrated in cities, and this proportion is predicted to increase to two-thirds by 2050. Cities expand near abundant water supplies. As population explodes, so does the demand. The environmental flows (E-Flows) remain much the same.
Some major cities are already under drought stress. Chennai and Hyderabad in South India had to be supplied with tankers in 2004 and 2005 and so was Delhi in North India. These cities in India depend on River Krishna and River Yamuna’s freshwater resources. Delhi also gets part of it’s supply from neighboring states of Rajasthan and Haryana.  São Paulo in Brazil in LatAm is now at a crisis point and cities in CA are struggling with unprecedented droughts.

Demand and supply:

Environmental scientist Julie Padowski and Steven Gorelick, director of the Global Freshwater Initiative at Stanford University in California, analyzed supplies to 70 cities in 39 countries, all of them with more than 750,000 inhabitants, and reliant on surface water.

City supply system vulnerability

City supply system vulnerability

Freshwater vulnerability:

Vulnerability is the failure of an urban supply basin to meet demands from human, environmental and agricultural users, and they set the supply target as 4,600 litres per person per day. This factors in “virtual water”, defined as the total volume of water needed to produce and process a commodity or service.
They proposed three different kinds of measure of supply.

  • If a city failed to meet one or two of these metrics, it was considered threatened.
  • If it failed to meet all three, it was rated as vulnerable.

The six cities that will begin to face water shortages are Dublin in Ireland, Charlotte in the US, Ouagadougou in Burkina Faso, and Guangzhou, Wuhan and Nanjing in China. Curiously, none of the cities in India have been listed to be freshwater vulnerable in the assessment.  Most of the cities that are already vulnerable rely on reservoirs, and the study implies that urban planners will need to think about

Precipitation data:
The article states that the scientists cannot rely on rainfall data because, the network of dedicated satellites “fails to meet operational data needs for flood management”.
4 of the 10 satellites have exceeded their design life. There are already weak spots in the network, especially in developing countries, which means that floods could take people by surprise. When 4 fail to deliver, the potential for catastrophe will be even worse. The scientists call for better international co-ordination of satellite replacement.

Vulnerable versus threatened status:

For both categories reservoir and river supplied cities were considered vulnerable, if all demand thresholds were violated. If a city had one or two but not all three metrics exceeding a threshold, then it was considered threatened, as these supplies may no longer be able to support urban demand without causing damage to agriculture, the environment, or both. The scientists referred to a city as susceptible if it has either threatened or vulnerable status; the number of susceptible cities is the sum of those that are threatened or vulnerable. Finally, cities were considered non-threatened if no demand thresholds were violated.

Future predictions:

Importantly, the scientists did not factor in climate change, which might make conditions worse. Instead, they simply considered current demand and supply, and then projected demand in 2040.
Of the 70 cities, they found that 25 (36%) could already be considered to have become vulnerable by 2010. By 2040, this number will grow to 31 (44%).

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