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Water scarcity is already a big problem for billions worldwide, study finds

Courtesy of Science Advances
Although much of the world may seem to have enough water to meet its needs on an annual basis, looking at month-to-month balances averaged from 1996 to 2005 shows that much of the planet experiences scarcity from one to five months of the year (yellow, orange, red) and a sizable portion runs short for half the year or more (dark red).

New research from the Netherlands challenges the common assumption that the world will face significant new water shortages in generations to come.

The scientists agree that scarcities will be deep and widespread. Where they differ is in concluding that the crisis has already arrived for much of the planet, much of the time:

As of today, they find, about two-thirds of Earth’s population experiences severe water scarcity – defined as demand that’s more than double the available supply – at some point during the year. That’s 3.97 billion people we’re talking about, which is more than double some generally accepted estimates.

Roughly half of them live in India or China, but no continent is immune; portions of the American Southwest are as out of balance as the deserts of northern Africa or the Australian interior.

And of those nearly 4 billion, about half – 1.78 billion – experience severe water scarcity for at least six months of every year, according to their paper published Friday in a journal of the American Association for the Advancement of Science.

A month-to-month view

This study departs from previous examinations of global water balances that relied on annual figures – an approach, the researchers say, that dramatically understates the problem because both sides of the supply/demand balance fluctuate significantly over the course of a year, and typically not in the same direction.

As Mesfin M. Mekonnen and Arjen Y. Hoekstra of the University of Twente put it,

The essence of global water scarcity is the geographic and temporal mismatch between freshwater demand and availability….

Various studies have assessed global water scarcity in physical terms at a high spatial resolution on a yearly time scale. Annual assessments of water scarcity, however, hide the variability within the year and underestimate the extent of water scarcity.

For this paper, researchers used monthly statistics for availability and consumption of “bluewater,” defined as freshwater from surface sources and aquifers.

They made adjustments for typical patterns of precipitation, evaporation and stream flow. Then they mapped the data at a very high geographic resolution, which enabled an especially close look at the impact of upstream water withdrawals on downstream populations.

(Note for the technically minded: Data were plotted on a grid whose cells were 30 by 30 minutes of arc; at the equator, such a square would be 30 nautical miles, or about 34 statute miles, on a side).

In addition to plotting areas of severe imbalance, they mapped zones of “moderate” water scarcity, defined as those with demand exceeding supply by 50 percent. As you’d expect, that drove up the numbers – but maybe not by as much as you’d think.

For the one-month measure, the population experiencing at least moderate scarcity was 4.26 billion, just 7 percent more than the population for whom the scarcity was severe. So in areas where significant water scarcity is already present, it would seem, the problem is much more likely to be severe than merely “moderate.”

Some 3.6 billion people experience moderate to severe scarcity at least one month out of four, and some 540 million endure moderate to severe scarcity all year round.

Where problems are, aren’t

Before looking at the geographic distribution of shortage, it’s interesting to consider which parts of the planet are still relatively problem-free:

Year-round low blue water scarcity can be found in the forested areas of South America (notably the Amazon basin), Central Africa (the Congo basin), and Malaysia-Indonesia (Sumatra, Borneo, New Guinea) and in the northern forested and subarctic parts of North America, Europe, and Asia. Other places with low water scarcity throughout the year can be found in the eastern half of the United States, in large parts of Europe, and in parts of South China.

As for patterns of high scarcity, they say, these “appear to prevail in areas with either high population density (for example, Greater London area) or the presence of much irrigated agriculture (High Plains in the United States), or both (India, eastern China, Nile delta).”

High water scarcity levels also occur in areas without dense populations or intense irrigated agriculture but with very low natural water availability, such as in the world’s arid areas (for example, Sahara, Taklamakan, Gobi, and Central Australia deserts). Water scarcity in the Arabian Desert is worse than that in other deserts because of the higher population density and irrigation intensity.

In many river basins, for instance, the Ganges basin in India, the Limpopo basin in Southern Africa, and the Murray-Darling basin in Australia, blue water consumption and blue water availability are countercyclical, with water consumption being highest when water availability is lowest. …

Groundwater depletion occurs in many countries, including India, Pakistan, the United States, Iran, China, Mexico, and Saudi Arabia. Direct victims of the overconsumption of water resources are the users themselves, who increasingly suffer from water shortages during droughts, resulting in reduced harvests and loss of income for farmers, threatening the livelihoods of whole communities [footnotes omitted].

Hard to shift the balances

The authors conclude that meeting humanity’s needs for freshwater will be “one of the most difficult and important challenges of the next century,” and they suggest that radical changes in agricultural practice to reduce irrigation withdrawals will be key.

But just as taking a month-to-month look reveals much larger imbalances than are apparent from annual statistics, their analysis suggests that even dramatic increases in water supply, or decreases in demand, won’t necessarily make a huge difference:

When we increase water availability estimates worldwide and for each month by 20%, the number of people facing severe water scarcity during at least 1 month of the year reduces by 2% (from 4.0 to 3.9 billion). Reducing water availability by 20% gives 4.1 billion.

Changing water footprints in the ±20% range results in the number of people facing severe water scarcity to be between 3.9 and 4.1 billion as well. Changing water availability in the ±50% range yields 3.8 to 4.3 billion people facing severe water scarcity during at least part of the year, whereas changing water footprints in the ±50% range yields 3.6 to 4.2 billion people.

The reason for the low sensitivity is the huge temporal mismatch between water demand and availability: Demand is generally much lower than availability or the other way around. Only in times wherein water demand and availability are of the same magnitude can changes in one or the other flip the situation from one scarcity level to another.

In other words, big boosts in supply and deep cuts in demand can only solve problems in places where there isn’t much of a problem to begin with.

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The full paper, “Four billion people facing severe water scarcity,” as published in the AAAS journal Science Advances can be read here without charge.

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