This map shows annual, seasonal and dry-year depletion for 15,091 watersheds worldwide. The inclusion of seasonal and dry-year depletion estimates increases by 15-fold the number of watersheds experiencing depletion of 75% or more. Source: Kate A. Brauman et al., in Elementa, January 20, 2016.
Earlier this month, the World Economic Forum in Davos, Switzerland, released its annual risk assessment
, which looks across the spectrum of threats to society and ranks them. This year, it declared water crises to be the top global risk to society over the next decade.
Just behind water crises were the failure to adapt to climate change, extreme weather events, food crises and profound social instability. Together, these five threats constitute an interwoven risk landscape that has water shortages, in one way or another, at its core.
The Forum is not predisposed to emphasize environmental concerns. Terrorism, fiscal crises and the spread of infectious diseases are among the risks it considers and ranks. The Forum’s members include heads of state, chief executive officers and civic leaders.
Water topped one of the risk categories in the 2015 assessment, as well.
If an alarm bell was needed to focus global attention on water security, it has rung.
Our tool, though developed with some complex modeling, is represented by a very straightforward global map that depicts the depletion of water at a high degree of spatial resolution. (By “depletion,” we mean the fraction of renewable surface water and groundwater available in a watershed that is consumed by human activities.) This tool differs from most others available in two important ways.
Whereas most scarcity maps examine trends at the scale of large watersheds – sometimes even as big as the entire Mississippi River basin, which drains 40 percent of the continental United States – our tool breaks the picture down to much smaller sub-watersheds. This enables users to get a more detailed and accurate picture of where problems of shortage are likely to arise.
Second, instead of depicting water scarcity only as an annual average problem, it sketches out where seasonal and dry-year shortages can be expected. The inclusion of these periodic episodes of depletion increases by a factor of 15 the number of watersheds experiencing 75 percent depletion or more.
We found that in two-thirds of the world’s river basins, water consumption remains within sustainable limits. But in the other third, water extractions are heavily depleting available supplies. In those areas, freshwater ecosystems and the species they support are likely in trouble, and shortages impacting farms, cities and businesses are more likely to occur.
Because growing crops requires a great deal of water, watersheds with extensive irrigated agriculture tend to be at higher than normal risk for shortages. Some 40 percent of the world’s food comes from the 18 percent of cropland that is irrigated, so threats to irrigated agriculture can quickly translate into threats to global food security.
Our study characterizes 71% of world-irrigated area and 47% of large cities as experiencing at least periodic water shortage.
While no tool can serve as a crystal ball, this one helps remove some uncertainties by pinpointing areas at risk. This can help farmers, city planners, conservationists and businesses take steps to build more resilience and security in the face of prospective shortages.
It can highlight, for example, where it could make sense to institute “dry-year options,” whereby a city might pay irrigators to improve irrigation efficiency in return for the water saved. Or it might help conservation groups target where water banks might be needed to secure flows for rivers and wetlands.
The map and water depletion data for all 15,091 watersheds are freely available at www.earthstat.org