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article imageWinter snowpack decline may cut water supply for many nations

By Karen Graham     Nov 12, 2015 in Environment
A decline in winter snowpacks in many mountainous regions of the world may soon lead to a reduction in many nations' water supplies, according to a new study.
Water from snow-dependent drainage basins in the Northern Hemisphere currently serving over two billion people run the risk of being critically imperiled in the coming years as winter snowpacks decline due to a changing climate.
In the Northern hemisphere, the maximum area usually covered by winter snow is about 45.2 million square kilometers (486,528,751 sq. ft.). In the U.S., snow melt from drainage basins in large parts of the west provides the major source of water for about 75 percent of the population. This is also true of drainage basins in southern Europe, the Mideast and central Asia.
As we have seen in the American west and in western Canada, winter snow pack is an important seasonal water source, not only for drinking water, but for farms, cities and ecosystems. Snow falling in the higher elevations of large mountain systems around the world gradually melts and the water runs into the lowlands during the spring and summer seasons.
This is when water is most needed by all of humankind, for drinking, storage, agriculture and our fragile ecosystems. But the study suggests that global warming is turning this system on its head, upsetting the balance. Studies are showing that winter precipitation is falling as rain, not snow in many parts of the world, and washing away, while snow that does fall is settling at higher elevations, yet melting earlier.
The study's lead author Justin Mankin, a postdoctoral fellow at Columbia University’s Earth Institute based jointly at the institute’s Lamont-Doherty Earth Observatory and its affiliated NASA Goddard Institute for Space Studies, says the new study estimates snow’s potential to "supply present human needs in both current and projected climates, taking both weather trends and population into account."
Mankin points out, "Snow is important because it forms its own reservoir. But the consequences of reduced snowpack are not the same for all places. It is also a function of where and when people demand water. Water managers in a lot of places may need to prepare for a world where the snow reservoir no longer exists."
With a warming world, scientists have been studying declining snowpacks in a number of regions across the world, from the disappearing snowfields in the Rocky Mountains from Colorado to northern Montana, as well as in the Himalayas and other areas.
The study also cites the California drought and the severe reduction in the Sierra-Nevada Mountains' snowpack, the greatest decline in 500 years.
The study focused on 97 out of 421 drainage basins in the Northern Hemisphere currently serving two million people. These drainage basins run a two-thirds chance of declines based on present water demands. The basins most sensitive to change and having the most at stake for humans are: "The basins of northern and central California, where much of U.S. produce is grown; the basins of the Colorado and Rio Grande rivers, which serve much of the American West and northern Mexico; the Atlas basin of Morocco; the Ebro-Duero basin, which feeds water to Portugal and much of Spain and southern France; and a series of basins covering eastern Italy, the southern Balkans, several Caucasus nations, and northern Turkey."
Projections of decreased potential for snow to supply spring and summer human water demand by mid-ce...
Projections of decreased potential for snow to supply spring and summer human water demand by mid-century (reddest areas are highest risk; yellow, lowest). In blue/white areas, future rainfall should be sufficient to supply present human needs; gray regions indicate basins where rainfall might be insufficient, potentially making them dependent on snow or other sources.
Mankin et al., Environmental Research Letters 2015
The volatile Shatt al Arab basin, which channels meltwater from the Zagros Mountains to Iraq, Syria, eastern Turkey, northern Saudi Arabia and eastern Iran is already impacted by a warming planet, with decreased rainfall and increasing evaporation across many parts of the eastern Mediterranean and into the Mideast.
"Managers need to be prepared for the possibility of multi-decadal decreases in snow water supply," said Mankin. "But at the same time, they could have large multi-decadal increases. Both of those outcomes are entirely consistent with a world with global warming."
This study, "The potential for snow to supply human water demand in the present and future," was published in the journal Environmental Research Letters on November 1, 2015.
More about declining snowpack, northern hemisphere, drainage beasins, seasonal water source, human water sources
 
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