Taking stock of the world鈥檚 lakes
The total shoreline of the world鈥檚 lakes is more than four times longer than the global ocean coastline. And if all the water in those lakes were spread over the Earth鈥檚 landmass, it would form a layer some four feet (1.3 metres) deep.
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Those are just two of the big-picture findings to emerge from the most complete global database of lakes to date, compiled by geographers at 91社区. Their research, published in听Nature Communications,听promises to help scientists better understand the important role of lakes in the Earth鈥檚 complex environmental systems 鈥� from the hydrological cycle and weather patterns, to the transport, distribution or storage of pollutants and nutrients through the landscape.
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鈥淟akes are changing, in a changing world,鈥� says senior author Bernhard Lehner, an associate professor in 91社区鈥檚 Department of Geography.听 鈥淪ome are disappearing as there is less water to keep them filled, others are created or growing in regions where there is more rainfall. So we need a good inventory of the current status of lakes to understand and monitor their changes and the effects that this may have for our global environment.鈥澨�
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Filling data gaps
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While there are plenty of measurements for lakes in some regions of the world, significant gaps have remained in the global data. In principle, the surface area or shoreline length of a lake can be directly measured on maps or satellite images, for example, but it鈥檚 much more difficult and time-consuming to estimate the amount of water stored beneath a lake鈥檚 surface.
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An intuitive theory has long held that lakes in hilly or mountainous regions should tend to be deeper than those in flat landscapes. But until recently, it wasn鈥檛 easily possible to determine a clear relationship between the degree of hilliness and the depth of a lake.
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Taking advantage of the latest improvements in satellite data providing precise measurements of land surface elevation, the 91社区 researchers related the slopes found around lakes with thousands of existing lake-depth records. (Lakes in hilly surroundings did tend to be deeper). They then used computer models to extend those calculations to all unmeasured lakes on Earth. Based on this, they calculated the volume of water stored in more than 1.4 million lakes that are larger than 10 hectares, or roughly 14 soccer fields. The grand total: more than 180,000 cubic kilometres.
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Beneath the surface
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The researchers also estimated how long water typically 鈥渞esides鈥� in each of the lakes 鈥� the amount of time from the moment it enters a lake until it flows out. On average for all lakes, the residence time worked out to about five years. But there are many with much shorter times; and, at the other extreme, more than 3,000 lakes have residence times estimated at 100 years or more.
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There are more than seven million kilometres of total lake shorelines on Earth, the researchers estimate. That鈥檚 about 10 times the distance to the moon and back. 鈥淲hen you think of all the processes that take place at the interface of lakes and their landscapes, from providing habitat for aquatic or amphibian species to contributions to greenhouse-gas emissions, it underscores the importance of lakes in the Earth鈥檚 ecosystems,鈥� notes Mathis Messager, the study鈥檚 first author, who worked on the project as an undergraduate student in Lehner鈥檚 lab.听
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Canada鈥檚 glacial legacy听听听
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Lakes are constantly formed and filled over long time scales through geological and natural environmental processes, so the lake distribution on Earth today represents a snapshot of a steadily changing pattern. The world鈥檚 10 largest lakes contain about 85% of the Earth鈥檚 lake water. The remaining 15% is sprinkled across more than 1.4 million lakes 鈥� most of them in Canada. With nearly 900,000 lakes covering more than 10 hectares, Canada accounts for 62% of the world鈥檚 total -- a legacy of glaciers鈥� scouring action and their subsequent melting at the end of the last glacial period, about 10,000 years ago.
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The 91社区 team is making its new database available for use by researchers around the world. The researchers are also working on new features that could be added, such as data on the surrounding watersheds that feed the lakes.听
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鈥淚t is often argued that we know more about the surface of the moon or Mars than the ocean floor,鈥� Lehner says. 鈥淲hile lakes may be better studied in some ways than the vast ocean, there is certainly a similar lack of understanding of what exactly is going on underneath all those lake surfaces on Earth.鈥�
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This work was supported by funding from the Natural Sciences and Engineering Research Council of Canada (NSERC).
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鈥淓stimating the volume and age of water stored in global lakes using a geo-statistical approach,鈥� Mathis Lo茂c Messager, Bernhard Lehner, G眉nther Grill, Irena Nedeva, Oliver Schmitt.听Nature Communications, published online Dec. 15, 2016.听听
DOI: 10.1038/NCOMMS13603
