Humanity is a geological force. Through its activities it is able to modify natural balances so vast that it is easy to deny the very possibility that such an influence exists. However, a team of researchers from Stanford University has just shown in a study published in Nature that human influence is pervasive and on a global scale. This study, the largest to date, shows a significant difference between the annual variability of the level of natural lakes and those managed by man.
Measuring the depth of a single lake is not easy. So, how to proceed to measure the depth of all the lakes in the world at the same time ? To measure the water depth of thousands of lakes and reservoirs, Stanford University researchers relied on the ICESat-2 satellite of the NASA, launched in 2018. This satellite is equipped with a laser altimeter, an instrument that allows it to accurately assess its distance from the ground, and thus to measure the changes in altitude of glaciers, ice floes or stretches of water. In this way it was possible to observe the seasonal variations of 227,386 natural or artificial lakes and basins for almost two years.
- Artist’s impression of the ICESat-2 satellite.
For natural lakes the seasonal variation is on average 0.22 meters, while in artificial lakes it is almost 0.86 meters, almost four times greater. This discrepancy is explained by the human use of water for hydroelectric power generation or by the irrigation of crops that mobilize large amounts of water.
All the reservoirs combined, the scientists estimated the seasonal variability at a volume of 3,496 km³, nearly forty times that of Lake Geneva. ! Of all these estimates, 57 The% of the variability counted directly depends on the artificial basins, which however represent only 3.9 % of the 227,386 basins considered. In some areas such as California, the seasonal variability of water stocks is up to 100 % humans. These data show that most of the global seasonal variability depends only on a minority of artificial lakes, with important consequences for the ecosystems downstream of these basins.
Artificialization of the water cycle
“ Due to anthropization, the storage of water, in lakes and soils, is today mainly of human origin. “, explains a Reporter Agnès Ducharne, researcher at the Metis laboratory (Environmental media, transfers and interactions in hydrosystems and soils) of the University of Paris.
Florence Habets, researcher CNRS from the geology laboratory of the École norma supérieure indicates: “ The flow of rivers dependent on these artificial lakes will be changed. This poses problems for the ecosystems that depend on these rivers. “
Groundwater pumping also plays a very important role in the management of this resource. “ In some highly irrigated regions, such as northern India, this extraction can also be seen from space thanks to gravimetry “, according to Florence Habets. Mankind’s handling of fresh water is equal an effect on sea level rise, says Agnès Ducharne: “ The creation of artificial lakes has significantly reduced sea level rise. With the pumping of the aquifers, however, the water accumulated for a long time is put back into circulation and then participates in the lifting of the seas. “ Therefore, the anthropization of water reserves on a planetary scale is part of a broader movement of artificialization of the water cycle.
Another parameter alters the cycle: global warming, even of human origin. A complete deregulation of this cycle would constitute the overcoming of one of the planetary limits. For Agnès Ducharne, “ exceeding one of these milestones endangers the entire system. The big debate is whether we have already passed it for the water cycle. “
Source: Fabien Houy-Delalande for Reporter
Satellite: public domain NASA Goddard / Wikimedia Commons
.Fort Peck – Public domain Harry Weddington, We Army Corps of Engineers / Wikimedia Commons