The huge Colorado River, which supplies water to seven American states, has seen its flow reduced by 20% in a century, and more than half of the decline is due to global warming, according to a study published in the journal this Thursday. Science.
Two researchers from the American Geological Research Bureau (USGS) finely modeled the movement of water (snow, rain, evaporation, river flow, etc.) throughout the upper river basin from 1913 to 2017, using readings from precipitation and temperatures, as well as satellite observations, to understand the effect of global warming on these movements.
Less snow
They observe that the warming has led to a reduction in the accumulation of snow and ice, from one winter to the next. Less snow means it melts a little earlier in the spring each year. A less snowy surface reflects less of the Sun’s rays, and therefore more rays are absorbed by the pool rather than being returned to space.
This additional absorbed energy causes more water to evaporate, Paul Milly, a scientist at the USGS, told AFP. Hence a reduced flow.
The evaporation due to warming is now quantified, thanks to this study: the annual flow has decreased by 9.3% per degree Celsius of warming.
Overexploitation of the river
The other factor reducing river flow is related to variations in precipitation.
Colorado is a 2,330 km long river, punctuated by numerous dams, overexploited and pumped to supply water to 40 million Americans, including the cities of Denver, Las Vegas and Los Angeles. When it reaches the end of the race in Mexico, in the Gulf of California, it is almost dried up.
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