The Colorado River is among the most important ones of the United States, rising in the Rocky Mountains of Colorado and then flowing westward to the Gulf of California. But that flow is now gradually being reduced because of climate change, according to a new study.

The river extends for 2,330 km and provides water to about 40 million people that live in the cities of Denver, Las Vegas, and Los Angeles, among others. That makes it very important for the communities around it, whose water availability could be altered if the river flow keeps changing.

Researchers from the Institute of Geological Studies of the United States (USGS) developed a detailed mathematical model of water movements (including snow, rain, evaporation and river flow, among others) in the upper river basin between 1913 and 2017, based on records of rainfall and temperature as well as satellite surveys.

They observed that the increase in temperature has caused a smaller accumulation of snow and ice in successive winters. Less snow means that melting occurs earlier in the spring. In turn, a surface with less snow reflects fewer sun rays, which means that more rays end up being absorbed by the basin.

This additional energy absorption causes greater water evaporation, said Paul Milly, one of the scientists at USGS, which results in the lower flow. Thanks to the study, evaporation due to heating could be quantified: the annual flow rate is reduced by 9.3% for each rise of one degree Celsius.

The difference is significant. For the 1913-2017 period analyzed in the study, the flow of the river decreased by about 20%, according to the researchers “More than half of this decrease was associated with warming. The rest was related to variations in precipitation,” Milly said in a press release.

Global warming has already exceeded the 1ºC compared to pre-industrial levels and the trend will continue going up without ambitious actions from every country. That’s bad news for the Colorado River, as there could be a higher risk of water shortages for the seven states connected by the river.

But climate change isn’t the only threat that the river has to cope with. The agricultural production in the area means lots of water is pumped from the river. Farmers use about 80% of Colorado River water to irrigate four million acres, providing 15% of the US crop output. This has led to reservoirs as the Lake Mead facing record-low levels.

Last year, the US government imposed mandatory cuts in water use from the river because of a record drought. Arizona, Nevada, and Mexico agreed to take less water from the river and implemented conservation measures such as replacing lawns with desert landscaping.

The study was published in Science.