From the University of Nevada Reno (Steph McAfee):
Drought on the Colorado River has been in the news this year. In August, the US Bureau of Reclamation’s water-level forecasts for January 2022 indicated that the water levels in Lake Mead, one of the system’s major reservoirs, would stay below 1,075 feet elevation. Because of how low water levels have fallen, a Tier 1 shortage was declared. This means that southern Nevada, which gets about 90% of its water from the Colorado, will have to make do with 7% less water.
But steadily dropping river flows and reservoir levels are not exactly a surprise. Scientists have long warned that higher temperatures, especially if they coincide with a drought, could stress water supply in the Colorado Basin and force us to be more thoughtful and creative in how we use and manage water. This year has underscored that message.
The Colorado River basin stretches from southwestern Wyoming to the Sea of Cortez in Mexico. Most of the water – about 90% — that feeds the Colorado River falls as rain and snow in the upper, or northern, part of the basin. The Upper Colorado River Basin drains the west side of the Rocky Mountains in Wyoming, Colorado and Utah, collecting a bit of water from northwest New Mexico. The Lower Colorado River Basin, which covers most of Arizona and extends just into New Mexico, Nevada, and California, contributes less water. The Colorado River Delta, where the river flows into the sea, is in northern Mexico.
In the 1980s and 1990s, the Lake Mead reservoir on the Colorado River was close to full, though water levels were sometimes a little lower and sometimes a little higher. Since then, less and less water has flowed out of the Rocky Mountains into the massive reservoirs that buffer the Southwest and Mexico against droughts, water the farm fields of southern California, and generate cheap electricity.
In 2008, Tim Barnett and David Pierce, researchers at the Scripps Institute of Oceanography, caused something of a furor by publishing a scientific paper titled, “When will Lake Mead go dry?” They concluded that there were even odds that Lake Mead would be essentially empty, with so little water that dam operators couldn’t send water downstream, in 2021. This finding was, as they put it, “startling” in how dire it was, but it was hardly the first time that scientists had discussed how climate impacts water supply and drought in the Colorado. In the decades before 2008 and since then, many scientists (myself included) have worked diligently to better understand how changes in climate affect water resources in the Southwest.
Not surprisingly, flow in the Colorado River drops when the mountains that feed it get less rain and snow. This kind of drought is not unusual. Historically, drought has impacted flow along the Colorado River during the 1930s, 1950s, 1960s, 1970s, in the late 1980s to early 1990s and again over the last 20 years. Tree-ring researchers have also identified long and sometimes severe droughts on the Colorado in the centuries before we started tracking flow with stream gages.
Currently, flow in the Colorado is very low – much lower than would be expected given that the Upper Colorado Basin has only been somewhat short of rain and snow. What’s different is that temperatures have been consistently high.
There are a number of ways that temperature can alter the amount of water that flows down the Colorado River. Warmer temperatures increase the atmosphere’s thirst for water, so it can pull more water from rivers, lakes, soils, and even snow. As temperatures rise, the growing season starts earlier and ends later, lengthening the window of time when plants are active and moving water from the soil through their leaves to the air. Getting more precipitation as rain instead of snow can also alter how much water evaporates, soaks into the soil, or runs off into streams.
It is now obvious that higher temperatures are already starting to stress water supplies, with further impacts on energy production, agriculture, the environment, and even recreation. What’s not currently clear is how much influence temperature has. Scientific studies using a variety of tools have come up with somewhat different answers about how big a role temperature plays. A recent review of the scientific literature led by the Wester Water Assessment found that temperature may have caused anywhere from a fifth to more than half of the recent drop in flow in the Upper Colorado River.
Scientists expect temperatures to rise in the coming decades. There is still some uncertainty about just how big a sip those higher temperatures will take out of the Colorado, but as it warms, the overall amount of water available will drop and droughts will be worse than they would otherwise have been. To meet Nevada’s current and new water demands, we will need innovation in water policy and management, new technology and conservation strategies, and the research to support those changes.