In a Lake Powell bay off the coast of Kane County floats a small, white rectangle that might be key to understanding the future of the nation’s second-largest reservoir.
That’s because it tells us how much of the dwindling water body is disappearing into thin air.
Since 2018, Chris Pearson, a research scientist with the Desert Research Institute in Nevada, has been studying how fast the massive lake’s water evaporates. His team uses two floating stations to track the rate of evaporation with high frequency sensors that capture a bunch of tiny snapshots showing how much vapor is moving upward and downward.
Then they piece together those snapshots into a puzzle that estimates the lake's flux — the amount of water evaporation off Lake Powell and how fast it’s leaving — over long periods.
Those estimates are essential for the Bureau of Reclamation’s management of the lake's inflow and outflow in the months and years to come.
“If we're not measuring and modeling what's currently going on very well,” Pearson said, “there's no chance we're going to do well into the future.”
The number of days with extreme heat in Utah could double by 2050, according to a climate projection from the National Oceanic and Atmospheric Administration. That warm-up would have far-reaching impacts on the state’s already precarious water supply — from speeding evaporation and threatening drinking water supplies to fueling drought and creating prime settings for wildfires.
So what have Pearson’s floating sensors found so far?
Lake Powell loses roughly 4 feet of water depth each year to evaporation. That might not sound like much, he said, but when you think of those 4 feet stretching across a 186-mile-long lake, it adds up.
“When you start to deal with these drought conditions where water supply is being stretched, 4 feet of water is a lot when it comes to farming or municipal water use,” Pearson said.
If temperatures go up 4 or 5 degrees by the 2050s, he said, that heat could evaporate an additional 5 or 6 inches of water depth per year on top of the usual 4 feet.
Pearson describes it as atmospheric thirst.
“As it gets hotter and drier, [the air] will naturally want to absorb more water from the surface.”
Evaporation on Lake Powell doesn’t happen in a vacuum. If an increase in extreme heat is frying the lake itself, it would also impact the regional water system upstream that flows into Powell.
Snow might evaporate before it melts. The snow that does melt might soak into dry dirt before it gets to a river or flow so quickly that reservoirs can’t take advantage of it. And more water would likely be diverted from the Colorado River system for agriculture before it gets to Powell
That could create a compounding effect where Lake Powell gets less inflow and more evaporation loss at the same time. And as the lake itself gets smaller and hotter, evaporation speeds up even more.
“A hotter water body leads to higher vapor pressure at the surface, which causes more evaporation,” Pearson said. “Hands down.”
Another concern is the hydropower that Lake Powell’s Glen Canyon Dam generates for 5 million people across the Southwest. As the lake hit record low levels earlier this year, it came dangerously close to hitting the minimum level needed to flow through the dam’s hydroelectric turbines, and increased evaporative loss in the future won’t help.
The loss of that dam’s power source would risk millions of Americans’ abilities to cool off in air conditioning during a time when extreme heat will already be threatening people’s health.
Utah Division of Water Resources director Candice Hasenyager said these are just some of the potential impacts that a hotter future would bring to Utah.
“Already we live in one of the driest states in the nation, and about 95% of our water supply comes from our snowpack,” Hasenyager said. “So there are a lot of concerns associated with increasing temperatures.”
A hotter Utah would also see increased demand for what little water it has. If the growing season becomes hotter, it also becomes longer, which means the sprinklers at farms and residential lawns would be pushed to run for more months of the year.
“So [there’s] a lot of concern on not only the supply side, but the demand side as well,” Hasenyager said. “It is a very connected system.”
While many parts of the Colorado River basin focus on conserving more of the water they use to irrigate their front yards, the Navajo Nation’s priority is getting tribal members water into their homes — a critical necessity to protect people’s health as extreme heat becomes more common.
Almost a third of Navajo people still don’t have clean, running water, and many have to travel long distances to pick some up. Crystal Tulley-Cordova is the principal hydrologist for the Navajo Nation Department of Water Resources.
“If you live in a home with no running water and you don't have a vehicle,” Tulley-Cordova said, “how do you get the water?”
The Navajo Nation has historically relied on groundwater for its taps, she said, but wants to diversify its sources to gain more water security for a hotter future. One example of that is the Navajo-Gallup Water Supply Project, which is being constructed to pipe and treat water from the San Juan River for tribal communities in New Mexico.
Tribal leaders reached an agreement with the federal government last year to potentially fund similar water projects in Utah, but Tulley-Cordova said the settlement’s funding is still unresolved as it makes its way through the legal process.
Even though Indigenous peoples have lived in the Colorado River basin since time immemorial, tribes have long been excluded from the agreements that dole out water rights on the river and have been striving for decades to get the access they deserve.
When it comes to securing the long-term future of water for the Navajo people, Tulley-Cordova said, any plans need to account for climate change. An agreement may give tribes the right to a certain amount of water on paper, but the actual amount of water that makes it downstream in a hotter, drier world could be far less.
“Even if we get the paper water, what is naturally provided by Mother Nature doesn't equate to what the paper water may say, especially with climate change impacts,” she said.
The best bet for preserving the future of water in southern Utah is to find ways to make sure more water actually arrives at reservoirs like Lake Powell by reducing the amount that farms and cities use upstream.
Recent legislative actions in Utah aim to make the watering methods farmers use more efficient — more than three-quarters of all the state’s water use goes to agriculture — and put meters on secondary water uses, such as irrigation and sprinkler systems. Just telling people how much water they’re using, Hasenyager said, could potentially help Utahns cut their use by 20%.
“It is daunting,” Hasenyager said. “And it may be different than how we've lived in the past, but I truly believe that if we all come together and work together, we'll get through it.”
