Despite best hopes, Southwest drought may be here to stay

If history is any indication, the Southwestern United States may one day see
the level of heavy precipitation it last experienced nearly 30 years
ago. But while some experts debate when that could happen, others
describe the last three decades as a slow and permanent shift to a new
normal of hotter and drier conditions. 

Scientists disagree as to
when the drought officially began, but most point to between 1994 and
1999. The Southwest has seen at-or-above-average precipitation in just
10 of the last 31 years, setting the clock back to 1993, when torrential
rain storms caused widespread power outages and infrastructure damage
in Arizona. In the following years, the region has remained dry, driven
by a warming Earth that reduces storm frequency and evaporates water
before it can recharge subsurface aquifers or runoff into the Colorado
River.

That river has been the bloodstream of the semi-arid
region for millennia, but is now depleting faster than the environment
and its inhabitants can adapt. 

Several consecutive years of heavy
rainfall can tip the average in favor of non-drought conditions,
according to Arizona State Climatologist Erin Saffell, who believes the
drought will end eventually.

But others say there’s no coming out of this one.

“I
don’t think we should be looking for any break in the severity of what
we’re experiencing,” Jay Famiglietti, a professor and climate researcher
at Arizona State University, told Courthouse News. “It’s only gonna
continue and it’s only gonna get worse.”

The arid and semi-arid
climate of Arizona, New Mexico and the southern portions of Utah,
Colorado, Nevada and California regularly fluctuate between short-term
drought and heavy precipitation. In the 20th century, the region saw
three significant droughts — the longest from 1942 to 1964.

Unlike droughts of the past, Famiglietti says the current conditions are “completely driven by climate change.” 

But
National Weather Service meteorologist Mark O’Malley said there isn’t a
direct cause-and-effect relationship between climate change and annual
rainfall.

There is, however, a clear relationship between warming
temperatures and a lack of moisture. Extreme heat has increased
evaporation, decreased soil moisture, and caused plants to increase
transpiration — the process of releasing water vapor through leaves.

Evapotranspiration
levels depend on the “potential evaporation” of the atmosphere,
Famiglietti explained. The hotter the air, the more water vapor it can
hold and the more moisture it can evaporate. With less moisture in the
soil, the energy from the sun that would have gone toward evaporation
instead heats the ground, which in turn heats the air, creating a
positive feedback loop.

At least half the loss in Colorado River water is because of
increased evapotranspiration, rather than a lack of precipitation,
according to a U.S. Geological Survey study conducted in 2020. Researchers came to the same conclusion
in November, finding that 61% of drought in the Colorado River Basin —
which covers parts of Wyoming, Colorado, Utah, New Mexico, Nevada,
California and Arizona — is heat-driven. 

That’s why Nolie
Templeton, a hydrologist and water planning analyst at the Central
Arizona Project, considers what most are calling a drought to be the
permanent aridification of the Southwest.

“We
may be seeing some hydrological impacts that are here to stay, rather
than it’ll be fixed by a couple of good snowpack seasons,” she said.
“That’s largely due to the increased temperatures that we’ve observed.”

The
Central Arizona Project is a system of canals that carry Colorado River
water through central and southern Arizona, bringing water to nearly
six million people and one million acres of farmland. 

The
Colorado River is fed almost exclusively by Rocky Mountain snowpack that
accumulates over the winter months and melts in the spring. Warmer
temperatures reduce snowfall, in turn reducing runoff into the Colorado
River and underground aquifers. Less snow on the groundmeans less reflected sunlight, and additional sunlight means more evaporation and decreased soil moisture.

Typically, Rocky Mountain snowpack melts slowly throughout spring and even into summer.

“Snow on the mountains is basically another form of a reservoir,”
Famiglietti said. “You’re literally putting water in the freezer.”

When
warming temperatures turn that precipitation from snow to rain, it runs
off much faster, causing intense floods and leaving the reservoirs
without a steady inflow. 

Researchers agree that winter precipitation is far more influential for hydrology than summer monsoon storms.

But
when a dry monsoon leaves topsoil dry heading into the winter, the
snowmelt operates at a deficit, as it soaks into the topsoil first,
resulting in less runoff into streams and underground aquifers.

In
recent winters, the region has received 100% or more of the average
expected snowfall, “yet we’re only experiencing 80% of historical
runoff,” O’Malley said.

However, in the months leading up to the
monsoon, the region needs to be hot and dry enough to draw moisture from
the Gulf of California, according to Northeastern University
postdoctoral researcher Somnath Mondal.

“It forces the moisture to
come inland and cause precipitation,” Mondal said. “But after
precipitation happens, the rule of the soil moisture is again
different.”

While increasing temperatures in the Upper Colorado Basin reduce
hydrology for much of the region, it may not be as much of a problem for
central Arizona, which is fed by the watersheds of the Salt and Verde rivers via a system of canals known as the Salt River Project.

Bo
Svoma, a climate scientist and meteorologist with the Salt River
Project, said soil moisture makes little to no impact on winter
precipitation runoff in the watersheds his team manages. 

“Our watershed’s so flashy, the storms are so wet and so big, you can easily wipe out the summer impacts,” Svoma said. 

Those types of intense winter rain events don’t happen as frequently further north, Svoma said.

From Svoma’s perspective, heat is an “overblown” contributor to a
system in which precipitation is key. But global climate change could
still affect precipitation in a major way. 

Climate scientists
agree that warming temperatures could push the winter storm track
further north and away from the Southwest. A warmer globe also means
more water vapor in the air, which would increase the intensity of the
storms that do hit the region. 

Svoma said he isn’t sure what will
play a larger role in the future climate, but one thing is certain:
“Our wet years will get wetter. Our dry years will get dryer.” 

Precipitation over the region heavily relies on a weather pattern over the Pacific Ocean known as the El Niño–Southern Oscillation.
In an El Niño event, warming ocean surface temperatures weaken trade
winds and lend to cooler, wetter winters across the Southwest. By
contrast, the ocean surface cools during a La Niña, causing dry, warm
winters. 

Though Saffel and O’Malley predict a dry winter driven
by a weak La Niña event on its way, it’s important to remember that
climate models aren’t forecasts, but rather conclusions drawn from
statistical relationships, Saffel added. 

Despite the uncertainty,
Svoma agrees with Saffell that the drought could end soon. Five of the
last eight winters in Arizona have been wetter than average, and the
state has seen wetter Marches in the past few years than in the two
decades prior.

“Recently, it hasn’t felt very droughty,” Svomasaid. 

But others remain skeptical that pre-drought conditions will ever return.

“I
don’t think there’s a happy note to really end on,” Famiglietti said.
“We’re in the throes of something very long term, and it’s gonna require
modifications to our lifestyle and our relationship with water.