The announcement earlier this year that the South African city of Cape Town could run out of water prompted many to ask, “How could this happen?”

Here at Bloomberg Environment, we started asking, “Who’s next?”

Many U.S. cities, particularly in the arid West, have their own unique problems that threaten the future of their water supplies.

Some city leaders are working on or have already developed concrete plans to address these threats. But it is unclear how well these plans will stand up to booming populations, meager funding for water infrastructure, and the still unknown consequences of climate change.

These are five of the U.S. cities that could soon be facing the same kinds of water shortages as Cape Town:

  • Why Santa Barbara WILL be the next Cape Town: This California community of 92,000 people is right on the Pacific Coast, which means its aquifers are at high risk of being contaminated with salty ocean water. To make matters worse, Santa Barbara is surrounded by mountains on three sides and has to pump in water from outside the city. “Santa Barbara is kind of siloed on the central coast,” Kellock Irvin, director of operations with water technology startup Dropcountr, told Bloomberg Environment.
  • Why Santa Barbara WON’T be the next Cape Town: Santa Barbara recently revived a desalinization plant, which had been mothballed since 1992. The plant now supplies the city with 3 million gallons of fresh water daily. Santa Barbara also can gain access to even more water if it can come to a reservoir-sharing agreement with suburban neighbors Montecito, Goleta, and others, which hold more senior water rights.
  • Why San Diego WILL be the next Cape Town: Also situated directly on the coast, San Diego’s lack of groundwater supplies is even more severe than Santa Barbara’s. “They’re relying basically on imported water,” Jay Famiglietti, a water scientist formerly at the University of California, Irvine, who recently moved to the University of Saskatchewan, told Bloomberg Environment. “If you’re just relying on surface water, you’re extremely vulnerable.”
  • Why San Diego WON’T be the next Cape Town: San Diego County has diversified its water sources, with less than half its supply coming from Northern California and the rest coming from the Colorado River Basin and elsewhere. As recently as 30 years ago, almost 95 percent of its water came from Northern California. San Diego also has a desalinization plant and is working on making its groundwater potable.
  • Why Las Vegas WILL be the next Cape Town: The population of what is without question the driest big city in America increased by more than 250,000 people just this decade alone. Demand for water in Las Vegas is expected to nearly double by around the middle of the century. And Nevada has the smallest claim to the waters of the Colorado River of any of the seven states within its watershed. Nevada gets roughly 300,000 acre-feet of water annually compared to the highest-user California, which gets 4.4 million acre-feet, Colorado River Commission of Nevada Executive Director Jayne Harkins told Bloomberg Environment.
  • Why Las Vegas WON’T be the next Cape Town: Almost no city outside the Middle East has done more than Las Vegas to perfect the art of water conservation. Between 2002 and 2014, per capita water use in the city declined by 40 percent. This was thanks in part to the city’s regional water authority, which offered rebates for installing more efficient appliances and replacing grass lawns with native plants. The authority has goals to reduce water use even more during the coming years. Also, in 2015, the Southern Nevada Water Authority completed a more than $800 million tunnel under the giant Lake Mead reservoir. “We went down and under,” Harkins said. “No matter what happens, if others can’t get water, we can.”
  • Why Miami WILL be the next Cape Town: Though the Florida city receives more than four feet of rain a year on average, its aquifers are extremely susceptible to pollution runoff because they’re made of porous limestone, according to Famiglietti. Additionally, this means its primary groundwater source also is susceptible to dangerous increases in salinity due to sea-level rise, which is expected to accelerate during the coming decades. “I think Miami is going to disappear,” he said.
  • Why Miami WON’T be the next Cape Town: The porousness of Miami’s aquifers comes with an upside: The water inside them cycles in and out more quickly than in other cities. Additionally, the city’s large average rainfall means it has more opportunities than Western cities to build new reservoirs.
  • Why Phoenix WILL be the next Cape Town: If water levels at Lake Mead drop below an elevation of 1,075 feet, Arizona will see a mandatory 11.4 percent cut in it the amount of water it receives from the country’s largest reservoir. Federal hydrological models predict that, factoring in climate change, this threshold will almost certainly be met by 2020, if not sooner.
  • Why Phoenix WON’T be the next Cape Town: The growing certainty that Lake Mead will fall below the 1,075-foot threshold is motivating farmers, businesses, mayors, and other stakeholders to come to the negotiating table and strike a water sharing bargain that would have been unthinkable before. Furthermore, if and when there are mandatory water cutbacks, the agriculture industry will likely feel their pain earlier and more acutely than the politically powerful metropolis of Phoenix, which isn’t as reliant on Lake Mead as are other big Southwestern cities.