Water is relatively cheap when it comes to food and housing, which is why it is sometimes ignored in discussions about public assistance for necessities.

But the cost of water is rising, and researchers at Michigan State University suggest water could become unaffordable to 36 percent of U.S. households in the next five years.

"In cities across the United States, water affordability is becoming an increasingly critical issue," Elizabeth Mack, an assistant geography professor, said in a news release.

Since 2010, water rates have increased 41 percent. The pressures of climate change and aging infrastructures are expected to accelerate rate increases in the coming decades. As global warming continues, extreme weather events will become more frequent. Droughts will tax reserves, while bigger storms and flooding will put pressure on water treatment facilities. Fixing systems of aging pipes will cost $1 trillion over the next 25 years, while modernizing wastewater facilities will add another $36 billion to the bill.

Water costs should only account for 4.5 percent of household income. Currently, 13.8 million U.S. households receive unaffordable water bills. If rising rates continue, as expected, Mack estimates the number of households struggling to pay for water will triple.

Mack hopes her work, published in the journal PLOS ONE, will inspire water supplies and governments to develop solutions for the problem of water affordability.

"Water is a fundamental right for all humans. However, a growing number of people in the United States and globally face daily barriers to accessing clean, affordable water," Mack said. "The hope is that enhanced awareness of this issue in the developed world will highlight the severity of the issue, which is not isolated to people in the developing world."