How much water could be saved by replacing non-efficient toilets with water-efficient ones in residential properties across five states? Enough water to fill 1,000 Rose Bowl Stadiums, or up to 170 billion gallons a year, according to a new research study by the Alliance for Water Efficiency (AWE) and Plumbing Manufacturers International (PMI).

The research looked at Arizona, California, Colorado, Georgia, and Texas, all of which have experienced severe water shortages and together made up 28 percent of the population in 2015.

Although California Governor Jerry Brown lifted the drought emergency in most of the state earlier this month, he reminded residents that the next one could be around the corner and that conservation must remain a constant. And California’s not alone in its concerns: According to a 2014  Government Accountability Office study, 40 of 50 state water managers expected shortages in the next 10 years.

Toilet flushing is the largest single indoor use of water, representing 24 percent of total use in single-family homes. Replacing non-efficient toilets with efficient ones is an important strategy to stretch available water supplies. In the five states examined in this new study, 13 mllion toilets (about 21 percent) were found to be non-efficient.

According to the study,  if these millions of toilets are replaced with water-efficient ones, then up to 170 billion gallons of water could be saved in these five states annually. That savings can be extrapolated to an estimate of up to 360 billion gallons of water per year nationally. However, at the current 4 percent annual replacement rate, the study says this potential savings will not happen 15 to 30 years unless toilet replacement programs are accelerated.

“This study affirms the important and sometimes overlooked role that water-efficient plumbing products—and programs such as the EPA WaterSense label—play in assuring water sustainability for our nation,” said Mary Ann Dickinson, AWE president and CEO, in a press release announcing the study’s findings. “We are nowhere near the potential of water savings we can achieve through water efficiency.”

According to the release, the results “also merit consideration within the current federal and state regulatory environment.” For example, proposed cost cutting at the Environmental Protection Agency threatens the WaterSense program, a voluntary public-private partnership initiative to encourage the use of water-efficient toilets, showerheads, faucets, and other plumbing products.

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