Raleigh, N.C. — As city officials grapple with trying to make it through a second summer marked by drought, they also face the prospect of demand outstripping the available supply of drinking water for years to come.

A long-term plan filed with the state Division of Water Resources shows that Raleigh officials expect water demand to equal supply by 2040 and to exceed supply by 21 percent a decade later.

"Our long-term concern, really, is about our long-term water supply," said Karen Rindge, chairwoman of WakeUP Wake County, which advocates managed growth. "The big issue is, 20 years from now, 30 years from now, 50 years from now, will we have enough water supply to meet our growing population?"

In the not-so-distant future, Raleigh officials expect by 2020 to top a threshold set by state regulators that calls for curbing water demand, according to the long-term plan, which is updated every five years.

Raleigh could be pumping almost 72 million gallons a day to the 570,000 people on the municipal water system in 12 years, which is about 88 percent of the daily supply the system should have available by then, plan details show. But the Division of Water Resources encourages water systems to remain below 80 percent of capacity.

"We have a planning process when a system gets to 80 percent. Once they get over (that level), we create a plan that will make them less than 80 percent," said Linwood Peele, director of the Water Supply Planning Section of the Division of Water Resources.

Raleigh officials expect to open a water treatment plant at Lake Benson in two years, which will add at least 15 million gallons a day to the city's drinking water supply. A new reservoir in eastern Wake County could add another 20 million gallons a day by 2025.

Raleigh Public Utilitites Director Dale Crisp said the city also has studied drawing water from Kerr Lake and is planning a reservoir on Middle Creek in Johnston County to boost water supply further.

But even with those additions, the city's long-term plan calls for tiered water rates to encourage conservation, targeting large consumers for increased efficiency and using treated wastewater for outdoor uses in case demand exceeds 80 percent of supply.

"I think that raises a red flag," Rindge said. "Down the line, if we don't make changes in how we use water and how we conserve water, we may not have enough water for our growing population."