FRANKLIN, N.C. — The abundant fresh water supply in the rainy Carolinas is apparently shrinking as demand has increased competition between a growing population and increased industrial pressure.

Researchers at the Coweeta Hydrologic Laboratory have found the mountain streams that are a key source of water for the region are losing flow and have been since the 1970s. More than a quarter of the previous run has been lost to date at the lab’s 200-acre mountainside research basin where measurements have been recorded since 1934.

“Stream flow increased through 1970, then decreased,” said Pete Caldwell, research hydrologist. “Some of it is forest change. More of it is climate change.”

Tests at the lab have shown that with the trend of warming air, trees are taking in more rain and releasing it as vapor before it can make it to the watershed.

“This has implications in drought years for folks who live downstream,” Caldwell said. “In drought years, water managers are going to recommend conservation by customers.”

As the climate warms, it will only get worse, officials predict, with computer modeling suggesting that droughts will become more frequent and more severe. Nearly two decades have been spent largely in drought, or on the cusp of it, in parts of South Carolina. Two-thirds of the state is currently in moderate drought or under the threat of drought, according to the S.C. Climate Office. The state’s Drought Response Committee is scheduled to meet next Wednesday to assess that status.

More than 960 billion of gallons are withdrawn in the state annually, according to recent S.C. Department of Health and Environmental Control statistics, though the total includes a massive amount pulled for cooling power plants that is rapidly returned to the same river.

Water streaming into South Carolina from the North Carolina mountains eventually flows into Lake Moultrie, the water source for much of the Charleston area. It became a supply-and-demand commodity — like fuel — when the severity of the drought at the turn of the century spurred a “water wars” dispute with North Carolina, becoming the sort of legal fight more associated with arid Western states.

South Carolina had to fight the case against a legal precedence of rulings that favor the upstream river user and against a state, North Carolina, that showed up in court with data from its own river basin computer modeling. The case was settled with few concessions to South Carolina.

In the aftermath of that dispute, state agencies working with Clemson University and a private contractor are developing a computer model to judge how much water is in a given river basin at any given point and time, and what might happen to that supply as various amounts of water are taken out. It could provide solid data for everything from drought management to deciding water-use permits.

Analysts expect that, more and more, water conservation measures will be put in place, managers will have to decide priorities among users and costs will rise.

“This is not 20 years down the road. We get to the next drought, we’re going to start having these conversations,” said James Fox, University of North Carolina at Asheville environmental modeling and analysis director, who has worked data from the lab and other research. “We probably have enough water, but not enough to keep supplying all the services we do now.”