Streams in Hawaii
show slow decline

Streams make news when they overflow during a deluge, but a new federal study reveals that the flow in island streams has declined over the past 90 years.

The trend may be bad news about a dwindling drinking water supply, according to the U.S. Geological Survey report on "Trends in Streamflow Characteristics" released this week.

"There has never been an analysis of this type," said hydrologist Delwyn Oki, who evaluated data from the Geological Survey network of stream gauge stations on the five major islands and 70 rain gauges. "It will be of great interest to people in the water resource business."

The study analyzed the "base flow" in streams during periods of no rainfall, when the sustained low level of water in a stream is coming from groundwater sources. It also evaluated information on the total flow in streams, including rainfall runoff.

"The most significant thing is, since 1913 there is a common downward trend," Oki said. "If ground water is reducing over time, that may mean a decline in ground water storage. Because ground water provides more than 95 percent of Hawaii's domestic drinking water, a reduction in groundwater storage has serious implications for long-term drinking water availability."

The trend toward diminishing levels in streams "would have an impact for farmers who rely on the resource," Oki said. It might also reduce habitat available for native fish.

"Flows in Hawaii's streams are related to the El Nino phenomenon," he said, and combined stream and rainfall data is the first long-term tracking of differences when the El Nino weather pattern brings drier winter months.

Rainfall records kept by the defunct sugar industry were a valuable resource for the Geological Survey scientist. "They kept clear records and collected a lot of important information. There are fewer gauges today than there were during the plantation period, and that's unfortunate," Oki said.

The study does not provide conclusions or predictions.

"When we look at the downward trend, it may be part of a normal long-term pattern," he said.

"If you look at the more recent data, over the last 30 or 50 years, the figures are not as pronounced as the earlier trend from 1913. It's safe to say the early part of the century -- 1913 to the early '40s -- was probably wetter."

"There were a few rainfall stations in operation from back to 1893 to 2002 that didn't show the same downward trend."

The lower flow in streams sometimes "could be from human-induced causes," he said. "If they are pumping ground water it could cause reduced groundwater storage. But most of the long-term gauges we have are in undeveloped areas."

"We have been collecting information for close to a century. There hasn't been an analysis of the long-term data until this," Oki said.

The report was prepared in cooperation with the state Commission on Water Resource Management and the Maui County Department of Water Supply. It can be found at hi.water.usgs.gov/.