It's taking a lot less water to quench O'ahu's thirst.

Since peaking in 2001, islandwide demand for drinking water has fallen about 7 percent to 136 million gallons a day, according to the Honolulu Board of Water Supply.

Last year, Honolulu residents saved enough water to fill about 4,000 Olympic-sized swimming pools when compared with 2001 consumption. Meanwhile, the number of homes and businesses on O'ahu connected to the water supply grew by more than 15,000, or nearly 10 percent.

But the lower consumption hasn't translated into lower water bills for most households. The Board of Water Supply has raised rates to compensate for the lower water sales and the increased cost of maintaining and expanding the system.

Officials aren't certain why consumers have turned down the spigot. Probable factors include the proliferation of water-saving devices such as low-flow toilets and showers, an ongoing decline in the state's agricultural sector and the higher prices.

Higher demand for bottled drinking water also was cited as a possible source.

Whatever the reason, there's widespread agreement that the less water wasted, the better.

"That's a good thing that we're using less water," said Randy Ching, former chairman of the Sierra Club O'ahu Group. "Actually, we should use a lot less. There are so many water-saving appliances and fixtures. Really, we don't have any excuse for wasting water."

Board of Water Supply officials cite public education and increased use of recycled and brackish water as other possible reasons for declining demand in urban Honolulu.

The use of low-flow toilets, which the city mandated in the 1990s, reduces the water consumption of a family of four by an estimated 22,000 gallons a year. On O'ahu, the average household uses about 156,000 gallons a year.

Additionally, nine golf courses and Hawaiian Electric's Kahe power plant are now users of recycled water.

LESS AGRICULTURAL DEMAND

The Board of Water Supply said only about 3 percent of its drinking water is used in agriculture. Still, some of the decline in water demand could be linked to the decline in Hawai'i's agriculture sector. In 2007 there were 58,630 acres of irrigated farmland statewide. That was 15 percent fewer acres than in 2002, according to the U.S. Department of Agriculture.

The elimination of Honolulu's sugarcane industry in the 1990s along with modern drip irrigation techniques also have resulted in less demand for water, said Dean Okimoto, president of the Hawaii Farm Bureau Federation.

"I would say it has to be playing some role because there's less agriculture," he said. "When sugar went out — that was major."

That decline in acreage probably has been partially offset by lower-than-normal rain in some areas, which puts pressure on some growers to use potable water on crops. According to the USDA's Drought Monitor, Honolulu is not experiencing drought conditions. However, the island has experienced abnormally low rain levels for at least a year.

Okimoto had another theory.

"I would venture a guess that most people are buying bottled water and don't drink tap water," he said. "I would bet that's the major reason."

Generally, demand for drinkable water on O'ahu has been flat for nearly 20 years, said Barry Usagawa, water resources program administrator for the Board of Water Supply. Annual demand fluctuates based on economic and weather conditions and in response to rate hikes.

Overall, reduced water use in Honolulu's dense, urban areas has helped offset growing demand for water from new developments in the 'Ewa plain, Usagawa said.

Lower per-capita demand for water, ironically, is a factor behind rising water rates. Reduced demand puts upward pressure on prices.

"Our infrastructure has increased, expanded with all the subdivisions they're building out in 'Ewa," Usagawa said. "There are rising operating and maintenance costs, but we're still selling the same amount of water.

"So it's a double-edged sword."

Still, "in comparison to what we pay for other commodities or other things that we use, water is so cheap," said the Sierra Club's Ching. "You can't really overemphasize just how inexpensive water is."

GUARANTEED — HIGHER RATES

One thing that's certain — water is going to get more expensive.

In 2006, the Board of Water Supply implemented a 58 percent rate increase spread out over five years. By July 2010, a single-family home that uses 13,000 gallons a month will be paying $78.38 per bimonthly billing, or $172 more a year than before the start of the rate increase. The rate hikes followed a decade of no rate hikes.

The city also is only halfway through a four-year rate hike that will increase the bimonthly sewer bill for an average household by 107 percent. The increases, which were pegged to a need to repair aging sewers, translate to an additional $900 per year in sewer charges by July 2010.

The combined hit of higher water and sewer fees also likely plays a role in reduced water use, said Phil Moravcik, a technology transfer specialist at the University of Hawai'i Water Resources Research Center.

Moravcik said it's good to save water, however, there's no shortage of drinking water on O'ahu.

"Certainly it's better not to waste it and it's better to keep it underground where we can get it later if we need it," he said.

"We're not in any immediate danger (of running out of drinking water), but it depends a lot on what happens in terms of climate change. If we go into a severe drought for the next 20 years, then we could be in some real trouble."