OREGON STATE UNIVERSITY

Good news, Hawaii: you're getting drier (at least, on the map)

12/15/1999

CORVALLIS, Ore. - Researchers at Oregon State University are putting the finishing touches on a new precipitation map for the state of Hawaii and they have come to the conclusion that parts of the islands just aren't as wet as advertised.

But other areas may be wetter than previously suspected.

One of their conclusions was to downgrade Maui's Mt. Waialeale from an average rainfall of 400 inches a year down to an estimated 315 inches a year.

"The change has to do with the averaging period used," said George Taylor, an OSU faculty member and the state climatologist for Oregon. "The official 'normal' period is now 1961 to 1990, and during that time Mt. Waialeale had some relatively dry years. But 315 inches is still a lot of rain."

It is, however, a long way from being the "wettest place on Earth," Taylor pointed out.

Cherrapunji, India, with its 460 inches a year, has been the reigning champ. But the OSU researchers think that a 15,000-foot mountain range in southeast Alaska, which ironically includes Mt. Fairweather, gets inundated by a whopping 500 inches of precipitation a year.

The OSU researchers have a contract with the Natural Resources Conservation Service to produce maps of all 50 states, as well as the U.S. territories. The Oregon Climate Service, based at OSU, has become a national leader in producing precipitation maps through its PRISM technology.

PRISM - Parameter-Elevation Regressions on Independent Slopes Model - is a new way to map precipitation. In addition to data generated from hundreds of rain gauges, PRISM technology looks at the effect of elevation, terrain, rain shadows, temperature inversions and coastal waters.

"The moisture in Hawaii is contained in a relatively shallow layer," Taylor said. "Above 1,500 meters, it begins to dry out. The tops of Mauna Loa and Mauna Kea are much drier than the top of Oregon's Mt. Hood. So in mapping, you have to be careful about inversions. Hawaii has a whopping orographic effect."

Orographics is the effect of the terrain on weather. The effect is illustrated on Oregon State University's PRISM map of Oahu's Koolau Range, where rainfall amounts of more than 300 inches - rivaling those on Kuaui, are now estimated.

It is the use of orographics that distinguishes the OSU-generated maps from previous precipitation models.

Chris Daly, the director of OSU's new Spatial Climate Analysis Service, said most rain gauges are on the wet side of mountains and many older maps generalize from the data those gauges produce. Consequently, they ignore rain shadows, coastal effects and other important factors.

"We've gone to a three-dimensional mapping process that is producing, we feel, some of the most accurate precipitation maps ever created," Daly said.

Hawaii has about 300 rain gauges, and accurate maps are critical for the state, Taylor said.

"Water is a real problem in Hawaii because they don't have many ways to 'catch' it, and it can't be piped in from anywhere," Taylor pointed out. "Knowing how much rain falls, and where, on a detailed basis helps decision-makers with their jobs."

The new maps should be completed early in 2000. For more information on PRISM, visit the Oregon Climate Service web site at: http://www.ocs.orst.edu/prism/prism_new.html.