A spot on the edge of Haleakala National Park overlooking Hana at about 5,400 feet elevation may be the wettest place in Hawaii and among the spots with the most rainfall in the world, said a University of Hawaii climatologist.
"Big Bog" had an estimated mean annual rainfall of 404.4 inches over the 30-year period of 1978 to 2007, surpassing the oft proclaimed wettest spot in Hawaii, Kauai's Mount Waialeale, according to Tom Giambelluca, who led a team of scientists that put together "Rainfall Atlas of Hawai'i."
Mount Waialeale logged 393.3 inches over the 30-year period in the study.
This is a rain gauge on Haleakala, part of a network of meteorological stations in the national park. One similar to this one is located nearby at “Big Bog,” which may be the wettest spot in Hawaii with an estimated mean annual rainfall of 404.4 inches over a 30-year period.
JOHN DELAY photo via “Rainfall Atlas of Hawai‘i”
And although Mount Waialeale has been mentioned by tour guides on Kauai as the "wettest spot in the world" that is also apparently incorrect. Giambelluca said that honor goes to either Mawsynram or Cherrapunji, two nearby towns in northeast India with more than 460 inches a year.
Despite the data his team collected, the professor in the UH Geology Department stopped short of declaring Big Bog the wettest place in Hawaii.
"I would not be comfortable saying that Big Bog is wetter (than Mount Waialeale)," he said. "Both are wet."
The wettest spot in the state may depend on how one looks at the data. Scientists in the Atlas looked at a 30-year mean; with such a small rainfall difference between the two places, positions could be switched using different time frames.
In addition, Mount Waialeale has a more complete record with U.S. Geological Survey data going back to 1950; the rain gauge at Big Bog went up in 1992, meaning that researchers had to make statistical adjustments for the study, he said.
And there is one more caveat: Big Bog and Mount Waialeale may be among the wettest places in the world - with a rain gauge, Giambelluca said. "There could be other places that are wetter" - without a gauge, he added.
Big Bog was not on the map as one of the wettest spots in the world until the climate station was put up in 1992. Scientists had estimated about 180 inches annually for the spot when installing the weather instruments.
That first year, 1993, the rain gauge at Big Bog collected 551 inches of rain, "one of the highest annual totals measured anywhere," he said.
To understand why Big Bog and Mount Waialeale are so wet, Giambelluca offered a lesson in orographic rainfall systems at work in the islands. He said that atmospheric circulation around the islands is relatively stable, meaning that the area around the Hawaiian Islands gets a relatively small amount of rainfall, less than 30 inches a year over the open ocean.
When the northwest trades hit the mountains, orographic lifting occurs. Air is forced to rise and cool water vapor condenses into clouds, which produces "tremendous amounts of rainfall" at the higher elevations of windward sides, Giambelluca explained.
Big Bog and Mount Waialeale are in areas with the "most consistent" and "maximum" orographic uplift, he said. In addition, both places are situated to receive precipitation from Kona or southerly storms as well.
There is another system in play on Haleakala and Big Bog. While the orographic uplift reaches the 5,150 foot summit of Mount Waialeale, the rain clouds on Haleakala cannot break the trade wind inversion layer at about 7,200 feet; the layer acts as a barrier to rising air.
This phenomena leads to shallow but persistent cloud generation. "We get rain almost everyday," Giambelluca said.
In a 1991 paper he co-authored on "Influence of the trade-wind inversion on the climate of a leeward mountain slope in Hawaii," Giambelluca described four atmospheric layers: A marine zone of moist well-mixed air in contact with oceanic moisture; a fog zone, where clouds are formed; a transitional zone or trade wind inversion layer with a highly variable climate; and an arid zone, usually above the inversion where air is extremely dry due to isolation from the oceanic moisture.
The Big Bog rain gauge, which is in a very remote location of the forest, is part of a network of stations on Haleakala that measures rainfall, temperature, solar radiation and wind direction and speed. The stations were installed to gather data for land management and protection of the biological ecosystem, not necessarily to find the wettest spot in the world, he said.
Scientists are using the meteorological instruments to monitor the inversion layer, in which the air is actually warmer than the cloud layer below. The barrier exists about 80 percent of the time on Haleakala, Giambelluca said. It is not present during major storms, the kind with thunder, lightning and lots of rain.
When there is no inversion, there could be more rainfall and at higher elevations, he said.
That's where global warming could come into play. The hypothesis is that global warming could push storm tracks away from the Hawaiian Islands and reduce precipitation. The major storms also disrupt the trade wind inversion; without those disruptions there could be less rain on Haleakala because the inversion layer will be present more often, he explained.
It is believed that the elevation of the forest line below the inversion level is controlled by the amounts of rainfall, he said. With the inversion present for longer stretches of time with global warming, rainfall will be reduced and the forest line will move lower on the mountain, he explained.
The elevation of the inversion layer and the forest line below is critical to the survival of native species, including the endangered and endemic apapane, iiwi, Maui creeper, Maui parrotbill and crested honeycreeper. They live in a safe zone in the cloud layer where temperatures are inhospitable to mosquitoes carrying avian malaria and infected non-native birds, he said.
That zone could be shrinking from above and below. There is a fear that climate change is "going to squeeze the birds into a smaller and smaller" safe zone, said Giambelluca.
While nonendemic birds such as robins have evolved a resistance to malaria, native birds have not, said state Forestry and Wildlife Division wildlife biologist Fern Duvall.
"Iiwi will die with a single (infected) mosquito bite within a week," said Duvall, adding that the lack of resistance to malaria is why there are no native birds in the lower elevations.
Changes affecting the inversion layer are "a very large fear" for bird management officials, Duvall said.
"Within 50 years, we might see a shift," he said, noting that the safe layer begins at about 3,600 feet.
To help save the birds and other endemic species, wildlife officials are looking at ways of killing the vector and finding ways to thwart the transmission of the disease, Duvall said.
Expanding the habitat of the native birds through reforestation efforts, especially on the leeward side from Polipoli to Kaupo, may create expanded "mosquito-free, malaria-free zones" as well as improving the watershed, he said.
The "Rainfall Atlas of Hawaii" was funded by the state Commission on Water Resource Management and the U.S. Army Corps of Engineers in Hawaii.
* Rainfall Atlas of Hawaii:
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