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Photo of Maunalua Bay, Oahu. (Photo by Scot Izuka)

HAWAII VOLCANIC-ROCK AQUIFER STUDY

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ABOUT THE PACIFIC ISLANDS WSC

USGS IN YOUR STATE

USGS Water Science Centers are located in each state.

There is a USGS Water Science Center office in each State. Washington Oregon California Idaho Nevada Montana Wyoming Utah Colorado Arizona New Mexico North Dakota South Dakota Nebraska Kansas Oklahoma Texas Minnesota Iowa Missouri Arkansas Louisiana Wisconsin Illinois Mississippi Michigan Indiana Ohio Kentucky Tennessee Alabama Pennsylvania West Virginia Georgia Florida Caribbean Alaska Hawaii New York Vermont New Hampshire Maine Massachusetts South Carolina North Carolina Rhode Island Virginia Connecticut New Jersey Maryland-Delaware-D.C.

HAWAII VOLCANIC-ROCK AQUIFER STUDY


STUDY AREA - CLIMATE

Photo of upper Koolau Range, Oahu, an example of the wet climate in windward and mountain areas of Hawaii (Photo by Scot Izuka).

Upper Koolau Range, Oahu, an example of the wetter climate in windward and mountain areas of Hawaii (photo by Scot Izuka).

Photo of Honolulu, Oahu, an example of the drier climate in leeward areas of Hawaii (Photo by Scot Izuka).

View of Honolulu, Oahu, an example of the drier climate in leeward areas of Hawaii (photo by Scot Izuka).

Hawaii lies in the trade-wind belt in the tropics of the North Pacific. The climate for most of Hawaii is characterized by mild temperatures, moderate humidity, and prevailing northeasterly trade winds. Rainfall can differ considerably between locations—mean annual rainfall ranges from less than 10 inches on some leeward coasts to hundreds of inches on the tops and windward slopes of mountains. The high contrast between windward and leeward rainfall results from the prevailing northeasterly trade winds and the orographic effect—rainfall is generated when the trade winds encounter mountains on the islands, resulting in uplift and cooling of moisture-laden air. The upper elevations of the tallest mountains are also dry because the trade-wind inversion limits the orographic effect (more information at Rainfall Atlas of Hawaii).

Climate varies seasonally with monthly mean rainfall in most areas highest in January and lowest in June to July. Climate also varies with multiyear ocean/atmosphere climate cycles such as the El-Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Climate data from Hawaii show a long-term-average drying trend over the last century and streamgage data show a concurrent downward trend in stream base flow (groundwater seepage to streams). Climate models vary widely in their predictions of future rainfall in Hawaii, but on average, they indicate that by the end of the 21st century, Hawaii will likely see a 5 to 10 percent decrease in wet-season rainfall and only a modest 5 percent increase in dry-season rainfall.

[References: Oki (2004), Chu and Chen (2005), Kruk and Levinson (2008), Timm and Diaz (2009), Chu and others (2010), Giambelluca and others (2011), Bassiouni and Oki (2012)]



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