Final Comprehensive Conservation Plan - U.S. Fish and Wildlife ...

Hakalau Forest National Wildlife Refuge
Comprehensive Conservation Plan
climate above the layer to be clearer, drier, and less humid than below the temperature inversion.
Orographic rain does not occur above the layer because air flows around the high elevation
mountains, rather than over the mountains. The temperature inversion is prevalent during the summer
months (Juvik and Juvik 1998).
Prevailing ocean currents surrounding the island also influence weather patterns by moderating the
surrounding surface air temperatures as a result of differential heat absorption and advection of heat.
Ocean currents in the Hawaiian Islands are moderated by the north Pacific anticyclone, a clockwise
gyre that extends from the tropics to the North Pacific (Juvik and Juvik 1998, Lau and Mink 2006).
The east-to-west-flowing North Equatorial Current splits at the Island of Hawai‘i, creating a northern
branch current that is 65 mi wide called the North Hawaiian Ridge Current.
3.1.1 Hakalau Forest Unit Climate
Climatic conditions at the HFU are largely shaped by elevation. Microclimatic shifts have also
occurred at and adjacent to the unit as a result of changing land use patterns. Clearing and grazing of
native vegetation allowed for the creation of large open grasslands. These areas increase wind speed,
reduce moisture, and result in more extreme temperature fluctuations (USFWS 1996b, USFWS
2002a).
Overall, the HFU is generally characterized by moderate temperatures and wet conditions. In lower
elevation areas, daily atmospheric temperatures are higher, with a mean annual temperature of
approximately 65°F. Higher elevation areas have lower temperatures around 53°F. At the upper
portion of the Refuge around 6,440 ft, temperatures have reached a maximum of 75°F. Night
temperatures during the winter can fall to 25°F, causing frost and soil ice (DHHL and USFWS 2003).
Generally, winds at the HFU arrive from the south southeast direction. Wind speed is approximately
5 mph (USFWS 2002a). Hawaiian Electric Company (2004) notes that mean annual wind speeds in
the area are less than 12.3 miles per hour (mph).
The windward HFU receives northeasterly tradewind-dominated rainfall. Rainfall varies along an
elevation gradient, with areas above 5,000 ft receiving less rainfall than lower elevation portions of
the Refuge. In the lower regions, annual rainfall is approximately 300 inches, compared to
210 inches at the upper elevations. Rainfall averages also vary within the upper elevations. The area
near the Hakalau Cabin at 6,100 ft receives about 20 percent less rain than at Pua ‘Ākala located at
6,300 ft elevation. Between 1989-1994, the average total rainfall at Pua ‘Ākala was 124.19 inches,
while Hakalau Cabin received 85.67 inches between 1990-2000 (USFWS 2002a). In general,
increased precipitation occurs between October-March (USFWS, unpubl.).
In addition to elevation and wind patterns, episodic events and vegetation differences also influence
rainfall variations. At HFU, El Niño years caused average rainfall to drop below historical records,
with an average of 55.57 inches each year (USFWS 2002a). Six drought periods (1992-93, 1995,
1998, 2000, and 2008-2010) have been recorded at HFU. Rainfall during these years ranged from
40–68 inches (DHHL and USFWS 2003).
Fog drip is estimated to account for approximately 35 percent of moisture amounts (USFWS 2002a).
Fog and mist are consistently present in the afternoons due to the inversion layer (Scowcroft et al.
2000). On the Hilo side of Hawai‘i, the average relative humidity remains fairly constant, ranging
from 77-81 percent throughout the year (Juvik and Juvik 1998). At HFU, average daily relative
3-2 Chapter 3. Physical Environment