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WRRCPR No.94-01 Evaporation Along A Transect Across Southern Oahu, Hawaii

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Title: WRRCPR No.94-01 Evaporation Along A Transect Across Southern Oahu, Hawaii
Authors: Ekern, Paul C.
Keywords: evapotranspiration
pan evaporation
water balance
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southern Oahu

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LC Subject Headings: Evaporation (Meteorology) -- Hawaii -- Oahu.
Issue Date: Aug 1993
Publisher: Water Resources Research Center, University of Hawaii at Manoa
Citation: Ekern PC. 1993. Evaporation along a transect across southern Oahu, Hawaii. Honolulu (HI): Water Resources Research Center, University of Hawaii at Manoa. WRRC project report, 94-01.
Series/Report no.: WRRC Project Reports
Abstract: Near O'ahu the annual oceanic evaporation is 65 in. (1651 mm) and the rainfall is 18 in.
(457.2 mm), giving a 47-in. (1 193.8-mm) deficit. In 1981, evaporation over central O'ahu was 27 in. (685.8 mm) and rainfall was 197 in. (5 003.8 mm), for a 170-in. (4 318-mm) surplus. The pattern of evaporation over an island differs from that over the open ocean, just as the pattern of rainfall and cloudiness is changed by island topography. Evaporation measurements that were begun in 1980 at five leeward sites were expanded to include measurements at four windward sites in southern O'ahu from October 1985 through September 1987. Specially designed small rain-shielded evaporimeters were calibrated against
class A pan evaporation. Such pertinent factors as rainfall, sunlight, wind, and air temperature were recorded for correlation with evaporation. Although monthly class A pan evaporation was 0.53 the evaporimeter measurement for the entire 1980-1987 period, a 0.55 ratio was finally adopted as a more reliable transformation value. Correlation coefficients of determination (r^2) values between monthly evaporimeter and pan evaporation readings ranged from 0.77 to 1.0, between pan evaporation and sunlight
readings from 0.79 to 0.87, and between evaporimeter and sunlight readings from 0.66 to 0.93. A fivefold increase in pan level wind across the transect only increased the sunlight energy used as latent heat by 50%.
Evaporation relative to the Manoa Mauka campus calibration site ranged from 0.79 on the windward coast, to 0.56 at the windward base of the Ko'olau Range, to 0.65 at the leeward base, to 1.2 at the extreme leeward site. Evaporation equivalent to 0.91 the windward net radiation but 1.23 the leeward value suggests negative advection onshore from the ocean and strong positive advection from the dry leeward surroundings. The annual water balance for October 1985 through September 1987 was a deficit of 20.4 to 32.4 in. (518.2 to 823.0 mm) at the windward coast, a surplus of 37.9 in. (962.7 mm) at the windward Ko'olau base, a surplus of 100 in. (2 540 mm) at the leeward Ko'olau base, and a deficit of 55.9 in. (1419.9 mm) at the leeward valley floor.
Sponsor: Board of Water Supply City and County of Honolulu Grant/Contract No. C12355
Pages/Duration: x + 23 pages
Appears in Collections:WRRC Project Reports

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