WRRCTR No.156 Measured Evaporation in High Rainfall Areas, Leeward Ko'olau Ranges, O'ahu, Hawai'i

Abstract

An evaporating surface made from a dark, porous carborundum stone, 6-in. (152.4-rnrn) in diameter, wetted by capillary rise from a plastic reservoir and shielded from rainfall by a clear plastic cover was designed for rainy areas. Class A pan evaporation at five different sites was 0.6 the water loss from corresponding evaporimeters. Water use by Californiagrass was 1.1 pan evaporation and 0.66 the water loss from the evaporimeter. Evaporation, rainfall, and pertinent meteorological factors were measured at one ridge and four valley sites on the Manoa transect in southern Oahu and at one valley and eight ridge sites on the Kipapa Ridge transect on the leeward Koolau Range slopes in central Oahu. Water use on the Kipapa transect was that predicted by the Priestley-Taylor relationship, while water use on the Manoa transect exceeded that predicted with positive advection from the urban surroundings and probable subsidence of the air from the Koolau crest. Measured rates exceeded those from empirical relationships based on extrapolation of pan evaporation measurements. Of the near normal 1981 rainfall of 149 in. (3784.6 mm), runoff was 27%, evaporation 28%, and deep percolate 45% for the Kipapa watershed water balance. Both runoff and evaporation were greater and percolate less than that given for the 1958 water balance. Direct interception of cloud water added an amount less than 2% of the rainfall on the watershed. Maximum short-term, deep percolation rates were no greater than 1.6 in. (40.64 mm)/day and runoff was 0.53 rainfall^1.28.