Climatic Controls on Evaporation in Hawaiʻi

Abstract

The primary aims of this dissertation are to identify and explain variation in potential evaporation at different elevations and different exposures on tropical high islands, to provide reference data for modeling evaporation, and to suggest how an evaporation model appropriate for tropical high islands might be developed. New evaporation measurements on Haleakala, Maui, existing data from climate stations on Haleakala (Mauinet), pan evaporation data, and rawinsonde data are analyzed to study the effects of radiation, advection, and high elevation on the evaporation rate. The most important contribution of this study is in quantifying the importance of advection in controlling the evaporation rate on tropical high islands. Sensible heat advection from the surrounding ocean moderates the evaporation rate at coastal sites. This effect ranged, on average, from 0.85 mm/day enhancement in November to 0.71 mm/day suppression in June. Heat advection from land sources increases the evaporation rate by as much as 2.8 mm/day in central Maui. Large-scale subsidence over the Hawaiian Islands region accounts for an additional source of advection enhancing evaporation above approximately above 1200 m on the mountain. Preliminary results indicate that this effect increases with elevation in conjunction with the night evaporation rate. Cloud patterns over the mountain slopes and optical air mass determine the solar radiation receipt (the primary source of energy for evaporation) pattern which ranged from 85% to 51% of clear-day radiation in summer and 80% to 63% in winter. Insolation declined with elevation over the study site, except above 1200 meters in winter where it increased with elevation. Results of the study indicate that climatic-average potential evaporation can be modeled using the Priestley-Taylor equation modified by advection approximations. Temperature and vapor pressure on the mountain can be estimated using rawinsonde data. Net radiation can be mapped from global radiation using a clear-day radiation baseline reduced to the open-ocean global radiation value, 80%, and further reduced based on a cloud index derived using a wind flow model. Ocean advection can be accounted for at the lower elevations by simply adding a monthly modifier based on results presented in the study. Land advection can be related to soil moisture, estimated using a water balance. The influence of the evaporation enhancement at high elevations would probably have to be determined empirically, and related to season and elevation.