Evapotranspiration and energy balance of native wet montane cloud forest in Hawai‘i

Abstract

Evapotranspiration (ET) and energy balance were observed, using eddy covariance and other micrometeorological measurements, at a native Metrosideros polymorpha forest site in Hawai‘i. Total rainfall for the 12-month study period (2401 mm) was close to the long-term mean (2500 mm year −1), but was highly variable from month to month with distinct dry periods in February, May, and December 2005. Measured annual ET of 996 mm year −1 was slightly higher than previous estimates for similar locations in Hawai‘i. However, the energy closure ratio was 0.784, leading us to adjust turbulent energy fluxes upward by 27.6%. The resulting adjusted annual ET of 1232 mm is significantly higher than previous forest ET estimates for Hawai‘i. Variations in canopy conductivity suggest that midday stomatal closure is occurring and/or that evaporation is high in the morning because of frequent wet canopy conditions early in the day. The daily fraction of turbulent energy flux used for ET was observed to be lower for days with high net radiation, indicating that transpiration is being limited under conditions of high evaporative demand. ET was generally higher during rainy periods than dry periods. After controlling for differences in available energy, ET is, on average, 29% higher on days with rain than days without rain. The fraction of energy used for ET follows a distinct annual pattern, roughly corresponding to the observed cycle in leaf area at the site, with a minimum in mid-April and a maximum in mid-October. Variations in rainfall appear to be the cause of deviations from the annual cycle of the fraction of energy used for ET. This suggests that ET is strongly controlled by variations in canopy wetness at this wet forest site.