Tropical montane forests with frequent fog are zones of great hydrological and ecological value. Within the cloud zone, vegetation strips droplets from the passing air, thereby increasing water input. Because of cloud water interception and suppressed transpiration, they generate high watershed yields, supporting water resource needs of downstream regions. In Hawai'i, wet montane forest areas are critically important sources of groundwater recharge and host threatened ecological resources. Planning for water development and protection of fragile ecosystems in Hawai'i depend on accurate quantification of hydrological processes in forests. However, no direct measurements of ET have ever been made in Hawai'i's forests, nor have any prior studies focused on the hydrological impacts of the loss of the epiphyte layer due to species invasion or other disturbances in Hawai'i's tropical montane cloud forests. Recent studies in similar maritime tropical environments suggest that forest ET in Hawai'i may exceed current estimates by 265-900 mm yr-1. Accurate forest ET measurements in Hawai'i are urgently needed to test the reproducibility of exceptionally high rates found elsewhere and to answer specific questions about hydrologic processes in Hawai'i. Canopy water balance observations suggest cloud water interception may be responsible for a larger proportion of the hydrological input in Hawai'i than previously thought. This project will contribute to a more comprehensive understanding of tropical forest hydrology generally, and address pressing needs for more accurate estimates of hydrologic fluxes in Hawai'i, by: (1) determining the rates of evapotranspiration, transpiration, interception evaporation, and soil/litter evaporation for wet forest sites in Hawai`i; (2) determining fog interception, interception evaporation, stem flow, stem storage, and transpiration in native forest and in an alien species stand; (3) testing alternative methods of estimating ET for Hawai'i forest sites; and (4) using alternative methods to estimate ET at existing observing sites on Maui. To address the objectives of this study, we will conduct comprehensive field measurements of all components of water flux into, out of, and within the vegetative layer at two forest sites on Hawai'i Island. The sites will include native forest and an invasive species site, both within the cloud zone. There will be three overlapping components of this research. In the first component, rainfall partitioning, evapotranspiration, transpiration, and fog interception will be intensively monitored at the two primary field sites. Field monitoring will continue throughout the three years of the study. Beginning in the second year of the study and intensifying in the final year, component two of the study will focus on analysis of the total evaporation and interception evaporation at each site, effects of species invasion on fog interception and interception evaporation, influences of SST variations on these processes, and performance of alternative methods of estimating ET in Hawai`i forests. In component three, knowledge gained in the first two phases will be applied to the problem of estimating ET over a range of Hawai'i's topoclimates.
