Potential evapotranspiration from forest and pasture in the tropics: A case study in Kona, Hawai‘i

SummaryForest conversion in tropical montane landscapes is widespread and has potentially large implications for both biological and physical processes. Understanding the ecohydrologic processes that affect water can help efforts to predict the downstream effects of parcel-scale land use change. Differences in evapotranspiration between trees and grasses in humid, low wind environments are understudied, however. We analyze predictions of the Penman–Monteith model of potential evapotranspiration (PET) based on hourly meteorological inputs and direct measurements of stomatal resistance for leeward Hawai‘i Island. While evapotranspiration is very low in all of these forest and pasture ecosystems, modeled PET from pasture is higher than is PET from forest. The balance between aerodynamically and stomatally controlled evapotranspiration differs significantly between the two vegetation types in such a way that the weighted sum of the two components yields lower overall PET at the forest sites. The interaction of aerodynamic and stomatal control on PET, in conjunction with tropical meteorology characterized by low wind speeds and low vapor pressure deficit (VPD) causes this unexpected phenomenon. Vegetation structure plays an important role: evapotranspiration from forest is increased considerably by contributions from the understory, while the shorter the stature of pasture grass, the higher its rate of PET. In tropical regions that do not experience water stress, grassland has the potential to transport as much or more water vapor to the atmosphere than does forest.

► Potential evapotranspiration (PET) is low on leeward Hawai‘i Island.
► In this tropical region without water stress, PET is higher in pasture than in forest.
► Aerodynamic and stomatal control of PET differ for the two vegetation types.
► Land use changes such as increased cattle grazing may impact rates of PET.
► Forest conversion in tropical montane landscapes could affect downstream water users.