Contribution of understory evaporation in a tropical wet forest during the dry season

Abstract. Tropical wet forests are complex ecosystems with a large number of plant species. These environments are characterized by a high water availability throughout the whole year and a complex canopy structure. However, how the different sections of the canopy contribute to total evaporation is poorly understood. The aim of this work is to estimate the total evaporation flux and differentiate the contribution among canopy layers of a tropical wet forest in Costa Rica. The fluxes were monitored during the dry season by making use of the energy balance to quantify the fluxes and stable water isotopes to trace the sources of water vapor. Total evaporation was 275.5  mm and represents 55.9 % of the recorded precipitation (498.8  mm ), with 11.7 % of the precipitation being intercepted and evaporated along the forest canopy. The understory beneath 8  m contributed 23.6 % of the evaporation, and almost half of it comes from the first 2  m of the understory. Stable water isotope signatures show different soil water sources depending on the plant type. Palms make use of a water source with an isotope signature similar to precipitation and throughfall. Soil water with a fractionated signature is used by trees, bushes and lianas. The isotope signature of water vapor samples overlap among different heights, but it was not possible to make use of the Keeling plot method due to the similar isotope signature of the possible sources of water vapor as well as the high water concentration even on the dryer days.