Evapotranspiration modelled from stands of three broad-leaved tropical trees in Costa Rica

To examine the impact of tree species on the water cycle in a wet tropical region, annual evapotranspiration (ET) was estimated in Costa Rican plantations of three native, broad-leaved tree species that contrasted strongly in leaf size, leaf area and phenology. Evapotranspiration was estimated using the Penman-Monteith equation for transpiration from the dry canopy, the equilibrium equation for evaporation from the understory and a modified Rutter model of interception for evaporation of water from the canopy when wetted by rainfall. Canopy conductance was estimated from stomatal conductance, leaf area and leaf boundary-layer conductance; canopy storage capacity and filling rate were estimated from throughfall measurements. Micrometeorological instruments were mounted on a scaffolding tower. Mean stomatal conductance, which ranged from 0?1 to 0?7 mol m-2 s-1, was similar to boundary-layer conductance, 0?1 to 0?5 mol m-2 s-1, indicating decoupling of stomata from atmospheric conditions. Mean canopy conductance varied from 0?6 to 0?7 mol m-2 s-1 in the 1994 wet season then dropped to 0?3-0?4 mol m-2 s-1 in stands of the two deciduous species, Cordia and Cedrela, as a result of reduced leaf area during the dry season. Despite increased understory evaporation, dry-season ET from these stands was only 78-81 f ET in stands of the evergreen species, Hyeronima. Maximum canopy water depth varied from 0?2 to 2?2 mm, causing modelled interception to vary from 6 o 25 f annual ET. Higher dry-season transpiration rates along with high rates of evaporation of intercepted rainfall in all seasons led to 14 igher annual ET in Hyeronima stands (1509 mm) than in stands of the species with lowest ET, Cedrela (1318 mm). Despite lack of strong physiological control on transpiration, tree species in wet tropical plantations can influence hydrological balanc