Energy dynamics and modeled evapotranspiration from a wet tropical forest in Costa Rica

The effects of albedo, net radiation (Rn), vapor pressure deficit (VPD), and surface conductances on energy fluxes and evapotranspiration (ET) were determined for a wet tropical forest in NE Costa Rica from 1997 to 2000. Sensible heat fluxes (H) were estimated by the combination of eddy-covariance and the change in below-canopy heat profiles. Above-canopy latent heat fluxes (λE) were estimated by the residuals from Rn and H, and below canopy λE fluxes. Surface reflectance (albedo) was ∼12% of incident solar radiation and did not differ seasonally. Rn was significantly different among years and explained ∼79% of the variation in H and λE fluxes. The effects of VPD did not explain any additional variation in heat fluxes. λE fluxes were always greater than H fluxes when Rn>40 W m−2. Understory heat fluxes were small and contributed little towards daily energy exchange, but may be significant when Rn is small. A dimensionless coefficient (Ω) was used to determine the relative importance of aerodynamic conductance (ga) and bulk canopy conductance (gb) on λE flux. During the day, Ω was >0.6 and peaked at 0.85 suggesting that the forest was decoupled from physiological controls, λE fluxes are more dependent on Rn than water availability, and ga exerts more control on λE fluxes than gb. Because of these results, both the Priestly–Taylor and the Penman–Monteith models performed well using only Rn. Because the canopy is wet ∼32% of the time, there was better precision in estimating λE fluxes using the Priestly–Taylor model (with an empirically estimated α=1.24), when the canopy was wet. Annual ET were 1892, 2292 and 2230 mm for 1998, 1999 and 2000, respectively. Annual ET ranged from 54 to 66% of bulk precipitation. Using a Rutter-type model, interception losses were 17–18% of bulk precipitation. The overall amount of energy needed for annual ET accounted for ∼88 to 97% of total Rn.