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

Abstract

The effects of albedo, net radiation ( R n), 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 R n and H, and below canopy λE fluxes. Surface reflectance (albedo) was ∼12% of incident solar radiation and did not differ seasonally. R n 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 R n>40 W m −2. Understory heat fluxes were small and contributed little towards daily energy exchange, but may be significant when R n is small. A dimensionless coefficient ( Ω) was used to determine the relative importance of aerodynamic conductance ( g a) and bulk canopy conductance ( g b) 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 R n than water availability, and g a exerts more control on λE fluxes than g b. Because of these results, both the Priestly–Taylor and the Penman–Monteith models performed well using only R n. 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 R n.