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

The effects of albedo, net radiation (R), 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 (lambda E) were estimated by the residuals from R-n and H, and below canopy lambda E fluxes. Surface reflectance (albedo) was similar to 12% of incident solar radiation and did not differ seasonally. R was significantly different among years and explained similar to 79% of the variation in H and E fluxes. The effects of VPD did not explain any additional variation in heat fluxes. lambda E 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 is small. A dimensionless coefficient (ohm) was used to determine the relative importance of aerodynamic conductance (g(a)) and bulk canopy conductance (g(b)) on lambda E flux. During the day, ohm was > 0.6 and peaked at 0.85 suggesting that the forest was decoupled from physiological controls, lambda E fluxes are more dependent on Rn than water availability, and g;, exerts more control on lambda E fluxes than g(b). Because of these results, both the Priestly-Taylor and the Penman-Monteith models performed well using only R. Because the canopy is wet similar to 32% of the time, there was better precision in estimating lambda E fluxes using the Priestly-Taylor model (with an empirically estimated alpha=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 similar to 88 to 97% of total R. (c) 2005 Elsevier B.V. All rights reserved.