Surface conductance for evapotranspiration of tropical forests: Calculations, variations, and controls

Tropical forests are responsible for the evaporation and transpiration of large quantities of water into the atmosphere annually. Surface conductance (g_s) is a poorly understood phenomenon that plays a central role in regulating this evapotranspiration. We studied the calculations, variations, and environmental factors controlling g_s based on eddy flux measurements from 10 tropical forest sites that covered a wide range of water gradients across continents. We found that boundary layer conductance (g_b) is comparable with aerodynamic conductance for momentum (g_aM) and thus, it should not be ignored in estimations of total aerodynamic conductance for water vapor (g_aV). Based on the findings, we have made some recommendations for g_aM estimation both with and without measurements of turbulence. The seasonal variation of g_s is low in humid sites but large in sites with a dry season. A value of 24.8 ± 13.8 mm s⁻¹ was suggested for maximum surface conductance (g_smax) for tropical forests. Both water vapor deficit (D) and radiation (Q) play an important role in controlling g_s. The model driven by both D and Q could capture the diurnal variations of g_s well and could be implemented in large-scale models in future. We believe the findings of this study could contribute substantially to our understanding of tropical forest g_s.