Canopy-scale biophysical controls of transpiration and evaporation in the Amazon Basin

Abstract. Canopy and aerodynamic conductances (gC and gA) are some of the key land surface variables determining the land surface response of climate models. Their representation is crucial for predicting transpiration (λET) and evaporation (λEE), which has important implications for global climate change and water resource management. Here, we present a novel approach to directly quantify the controls of the canopy-scale conductances on λET and λEE over multiple plant functions types (PFTs) in the Amazon Basin. Combining data from six LBA (Large-scale Biosphere-Atmosphere Experiment in Amazonia) eddy covariance tower sites and a physically-based modeling approach, we identified the canopy-scale feedback-response mechanism between gC, λET, and atmospheric vapor pressure deficit (DA), which was originally postulated to occur at the leaf-scale. We show minor biophysical control on λET under wet conditions where net radiation (RN) determines 75 % to 80 % of the variances of λET. However, biophysical control on λET is amplified during the drought year (2005) and dry conditions, explaining 50 % to 65 % of the variances of λET. Despite substantial differences in gA, nearly similar “coupling” was found in forests and pastures due to the increase of gC induced by soil moisture. This suggests that the relative response of gC to per unit change of wetness is significantly higher compared to gA. Our results reveal the occurrence of a larger magnitude of hysteresis between λET and gC during the dry season for the pasture sites, which is attributed to relatively low soil water availability compared to the rainforest. Evaporation was significantly influenced by gA for all the PFTs and across all wetness conditions. Our analytical framework faithfully captures the responses of gC and gA to changing atmospheric radiation, DA, and surface skin temperature, and, thus appears to be promising for the improvement of existing land surface parameterisations at a range of spatial scales.