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

Abstract

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.

Insights Into the Aerodynamic Versus Radiometric Surface Temperature Debate in Thermal‐Based Evaporation Modeling

“…With the initial estimates of these variables, first estimates of the conductances, T 0 , F E , H, and E are obtained. The process is then iterated by updating e 0 *, e 0 , I SM , and α (using Equations A9, A10, A11, A17, A16, and A15 in Mallick et al., 2016), with the first estimates of g cs , g a , T 0 , and E . This is followed by recalculation of G , g cs , g a , T 0 , F E , H , and E in the subsequent iterations with the previous estimates of e 0 *, e 0 , I SM , and α until the convergence of E is achieved.…”

“…The state equations are connected with T r through I SM , and the effects of T r are subsequently propagated into their analytical solutions (Equations below). These equations are based on the aerodynamic bulk transfer hypothesis, advection‐aridity hypothesis (Brutsaert & Stricker, 1979), and evaporative fraction ( F E ) theory (Mallick et al., 2016; Shuttleworth et al., 1989) with the detailed information for solving these equations provided in Mallick et al. (2016) and Section S1 of the Supporting Information .…”

“…These equations are based on the aerodynamic bulk transfer hypothesis, advection‐aridity hypothesis (Brutsaert & Stricker, 1979), and evaporative fraction ( F E ) theory (Mallick et al., 2016; Shuttleworth et al., 1989) with the detailed information for solving these equations provided in Mallick et al. (2016) and Section S1 of the Supporting Information . $$${F}_{E}=\frac{2\alpha s}{2s\,+\,2\gamma \,+\,\gamma \left(1+{I}_{SM}\right)\frac{{g}_{a}}{{g}_{cs}}}$$$ $$${T}_{0}={T}_{a}\,+\,\frac{\left({e}_{0}-{e}_{a}\right)}{\gamma }\frac{\left(1-{F}_{E}\right)}{{F}_{E}}$$$ $$${g}_{a}=\frac{{R}_{N}\,-\,G}{\rho {c}_{p}\left[\left({T}_{0}-{T}_{a}\right)\,+\,\frac{\left({e}_{0}-{e}_{a}\right)}{\gamma }\right]}\,$$$ …”

“…Such a non‐parametric formulation, called Surface Temperature Initiated Closure (STIC) has been shown to provide reasonable estimates of evaporative fluxes across contrasting biomes and aridity in the northern and southern hemispheres (Bai et al., 2021; Bhattarai et al., 2018, 2019; Mallick et al., 2014, 2016, 2018; Trebs et al., 2021). However, it is equally important to explain the long‐debated T r versus T 0 differences from the theoretical standpoint to simplify the operational evaporation retrievals from current and future TIR satellite missions.…”

Estimation of Evapotranspiration of Amazon Rainforest Using the Maximum Entropy Production Method

“…Specifically, in the context of the MEP approach, the energy conservation statement as in equation is automatically satisfied, while the sum of EC‐measured latent and sensible heat flux is often lower than R n . The Bowen ratio closure method (Mallick et al, ; Twine et al, ) is used here to address this problem. Bowen Ratio ( Bo ) obtained directly from the measurements of λE OBS and H OBS is assumed to be valid, and the net energy imbalance is redistributed to the surface heat fluxes: where “OBS” refers to observed fluxes and is the corrected flux.…”