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

Abstract

Abstract. Canopy and aerodynamic conductances (g C and g A ) are two of the key land surface biophysical variables that control the land surface response of land surface schemes in climate models. Their representation is crucial for predicting transpiration (E T ) and evaporation (λE E ) flux components of the terrestrial latent heat flux (λE), which has important implications for global climate change and water resource management. By physical integration of radiometric surface temperature (T R ) into an integrated framework of the Penman-Monteith and Shuttleworth-Wallace models, we present a novel approach to directly quantify the canopyscale biophysical controls on λE T and λE E over multiple plant functional types (PFTs) in the Amazon Basin. Combining data from six LBA (Large-scale Biosphere-Atmosphere Experiment in Amazonia) eddy covariance tower sites and a T R -driven physically based modeling approach, we identified the canopy-scale feedback-response mechanism between g C , λE T , and atmospheric vapor pressure deficit (D A ), without using any leaf-scale empirical parameterizations for the modeling. The T R -based model shows minor biophysical control on λE T during the wet (rainy) seasons where λE T becomes predominantly radiation driven and net radiation (R N ) determines 75 to 80 % of the variances of λE T . However, biophysical control on λE T is dramatically increased during the dry seasons, and particularly the 2005 drought year, explaining 50 to 65 % of the variances of λE T , and indicates λE T to be substantially soil moisture driven during the rainfall deficit phase. Despite substantial differences in g A between forests and pastures, very similar canopy-atmosphere "coupling" was found in these two biomes due to soil moistureinduced decrease in g C in the pasture. This revealed the pragmatic aspect of the T R -driven model behavior that exhibits a high sensitivity of g C to per unit change in wetness as opposed to g A that is marginally sensitive to surface wetness variability. Our results reveal the occurrence of a significant hysteresis between λE T and g C during the dry season for the Published by Copernicus Publications on behalf of the European Geosciences Union. 4238 K. Mallick et al.: Canopy-scale biophysical controls of transpiration and evaporation in the Amazon Basin pasture sites, which is attributed to relatively low soil water availability as compared to the rainforests, likely due to differences in rooting depth between the two systems. Evaporation was significantly influenced by g A for all the PFTs and across all wetness conditions. Our analytical framework logically captures the responses of g C and g A to changes in atmospheric radiation, D A , and surface radiometric temperature, and thus appears to be promising for the improvement of existing land-surface-atmosphere exchange parameterizations across a range of spatial scales.

Estimating the Effect of Precipitation and Vegetation on Water Yield in Northern China

“…Forest have a higher capacity for water consumption, associated with the higher leaf area index (LAI) of the higher stature vegetation [24], and tend to have better access to water sources through accessing deep water or drawing on stored soil water [25]. Transpiration is traditionally considered the most important component of forest evapotranspiration, but interception and subsequent evaporation from the canopy can also increase substantially, particularly with conifers [26]. The evaporation of intercepted precipitation can account for 10-20% of the rainfall for broadleaf trees and 20-40% for conifers [27].…”

Regional evapotranspiration from image-based implementation of the Surface Temperature Initiated Closure (STIC1.2) model and its validation across an aridity gradient in the conterminous United States

“…This could largely be attributed to the fact that the model is only used for understanding ecosystem-scale ET partitioning and their biophysical controls at the eddy covariance (EC) footprints (Mallick et al, 2015;Mallick et al, 2016), where all the necessary forcing variables were measured at the flux tower sites. In this paper, we present the first ever implementation of the STIC1.2 model using MODIS LST and associated land surface products, and its validation in thirteen core AmeriFlux sites Hydrol.…”

“…Somewhat surprisingly, the idea of integrating TR into the PM model was never attempted because of complexities associated with gC 20 parameterization (Bell et al, 2015;Matheny et al, 2014), until the concept of STIC was formulated (Mallick et al, 2014;Mallick et al, 2015). The recent version of STIC, STIC1.2, combines PM with the Shuttleworth-Wallace (SW) model (Shuttleworth and Wallace, 1985) to estimate the source/sink height temperature and vapour pressure (T0 and e0) (Mallick et al, 2016). By algebraic reorganization of aerodynamic equations of H and E, Bowen ratio evaporative fraction hypothesis (Bowen, 1926) and modified advection-aridity hypothesis (Brutsaert and Stricker, 1979), STIC1.2 formulates multiple state 25 equations where the state equations were constrained with an aggregated moisture availability factor (M).…”

“…On the contrary, STIC1.2 directly estimates H and λE through the PM equation (Mallick et al, 2016) by solving state equations for the conductances. MOD16 estimates λE directly using a modified PM framework (Mu et al, 2007;Mu et al, 2011), where the conductances are estimated based on a biome property look up (BPLUT) and meteorological scaling functions.…”

“…The prime focus of STIC (Mallick et al, 2014;Mallick et al, 2015;Mallick et al, 2016) is based on physical integration of 15 TR into the Penman-Monteith (PM) equation, which is fundamentally constrained to account for the necessary feedbacks between ET, TR, DA, gA, and gC (Monteith, 1965). Monteith (1981) highlighted the fact that the biophysical conductances (i.e., gA and gC) regulating ET are heavily temperature dependent, after which a stream of research demonstrated the dominant control of TR into gC and associated canopy-scale aerodynamics (Moffett and Gorelick, 2012;Blonquist et al, 2009).…”