What Controls the Geographic Distribution of Evapotranspiration in the Amazon River Basin?

Amazonia hosts the largest block of tropical rain forest, considered as the prime contributor to land surface evapotranspiration (ET) (Choudhury et al. 1998). In the dry season, forest root uptake from deep soil provides enough water for plant photosynthesis, and the geographical distribution of evapotranspiration (ET) is largely controlled by surface radiation or surface meteorological conditions. Recently, Negron Juarez et al. (2007) compared in situ observations from 10 sites in Amazonia and suggested that the geographical distribution of dry season ET are mainly determined by the spatial variation of surface radiation. However, in their study, the available stations cover only part of the Amazon basin. The observation period (generally a couple of years in each station) varies from station to station, making the results suspectible due to the large interannual variability in the Amazon basin. Moreover, the correlation between ET and radiation was evaluated only qualitatively (see their Figure 7).

In this study, a comprehensive evaluation of water and energy budgets using available observational data and observation-constrained land model simulations is made. Our goal is to provide a self-consistent analysis of the surface water and energy fluxes over the Amazon Basin during 1979-2004. Because of incomplete observations of surface water and energy fields, we employ a comprehensive land surface model, namely the new Community Land model (CLM3.5) with an improved terrestrial water cycle (Oleson et al. 2008), forced with observation-based precipitation, temperature and other atmospheric forcing to simulate historical land surface conditions and study the factors which control the spatial variation of ET.

In the dry season, the latent heat flux distribution does not follow the net radiation distribution as suggested by Negron Juarez et al. (2007). In fact, the sensible heat flux plays an important role in the surface energy budget, and is negatively correlated with the latent heat flux, leading to a spatial variability of the Bowen ratio. The ET maximum in the southwestern part of the basin is mainly in the form of canopy transpiration and is contributed almost equally by the precipitation and the water storage loss. In the dry season, the distribution of latent heat flux might be also affected by surface meteorological conditions.

Sensitivity experiments reveal that ET distribution is firstly dominated by atmospheric specific humidity, secondly by net radiation through the contribution from transpiration. Precipitation only affects the canopy evaporation distribution. Vegetation cover only affects the ground evaporation.