The land–atmosphere water flux in the tropics

Abstract

Tropical vegetation is a major source of global land surface evapotranspiration, and can thus play a major role in global hydrological cycles and global atmospheric circulation. Accurate prediction of tropical evapotranspiration is critical to our understanding of these processes under changing climate. We examined the controls on evapotranspiration in tropical vegetation at 21 pan-tropical eddy covariance sites, conducted a comprehensive and systematic evaluation of 13 evapotranspiration models at these sites, and assessed the ability to scale up model estimates of evapotranspiration for the test region of Amazonia. Net radiation was the strongest determinant of evapotranspiration (mean evaporative fraction was 0.72) and explained 87% of the variance in monthly evapotranspiration across the sites. Vapor pressure deficit was the strongest residual predictor (14%), followed by normalized difference vegetation index (9%), precipitation (6%) and wind speed (4%). The radiation-based evapotranspiration models performed best overall for three reasons: (1) the vegetation was largely decoupled from atmospheric turbulent transfer (calculated from X decoupling factor), especially at the wetter sites; (2) the resistance-based models were hindered by difficulty in consistently characterizing canopy (and stomatal) resistance in the highly diverse vegetation; (3) the temperature-based models inadequately captured the variability in tropical evapotranspiration. We evaluated the potential to predict regional evapotranspiration for one test region: Amazonia. We estimated an Amazonia-wide evapotranspiration of 1370 mm yr À1 , but this value is dependent on assumptions about energy balance closure for the tropical eddy covariance sites; a lower value (1096 mm yr À1 ) is considered in discussion on the use of flux data to validate and interpolate models.

Mechanisms of water supply and vegetation demand govern the seasonality and magnitude of evapotranspiration in Amazonia and Cerrado

“…2 shows that equatorial forests exhibit a seasonal cycle of E peaking with net radiation during the dry season, transitional southern forests show either a flat seasonal cycle (due to less seasonality in available light) or a slight dry season depression (some degree of water limitation), and Cerrado demonstrates a strong dry season depression (both due to reductions in light and water). These results corroborate those of previous work which showed a general trend of increasing water limitation from north to south (Hasler and Avissar, 2007;Juarez et al, 2007;Borma et al, 2009;da Rocha et al, 2002da Rocha et al, , 2009Fisher et al, 2009). While BAN and RJA differed slightly in their respective seasonalities of E (BAN has a more pronounced dry season depression compared to RJA), overall the individual site E seasonalities corresponded to the mean E seasonality of the grouped sites (see Appendix D of the supplement for individual site seasonalities).…”

“…We interpreted consistency between model-derived and observation-derived R 2 and slope values as one metric of realism of modeled controls on E. For these regressions, we approximated available energy with the sum of latent and sensible heat (LE + H). Using LE + H as an estimate of available energy instead of R n is an approach recently adopted by a pan-tropical review of LE (Fisher et al, 2009) as an alternative to filtering out periods of poor energy budget closure (periods when LE + H fall short of net radiation, R n ), which can reduce the number of daily replicates comprising a monthly mean (Costa et al, 2010). We recognize that such an approach inflates R 2 values and increases the slope, but absolute values are not the emphasis here.…”

“…The degree to which stomata regulate transpiration (E t ) independent of environmental conditions in the Amazon has been the topic of debate (Avissar and Werth, 2004;Costa et al, 2004). The conclusions of syntheses of eddy covariance measures of the seasonality of E in the Amazon have largely emphasized the secondary role of vegetation demand across a range of forest types (Costa et al, 2004;Juarez et al, 2007;da Rocha et al, 2002da Rocha et al, , 2009Fisher et al, 2009), but recent work suggests that forests indeed exhibit varying degrees of control on the seasonal exchange of water in their canopies (Costa et al, 2010). Much of what is known about the functioning of stomata remains phenomenological; at the leaf-level, attempts at forming a solid mechanistic basis of stomatal function have proven to be a challenge (Buckley, 2005;Peak and Mott, 2011).…”

“…Recent syntheses using data from eddy covariance measures of carbon, water, and energy exchange across Amazonia indicate a simple dependency of E on net radiation (R n ) for forest types ranging from seasonally wet to seasonally dry forests (Shuttleworth, 1988;Hasler and Avissar, 2007;Juarez et al, 2007;da Rocha et al, 2002da Rocha et al, , 2009Fisher et al, 2009). However, this stands in stark contrast to many model predictions which instead have historically simulated an annual E cycle in phase with precipitation (P) (Shuttleworth, 1991;Bonan, 1998;Dickinson et al, 2006), suggesting that E is limited by water availability.…”

Do soil fertilization and forest canopy foliage affect the growth and photosynthesis of Amazonian saplings?

“…It is particularly important because of its prominent role in carbon and water cycles. Its vegetation stores about 90 Pg (1 Pg = 10 15 g) of carbon (Saatchi et al, 2007), and about 50 % of the rainfall in the region comes from water cycled via transpiration (Salati, 1987;Fisher et al, 2009). Net productivity of tropical forests may be limited by several factors, including nutrient defi ciency (Vitousek et al, 2010), sunlight (Graham et al, 2003) and soil water content (Wagner et al, 2012).…”