Control of Dry Season Evapotranspiration over the Amazonian Forest as Inferred from Observations at a Southern Amazon Forest Site

Abstract

The extent to which soil water storage can support an average dry season evapotranspiration (ET) is investigated using observations from the Rebio Jarú site for the period of 2000 to 2002. During the dry season, when total rainfall is less than 100 mm, the soil moisture storage available to root uptake in the top 3-m layer is sufficient to maintain the ET rate, which is equal to or higher than that in the wet season. With a normal or less-than-normal dry season rainfall, more than 75% of the ET is supplied by soil water below 1 m, whereas during a rainier dry season, about 50% of ET is provided by soil water from below 1 m. Soil moisture below 1-m depth is recharged by rainfall during the previous wet season: dry season rainfall rarely infiltrates to this depth. These results suggest that, even near the southern edge of the Amazon forest, seasonal and moderate interannual rainfall deficits can be mitigated by an increase in root uptake from deeper soil.How dry season ET varies geographically within the Amazon and what might control its geographic distribution are examined by comparing in situ observations from 10 sites from different areas of Amazonia reported during the last two decades. Results show that the average dry season ET varies less than 1 mm day Ϫ1 or 30% from the driest to nearly the wettest parts of Amazonia and is largely correlated with the change of surface net radiation of 25% and 30%. Thus the geographic variation of the average dry season ET appears to be mainly determined by the surface radiation.

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).…”

“…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.…”

“…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).…”

Effects of RegCM3 parameterizations on simulated rainy season over South America

“…Observed precipitation, net radiation (R n ), and sensible (H) and latent (λE) heat fluxes are used to evaluate the simulated surface energy budget. The H and λE fluxes are derived from the eddy correlation technique, and a summary of these micrometeorological observations can be obtained in Negrón-Juárez et al (2007). The climatologies of several variables observed at the sites are shown in da Rocha et al (2009b).…”

Potential negative effects of groundwater dynamics on dry season convection in the Amazon River basin

“…The response of ET to groundwater depends on the seasonal amplitude of the atmospheric precipitation and temperature, and land surface conditions, resulting in a substantial increase in ET in the dry season. Using an observational dataset, Juárez et al (2007) showed that, during the dry season, the deep soil provides a sufficient supply of water to account for ET because of the extensive and deep root system of trees in the ARB (da Rocha et al 2009). Hydraulic redistribution (HR), a phenomenon of tree root redistributes soil water from wet to dry areas at night and normally redistributes the water from deep soil, where water was stored in raining season, to surface soil (Lee et al 2005).…”