Analysis of biological and meteorological controls of evapotranspiration in pristine forests and a pasture site in Amazonia

Abstract This work studied the behavior and seasonality of evapotranspiration influenced by biotic and abiotic factors through analysis of diurnal variation of aerodynamic resistance (ra), stomatal resistance (rs) and decoupling factor (Ω). This index was proposed by Jarvis and McNaughton (1986) as an indicative of the control of these resistances on the evapotranspiration of vegetation. Selection of representative data from wet and dry seasons from a primary forest in Central Amazonia and a primary forest and a pasture sites in Southwestern Amazonia had shown that: (i) rais about 20 s.m-1in both forests in both seasons, and ranges from 70 to 100 s.m-1in the pasture site; (ii) rsvaries both throughout the day and seasonally, with medians increasing from 40 in the morning, to 150 s.m-1in late afternoon, in the wet season in the forests and from 50 to 160 s.m-1in the pasture. These values increase in the dry season, with the forests rsranging from 50 up to 500 s.m-1and pasture rsstarting from 140 s.m-1and reaching up to more than 1800 s.m-1in the dry afternoons; (iii) Ω ranges from 0.5 to 0.8 during the wet season, and reduces to values below 0.5 in the afternoons during the dry season, indicating that, although a strong influence of net radiation in the evaporative loss is present, to a large extent the evapotranspiration fluxes are coupled to the biotic control of stomatal closure in the vegetation, especially in the pasture and during dry periods.