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) ra is about 20 s.m-1 in both forests in both seasons, and ranges from 70 to 100 s.m-1 in the pasture site; (ii) rs varies both throughout the day and seasonally, with medians increasing from 40 in the morning, to 150 s.m-1 in late afternoon, in the wet season in the forests and from 50 to 160 s.m-1 in the pasture. These values increase in the dry season, with the forests rs ranging from 50 up to 500 s.m-1 and pasture rs starting from 140 s.m 1 and reaching up to more than 1800 s.m-1 in 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.

Keywords

• decoupling factor
• aerodynamic resistance
• stomatal resistance