TRANSPIRATION FROM AN AMAZONIAN RAIN-FOREST CALCULATED FROM STOMATAL CONDUCTANCE MEASUREMENTS

A multi-layer combination equation has been used to calculate transpiration from a tropical rainforest in the central Amazon, Brazil. The five-layer formulation used through-canopy measurements of temperature and humidity deficit with leaf stomatal and boundary layer conductances scaled up to canopy layer values used a profile of canopy leaf area index derived from the literature. Net radiation input to each layer was estimated from an above canopy measurement using an extinction function and leaf area index. Transpiration calculated for hourly and daily periods in a range of seasonal conditions agreed well with those measured directly with an eddy correlation device. Four simplified forms of the transpiration calculation were also compared with the direct measurement. These simplified forms used the above-canopy or the average of the through-canopy values of weather variables and used either the average of leaf conductances multiplied by total leaf area index or the average of the layer conductances. These simplified forms of the transpiration calculation gave poorer agreement with measured transpiration.