Evaporation from Amazonian rainforest

Abstract The results of a two-year Anglo-Brazilian study of the micrometeorology and process hydrology of undisturbed tropical rainforest at a site in central Amazonia are described. Sample measurements of dry-canopy evaporation from three intensive field campaigns and continuous measurements of rainfall interception loss are used to calibrate a micrometeorological model of evaporation from the forest canopy. Evaporation at this site is calculated by the model from September 1983 to September 1985 by using hourly-average meteorological measurements routinely taken above the canopy, and regular measurements of soil water tension averaged to a depth of one metre. The monthly evaporation derived in this way is compared with the measured precipitation and radiant energy input to the forest, and with calculated potential evaporation rates. Over the whole study period, approximately 10% of rainfall was intercepted by the forest canopy, and this accounted for 20-25 % of the evaporation. The remainder occurred as transpiration from the trees. Over this same period, about one half of the incoming precipitation is returned to the atmosphere as evaporation, a process which requires 90% of the radiant energy input. These proportions exhibit some seasonal behaviour in response to the large seasonal variation in rainfall. The average evaporation over two years was within 5% of potential evaporation. Monthly-average evaporation exceeds potential estimates by about 10% during wet months, and falls below such estimates by at least this proportion in dry months. These data provide the hitherto missing numerical basis for assessing the likely consequences of Amazonian deforestation on surface water and energy balances.