Evaporation from young secondary vegetation in eastern Amazonia

The fallow vegetation of the slash and burn agriculture in eastern Amazonia is dominated by shrubs and trees. This study of evaporation from such secondary vegetation started when the aboveground parts of the vegetation were approximately 2.5 years old. The results are based mainly on a data set containing 231 days of micrometeorological observations in the period from April 1992 to April 1993. Evaporation values obtained with the Penman open water formula ranged from 1.1 to 7.2 mm d(-1), with an overall mean of 4.6 mm d(-1). Actual evaporation, calculated with the Bowen ratio approach, varied from 1.2 to 5.9 mm d(-1), with an overall mean of 3.9 mm d(-1). Due to the high net radiation and vapour pressure deficit, and the evenly distributed moderate rainfall, the actual evaporation was constantly high during the transition between the rainy and dry seasons. In a relatively dry period, water limitations were indicated by a decrease in the actual evaporation compared with the Penman open water evaporation, Day-to-day variability was pronounced in the rainy season. An overall average of 79% of the net radiation was converted to latent heat flux. The annual evaporation was calculated by an interpolation of missing data with the continuously observed net radiation. The total actual evaporation was estimated to be 1364 mm a(-1), against rainfall of 1819 mm a(-1); the remaining 455 mm were allocated to drainage. When actual evaporation exceeded rainfall during the dry season, there had to be access to water storage down to depths of more than 3 m. We conclude that the young secondary vegetation can re-evaporate an important part of the rainfall input in spire of the marked seasonal distribution of rainfall. Possible regional climatic changes due to deforestation may be less severe in areas where woody secondary vegetation plays an important role in land cover. (C) 1997 Elsevier Science B.V.