EVAPORATION FROM YOUNG SECONDARY VEGETATION IN EASTERN AMAZONIA

The fallow vegetation of the slash and burn agriculture in eastern Ama zonia is dominated by shrubs and trees. This study of evaporation from such secondary vegetation started when the aboveground parts of the v egetation were approximately 2.5 years old. The results are based main ly on a data set containing 231 days of micrometeorological observatio ns in the period from April 1992 to April 1993. Evaporation values obt ained 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, calculat ed with the Bowen ratio approach, varied from 1.2 to 5.9 mm d(-1), wit h an overall mean of 3.9 mm d(-1). Due to the high net radiation and v apour pressure deficit, and the evenly distributed moderate rainfall, the actual evaporation was constantly high during the transition betwe en the rainy and dry seasons. In a relatively dry period, water limita tions were indicated by a decrease in the actual evaporation compared with the Penman open water evaporation, Day-to-day variability was pro nounced in the rainy season. An overall average of 79% of the net radi ation was converted to latent heat flux. The annual evaporation was ca lculated by an interpolation of missing data with the continuously obs erved 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 t o depths of more than 3 m. We conclude that the young secondary vegeta tion can re-evaporate an important part of the rainfall input in spire of the marked seasonal distribution of rainfall. Possible regional cl imatic 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.