Latent and sensible energy flux over deforested land surfaces in the eastern Amazon and northern Thailand

Land cover change may impact watershed hydrology and regional climate by al tering land-atmosphere exchanges of energy and water. Conversion of forest to pasture has previously been shown to decrease the rate of evaporation (i ncluding transpiration) because of reduced aerodynamic roughness, less cano py rainfall interception and greater canopy resistance during dry periods. However, less is known about the effects of forest replacement land covers other than pasture. In this study, field measurements of meteorological pro cesses, stomatal resistance, and Leaf Area Index (LAI) were taken over vari ous deforested land surfaces in the eastern Amazon Basin and in northern Th ailand. Based on these measurements, evaporation and sensible heat flux wer e estimated over each site. Results show the degree to which different land covers replacing tropical forest affect energy partitioning. In comparison with forest, non-irrigated, actively and recently cultivated sites had sha rply lower rates of evaporation. Secondary vegetation generally had higher rates of evaporation than actively or recently used sites. The proportion o f energy used for evaporation increased rapidly with age in secondary veget ation. Evaporation rates at sites eight and 25 years after abandonment in t he Thailand study area were enhanced by positive sensible energy advection from nearby cultivated areas, and were similar to those of primary forest. Differences in dry season evaporation rates of 3-, 8- and 25-year secondary vegetation are explained, in part, by the differences in canopy resistance . Minimum canopy resistance was 223, 61 and 58 s m(-1), respectively at the 3-, 8 - and 25-year sites.