OBSERVATIONS OF ALBEDO AND RADIATION BALANCE OVER POSTFOREST LAND SURFACES IN THE EASTERN AMAZON BASIN

Regional climatic change, including significant reductions in Amazon B asin evaporation and precipitation, has been predicted by numerical si mulations of total tropical forest removal. These results have been sh own to be very sensitive to the prescription of the albedo shift assoc iated with conversion from forest to a replacement land cover. Modeler s have so far chosen to use an ''impoverished grassland'' scenario to represent the postforest land surface. This choice maximizes the shift s in land surface parameters, especially albedo (fraction of incident shortwave radiation reflected by the surface). Recent surveys show sec ondary vegetation to be the dominant land cover for some deforested ar eas of the Amazon. The characteristics of secondary vegetation as well as agricultural land covers other than pasture have received little a ttention from field scientists in the region. This paper presents the results of field measurements of radiation flux over various deforeste d surfaces on a small farm in the eastern Amazonian state of Para. The albedo of fields in active use was as high as 0.176, slightly less th an the 0.180 recently determined for Amazonian pasture and substantial ly less than the 0.19 commonly used in GCM simulations of deforestatio n. For 10-yr-old secondary vegetation, albedo was 0.135, practically i ndistinguishable from the recently published mean primary forest albed o of 0.134. Measurements of surface temperature and net radiation show that, despite similarity in albedo, secondary vegetation differs from primary forest in energy and mass exchange. The elevation of midday s urface temperature above air temperature was found to be greatest for actively and recently farmed land, declining with time since abandonme nt. Net radiation was correspondingly lower for fields in active or re cent use. Using land cover analyses of the region surrounding the stud y area for 1984, 1988, and 1991, the pace of change in regional-mean a lbedo is estimated to have declined and appears to be leveling at a va lue less than 0.03 above that of the original forest cover.