Dry-season radiation balance of land covers replacing forest in northern Thailand

Abstract

The results of numerous general circulation model (GCM) experiments of climatic effects of extensive tropical deforestation include significant reductions in regional precipitation. However, the simulated precipitation decrease is highly dependent on the large albedo shift associated with the assumed conversion of forest into grassland. Land cover change surveys of the Amazon and Thailand suggest that secondary vegetation at various growth stages, rather than grassland, is the dominant feature of deforested land. In this paper, we present field measurements, taken during the dry season, of radiative characteristics of various deforested land surfaces in montane northern Thailand, including secondary vegetation. Dry-season albedo at nine sites in the village of Pang Khum, Chiang Mai Province, ranged from 0.085 for irrigated bare soil to 0.171 for 3-year secondary vegetation. As a result of increased albedo and higher daytime surface temperature at exposed dry soil sites, net radiation is reduced substantially. Regional mean dry-season albedo is estimated to have increased from 0.13 to 0.144 by 1980 for all of northern Thailand, and from 0.13 to 0.14 by 1983 for the southern portion of Sam Mun watershed, the 10 000 ha area immediately surrounding the study site. Continuing significant upward trends in regional albedo at both spatial scales are suggested by the estimates. However, our measurements suggest a maximum deforestation-induced albedo increase of about 0.04, half that used to simulate the effects of deforestation in most GCM experiments. It is likely, therefore, that simulated reductions in precipitation in the region due to deforestation will not be seen in model runs using more realistic scenarios of post-deforestation land cover characteristics.