Environmental dependence and seasonal variation of diffuse solar radiation in tropical peatland

Changes in cloud cover and atmospheric aerosol loading strongly affect the diffuse proportion of solar radiation (Rd/Rg). It has been reported that plant photosynthesis is more efficient under diffuse light conditions, but diffuse radiation (Rd) regimes in tropical peatland frequently subjected to largescale fires are poorly described, and there are few or no site-based datasets to date. Therefore, we continuously measured Rd for more than three years at a burnt ex-forest on tropical peatland in Central Kalimantan, Indonesia. Daily Rd/Rg was significantly related to the clearness index (Rg/Ro) with a linear threshold model. The model parameters showed that Rd/Rg under a clear sky is larger in tropical peatland with a humid climate than in Australia. Using the model, long-term variation in Rd/Rg for more than 12 years was estimated from Rg measured above a nearby forest. As a result, Rd/Rg showed a seasonal variation with its minimum of 0.51 in June (the transition between the wet and dry seasons) and its maximum of 0.68 in October (the late dry season) on a monthly basis. The decreasing pattern from the late wet season to the early dry season corresponded to decreasing precipitation due to fewer clouds. In contrast, the increasing pattern through the latter half of the dry season was due to shading by smoke emitted through burning biomass and peat fires. In particular, during the El Niño droughts in 2002, 2006 and 2009, the monthly mean Rd/Rg rose above 0.72, because the ground was densely covered with smoke from large-scale fires.