关键词: SiB2 模型, 地表能量平衡, 反照率, 土地利用和覆被变化, 叶面积指数

Abstract: The influence of land use and cover change (LUCC), by means of biogeophysical processes, on surface energy balance and climate is highly concerned by climate scientists. Driven by spatially interpolated meteorological data and remote-sensing derived leaf area index validated by on-site observations, the Simple Biosphere model (SiB2) was used to simulate the surface energy balances for different land use types, i.e. farmland, grassland and forest in northeast China, from 2001 to 2010. The SiB2 model validation was done by comparing simulation results with observed surface energy balance components and temperature in 2003 for the three land use types. The effects of LUCC on surface energy balance and climate was based on the average simulation outcomes in 2001-2010. Biogeophysical parameters (including albedo and roughness length), surface energy balances (net radiation flux, latent heat flux and sensible heat flux) and climate (canopy temperature) were analyzed to demonstrate the influences of LUCC. The results show that: (1) The simulated and observed data have similar annual trends (R²>0.42) but there is a gap of ±30 W/m² for energy balance components and ±4 ℃ for temperature, based on annual averages. The model imitated the surface energy balance and temperature very well, and was used in the simulation of surface energy balance in northeast China. Sensitivity analysis revealed that the major influencing factor of surface energy partitioning was leaf area index, not vegetation cover as set in the SiB2 model. (2) The impacts of different land use types on surface energy balance and climate through biogeophysical processes were simulated using the same meteorological data for the purpose of eliminating the effects of meteorological conditions. The 10 year averages showed that albedo was lowest in forest, followed by grassland and farmland, while forest > farmland > grassland for leaf area index, forest < grassland < farmland for Bowen Ratio and forest > farmland > grassland for roughness length. (3) Trapped net radiation by ecosystem was primarily determined by albedo. Forest traps the highest net radiation, which is mainly allocated into latent heat flux; farmland traps the lowest net radiation, which is mostly channeled into sensible heat flux, and grassland is in between for both net radiation and its partitioning. (4) Canopy temperature (℃) was determined by the relative importance of albedo and the partitioning of net radiation into latent and sensible heat fluxes: forest (7.7)>farmland (7.64) > grassland (6.67), on annual average. The highest temperature of forest was decided by the lowest albedo, but the higher temperature of farmland as compared to grassland was caused by the higher Bowen ratio of farmland. (5) Precipitation was the predominant impact variable on surface energy balances, and was the main cause of the deviation of SiB2 simulation for forest. With the increment of precipitation in all three land use types, the net radiation flux partitioning into latent heat flux was enhanced, and that into sensible heat flux was weakened, as rain increases soil water content; at the same time, canopy temperature also decreased. The results of this research support the warming effect of farmlands converted from grasslands, which have taken place in northeast China over the past few decades.

Key words: albedo, leaf area index, LUCC, SiB2 Model, surface energy balance