Abstract: In order to discriminate the canopy’s absorption of direct solar radiation and scattered radiation and the influence to total FPAR (fraction of absorbed photosynthetically active radiation), the SAIL (scattering by arbitrarily inclined leaves) model is used to simulate the direct FPAR, scattering of FPAR and total FPAR under different weather condition. The main factors influencing the change of total FPAR and the difference between direct FPAR and diffuse FPAR are analysised. The results show that with the change of LAI, FPAR also has the saturation phenomenon. When the visibility is set as 5, 15 and 30 km, the contribution of scattering of FPAR on the total FPAR is 52.6%, 29.3% and 21.7%. The error between total FPAR and direct FPAR is reduced with the increasing of visibility and increased with the reducing of LAI. The maximum relative error is 13.2%. With the change of the solar zenith, the maximum relative error is 10.29% between direct FPAR and total FPAR. From the simulation analyses, it can be see that direct and diffuse FPAR are different with the changes of environment variables. So when modeling of FPAR, the diffuse part can not be ignored. Direct FPAR and diffuse FPAR must be modeled respectively. The total FPAR need to be calculated by the proportion of scattered radiation which is determined by the weather condition.

Key words: FPAR, SAIL model, direct radiation, diffuse radiation

摘要： 为明确太阳直接辐射及散射辐射在进入冠层被冠层吸收以及两者对总光合有效辐射吸收比例(FPAR)造成的影响, 利用SAIL模型模拟不同条件下的直射FPAR、散射FPAR以及总FPAR, 分析影响总FPAR变化的主要因素以及直射FPAR与总FPAR之间的差异。结果表明: FPAR随叶面积指数(LAI)的变化存在饱和现象; 能见度为5, 15, 30 km时散射FPAR对总FPAR的贡献分别是52.6%, 29.3%和21.7%, 随能见度的变化, 直射FPAR与总FPAR间相对误差最大达到13.2%; 随太阳天顶角的变化, 直射FPAR与总FPAR间相对误差最大达到10.29%, 这一误差是由散射FPAR引起的。因此, 在进行植被冠层FPAR遥感反演建模时, 直射FPAR与散射FPAR需要单独建模, 总FPAR则需要根据由天气条件决定的直、散射辐射比例计算得到。

关键词: FPAR, SAIL模型, 直接辐射, 散射辐射