城市人工林LAI季节变化对地表水热通量的影响

Effects of seasonal LAI change on surface water and heat fluxes over urban forest

为定量评价北京城区绿化人工林叶面积指数LAI变化对改善生态环境效果的影响。2006年4月～2007年3月在北京海淀公园人工林架设涡度相关系统连续观测地表潜热和显热通量,提出LAI-2000实验观测LAI与遥感测量林地面积的加权平均法以估算整个林地LAI,研究LAI季节变化对通量的影响。结果表明:随LAI的增长,潜热占净辐射的比例增加而显热占净辐射的比例减少,具有明显的月变化特征;6～8月LAI>2.0,潜热占净辐射的比例最高,晴天在60%以上,阴天在45%以上,而显热占净辐射的比例最低,晴天低于10%,阴天约5%。12～2月LAI<0.5,潜热占净辐射的比例最低,晴天低于10%,阴晴天的差异很小;显热占净辐射的比例最高,晴天高于40%,而阴天略低于晴天。

To evaluate the effect that leaf area index（LAI） change of forest impact on improving entironment in downtown Beijing,surface latent heat（LE） and sensible heat（H） fluxes were measured with eddy covariance over the forest in Beijing Haidian Park from April 2006 to March 2007.Weighted average method was proposed to estimate the whole forest LAI with LAI-2000 measurements and forest areas from remote sensing image.Result shows that the proportion of LE to net radiance（LE/Rn） increases and the proportion of H to net radiance（H/Rn） decreases with LAI increase.The effect appears visible monthly change.As LAI is above 2.0 during June to August,LE/Rn reaches its peak and exceeds 60% in a clear day or 45% in a cloudy day.Meanwhile,H/Rn drops to its bottom and is below 10% in a clear day or about 5% in a cloudy day.However,while LAI is below 0.5 during December to February,LE/Rn falls to minimum and is below 10% in a clear day,shows little difference between a clear day and a cloudy day.H/Rn reaches maximum and is above 40%,a little higher in a clear day than in a cloudy day.