基于野外观测和TM数据的锡林浩特典型草原MODIS/LAI产品验证

Research on validating MODIS/LAI product in a typical semi-arid steppe of Xilinhot in Northern China

通过2008年夏季生长季的3期LAI野外观测和TM图像的处理分析,对锡林浩特国家观象台附近24 km2典型草原区的MODIS/LAI产品精度从时空2种维度进行了评估。结果显示,在时间维度上,MODIS/LAI产品可以较为准确的体现草地植被生长轨迹;在空间维度上,MODIS/LAI产品与TM/LAI反演结果有相似的变化趋势。以TM/LAI结果为参考真值,MODIS/LAI产品的相对均方根误差约为24%,并存在10.8%的正偏差,表明有一定程度的高估。误差分析结果显示,MODIS/LAI产品精度主要受到土壤背景光谱以及地表均匀程度的影响,在LAI较小的区域,以及在草地空间异质性因受人类活动而增强的7月初和8月末,MODIS/LAI产品误差更大。

MODIS/LAI product in a typical semi-arid steppe of Xilinhot in Northern China was validated based on three comprehensive field surveys which were carried out during July-August 2008 in the study area.LAI was measured in 25 m×25 m plots by a destructive harvest method of 80 intensive plots and 20 scattered plots.Three TM/ETM＋ images acquired during the field survey periods were collected and processed.The validation work covered two parts： Time series analysis and spatial pattern analysis.In time series analysis,MODIS/LAI track in the intensive sampling region extracted from DOY169 to DOY241 were compared with ground measured values.In spatial pattern analysis,LAI map with 30 m spatial resolution were first derived by building a regression line between LAI and TM band 5（1.6 μm）,and then aggregated to 1 km spatial resolution based on MODIS grids.Results indicated that in the temporal domain MODIS/LAI product can correctly reflect grass growth trends;In the spatial domain it showed a similar pattern with TM/LAI（1 km）.Using the TM/LAI as reference,the relative RMSE of MODIS/LAI was about 24% and a positive bias（10.8%） existed,suggesting slight over-estimation.Error distribution analysis revealed that the MODIS/LAI product was less accurate in regions with lower LAI values,probably due to soil contamination.Moreover,the relative RMSE of the MODIS/LAI product was higher at the beginning of July and the end of August when the heterogeneity of grass cover increased as a result of human activities.