1988-2013年基于BP神经网络的植被叶面积指数遥感定量反演

摘要

Leaf Area Index (LAI) can reflect the horizontal coverage,vertical structure of vegetation,the thickness of the litter layer and the amount of underground biomass,which is the main aspect of vegetation affecting soil erosion.It is very important to monitor the changes in the amount of soil erosion,for useful inforrnation to guide the planning of soil and water conservation,protect the soil and water resources and control the soil erosion.Therefore,the method by which we obtain high quality and long sequential LAI at a regional scale is very important for analyzing the relationship between the dynamic changes in soil erosion and vegetation.Previous studies showed that the neural network had an incomparable superiority in terms of complex,nonlinear data fitting and pattern recognition,and had been successfully applied to inverse the LAI in Nanjing based on the multi-spectral remote sensing data derived from the Landsat 8 Operational Land Imager(OLI),four types of vegetation indices (Normalized Difference Vegetation Index,NDVI;Ratio Vegetation Index,RVI;Soil-adjusted Vegetation Index,SAVI;Modified Soil adjusted Vegetation Index,MSAVI),and measured LAI data.The results showed that the accuracy of retrieval was good.In this paper,we used the Back Propagation(BP) neural network model to inverse the LAI in Nanjing during 1988-2013 based on the data derived from Landsat 8OLI and Landsat 5 Thematic Mapper (TM).Based on the measured values of LAI in 2009 and 2010,the evaluation accuracy and adaptability of the model were verified and discussed.The results showed that:(1) The model had a fitting of higher degree,and average relative errors (MAPE),root mean square errors (RMSE),and correlation coefficients (R) in 2009 and 2010 of 0.2395 and 0.2174,0.2962 and 0.2581,and 0.7713 and 0.6844,respectively.Each accuracy evaluation index was good.(2) Following statistical analysis,we found that the low vegetation coverage area (LAI ＜ 2) exhibited an increasing trend,the high vegetation coverage area(LAI ＞ 3) presented a first decreasing and then increasing trend,while the cultivated land area decreased with the rapid development in Nanjing.(3) To analyze accurately the LAI,we extracted the LAI in the main urban area,and found that there was a relatively high inversion value,and the inter-annual change in LAI was consistent with the change in vegetation coverage in Nanjing reported by other studies.Therefore,we could see that the BP neural network model had a high accuracy for the time series LAI inversion.It provides a new way for quantitative remote sensing monitoring of regional soil erosion.Moreover,because of other potential limiting factors,such as the errors produced by the BP neural network model,the large area of the inversion area,the complexity of vegetation types and community structure,etc.,the inversion accuracy of LAI through remote sensing still needs to be explored,and the inversion method improved.We will try to establish a multi-angle LAI inversion method to construct the coupling model of LAI and soil erosion or quantitative fusion of multi-source remote sensing images in the future study.%叶面积指数(Leaf Area Index,LAI)高度综合了植被水平覆盖状况和垂直结构,以及枯枝落叶层厚薄和地下生物量多少,是植被影响土壤侵蚀的主要方面.区域尺度的时间序列叶面积指数揭示了区域土壤侵蚀的演化过程.因此,及时准确地掌握区域尺度上长时间序列的植被LAI,对研究土壤侵蚀动态变化与植被的关系至关重要.选择南京市1988-2013年10期遥感影像,基于反向传播(Back Propagation,BP)神经网络构建LAI反演模型,进行了长时间序列的叶面积指数反演.结合2009和2010年LAI实测值,验证与探讨了该模型的评价精度与适应性.结果表明:(1)该模型拟合度较高,2009和2010年平均相对误差、均方根误差、相关系数分别是0.2395和0.2174,0.2962和0.2581,0.7713和0.6844,各项精度评价指标均较好;(2)统计分析去除耕地后全市LAI变化,低植被覆盖(LAI＜2)面积不断增加,高植被覆盖区(LAI＞3)面积先减少后增加,耕地面积不断减少,符合南京市的发展变化规律;(3)主城区LAI年际变化与其他学者得到的南京市植被盖度变化趋势一致,反演结果的时序性较高.本文提出的基于反向传播神经网络模型反演长时间序列LAI是可行的,为区域尺度土壤侵蚀定量遥感监测提供新途径.