用于植被变化归因的区域机器学习残差趋势法
Residual trend method based on regional modeling and machine learning for attribution of vegetation changes
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摘要: 现有的残差趋势法采用逐像元建模策略, 利用普通最小二乘法构建模型, 存在着一定的局限性: 一方面, 逐像元建模策略使每个模型都包含了局部空间内的人类活动信号干扰; 另一方面, 普通最小二乘法不利于模拟普遍存在的非线性特征。因此, 该文提出一种全新的基于区域建模策略和机器学习算法的残差趋势法, 并对比了用于表达空间异质性的2种环境变量: ①地形、水文和土地利用等直接环境变量(direct-environmental variables, DEVs); ②植被和气候时空序列组合的代理环境变量(proxy-environmental variables, PEVs)。首先, 采用区域建模策略, 分别引入DEVs和PEVs, 使用机器学习算法构建植被-气候关系模型; 其次, 根据残差趋势法的定义得到残差值; 最后, 评估气候和人为因素对植被变化的贡献。结果表明: ①相比以往的逐像元普通最小二乘残差趋势法, 所提方法的优势不仅表现为机器学习能够模拟植被-气候关系的非线性特征, 还表现为区域建模具备更强的抗人类信号干扰能力; ②新方法中, 使用PEVs明显优于使用DEVs, 前者充分利用了原有建模数据, 没有增加数据获取难度, 也避免了引入额外的数据误差。该文提出的区域机器学习残差趋势法可以实现更有效的植被变化归因。Abstract: Existing residual trend methods utilize a pixel-by-pixel modeling strategy, in which the ordinary least squares method is employed. These methods suffer certain limitations. On the one hand, the pixel-by-pixel modeling strategy causes each model to contain signal interference from human activities in local space. On the other hand, the ordinary least squares method is unfavorable for simulating commonly observed nonlinear characteristics. This study proposed an entirely new residual trend method based on regional modeling and machine learning. Besides, this study compared two types of environmental variables used to express spatial heterogeneity: ①direct-environmental variables (DEVs) such as terrain, hydrology, and land use; and ②proxy-environmental variables (PEVs) that combine the spatiotemporal series of vegetation and climate. First, a regional modeling strategy was adopted. After DEVs and PEVs were introduced individually, models for the vegetation-climate relationship were built using machine learning. Second, residuals were determined based on the definition of the residual trend method. Finally, the contributions of anthropogenic and climatic factors to vegetation change were assessed. The results indicate that compared to the previous pixel-by-pixel residual trend method that utilizes ordinary least squares, the new residual trend method can simulate the nonlinear features of the vegetation-climate relationship and exhibits enhanced resistance to human signal interference. For the new method, significantly higher performance can be achieved using PEVs compared to DEVs. PEVs can fully utilize the original modeling data, without increasing difficulties with data acquisition and avoiding additional data errors. The residual trend method based on regional modeling and machine learning proposed in this study allows for more effective attribution of vegetation changes.
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