基于Google Earth Engine分析黄土高原植被覆盖变化及原因

为探明黄土高原植被覆盖时空变化及其原因，基于Google Earth Engine（GEE），采用Landsat Surface Reflectance data（陆地卫星地表反射率数据）分析了黄土高原1987~2015年间植被覆盖度的时空变化规律，并借助累积量斜率变化率方法对引起植被覆盖度变化的气候和人为因素进行了量化分析.结果表明：黄土高原年均植被覆盖度由1987年的41.78%增加到2015年的53.23%，增速为0.38%/a（P<0.05）.其中，1987~1999年年均植被覆盖度变化趋势不显著（P>0.05）；而退耕还林还草工程实施以来（2000~2015年），年均植被覆盖度显著增加（P<0.05），增速达到0.59%/a.由像元尺度分析，黄土高原72.93%的区域植被覆盖度呈增加趋势，其中38.31%的区域增加趋势显著（P<0.05）.植被覆盖度的变化受气候和人为因素的共同影响，以1987~1999年为基准期，气候变化和人类活动对黄土高原2000~2015年间植被覆盖度变化的相对贡献率分别为23.77%、76.23%，人类活动为引起黄土高原植被覆盖度变化的主要原因.退耕还林还草工程极大地改善了黄土高原的植被覆盖状况，但是城市的扩张使得部分地区的植被覆盖呈显著退化现象.

Analyzing vegetation coverage changes and its reasons on the Loess Plateau based on Google Earth Engine

Based on Google Earth Engine, this study conducted a quantitative analysis for the spatiotemporal variation of vegetation coverage from 1987 to 2015 on the Loess Plateau using Landsat Surface Reflectance data, and the method of slope change ratio of cumulative quantity was used to evaluate the relative contributions of climate change and human activities to it. The results revealed that:the annual vegetation coverage significantly increased from 41.78% in 1987 to 53.23% in 2015 (P<0.05), especially after the implementation of the Grain for Green Program (2000~2015) with a significant increase of 0.59%/a (P<0.05). However, no significant change was observed for the vegetation coverage during 1987~1999 (P>0.05). At a pixel scale, about 72.93% of the research area showed an increasing vegetation coverage and 38.31% of the research area had a significant increase in vegetation coverage (P<0.05). Vegetation coverage changes was affected by both climate and human factors. Taking 1987~1999 as reference period, the relative contributions of climate change and human activities to vegetation coverage changes were 23.77% and 76.23% during 2000~2015, respectively. Therefore, human activities were the dominant factors for vegetation coverage changes on the Loess Plateau. The Grain for Green Program has greatly improved the vegetation status on the Loess Plateau, but the expansion of city has caused significant degradation of vegetation coverage in some areas.