黄土残塬沟壑区范围界定及近21年来NDVI时空变化特征

为探究黄土残塬沟壑区退耕还林（草）工程等林业生态工程实施后林业资源恢复情况,以黄土高原DEM数据及2000—2020年归一化植被指数NDVI数据为基础,采用地形因子计算、水文分析、空间叠加分析等方法,划分了较为完整的黄土残塬沟壑区的范围,并利用趋势分析法、变异系数的相关理论与方法,分析了近21年来黄土残塬沟壑区NDVI时空变化特征。结果表明：（1）黄土残塬沟壑区横跨山西、陕西和甘肃3省,面积约为2.99万km²,沟壑密度为1.91~3.21 km/km²;（2）黄土残塬沟壑区NDVI从时序变化上看,全区21年总平均NDVI值为0.711,2000—2020年该区植被NDVI变化趋势呈快—慢—较快增长;从空间分布来看,植被覆盖度NDVI值总体为0.6~0.9,研究区中部地区植被覆盖度较其他地区高;（3）在时空趋势特征方面,研究区西部和东部部分地区植被覆盖改善程度明显,中部地区植被覆盖情况较为稳定;在时空波动特征方面,该区植被NDVI时序波动稳定,空间波动差异性较大,低波动区域面积占比为48.06%。整体而言,黄土残塬沟壑区主要分布在山西、陕西和甘肃3省,该区2000—2020年间植被NDVI整体呈上升态势,植被覆盖水平整体较高且波动较低,并呈持续改善趋势。研究结果可为黄土残塬沟壑区水土保持工作提供理论基础和科学依据。

Determining the Scope of the Residual Gully Region on the Loess Plateau and the Characteristics of Temporal and Spatial Changes of NDVI

In order to explore the restoration of forest resources after the implementation of forestry ecological projects such as Grain for Green Project in the residual gully region of the Loess Plateau, the calculation was based on the DEM data of the Loess Plateau and the normalized vegetation index NDVI data from 2000 to 2020. The calculation of topographic factors, hydrological analysis, spatial overlay analysis, etc, were used to divide the relatively complete range of the residual gully region of the Loess Plateau area. The trend analysis and coefficient of variation were used to analyze the characteristics of the temporal and spatial changes of NDVI in the residual gully region of the Loess Plateau area. The results showed that: (1) The residual gully region of the Loess Plateau area spanned three provinces of Shanxi, Shaanxi, and Gansu, covering an area of approximate 29 900 km², and the gully density was between 1.91~3.21 km/km². (2) The NDVI of residual gully region of the Loess Plateau area varied with time, the total average NDVI value of the region for 21 years was 0.711, and the change trend of vegetation NDVI in this area from 2000 to 2020 showed a fast-slow-fast growth. In terms of spatial distribution, the vegetation coverage was generally in the range of NDVI value 0.6~0.9. The vegetation coverage in the central part of the study area was higher than those of other areas in the study area. (3) In terms of temporal and spatial trend characteristics, the vegetation coverage in the western and eastern parts of the study area had improved significantly, and the vegetation coverage in the central area of the study area was relatively stable. In terms of temporal and spatial fluctuation characteristics, the vegetation NDVI in this area fluctuated steadily in time series, and the difference in spatial fluctuation was large, and the area of low fluctuation area accounted for 48.06%. On the whole, the residual gully region on the Loess Plateau was mainly distributed in three provinces: Shanxi, Shaanxi and Gansu. From 2000 to 2020, the NDVI of vegetation in this area showed an overall upward trend. The overall vegetation coverage level was relatively high with low fluctuations, and showed a trend of improvement. These results could provide theoretical and scientific basis for soil and water conservation in the residual gully region on the Loess Plateau.