Since implementation of the project of returning farmland to forest and grassland in the Loess Plateau, vegetation cover of the region has been greatly changed. To effectively monitor variation of the vegetation cover, this paper constructs a yearly maximum normalized difference vegetation index (NDVI) sequence covering Loess Plateau from 1982 to 2014 using the per-pixel unary linear regression model (LRM). The work is based on AVHRR GIMMS NDVI datasets from 1982 to 2006 and MODIS NDVI datasets from 2001 to 2014. The constructed NDVI has passed consistency check, and is used to analyze spatial-temporal characteristics of vegetation NDVI in the past 33 years. The following results are obtained. The overall vegetation NDVI of the Loess Plateau in the past 33 years has been increasing. Spatial distribution of the vegetation cover in the regions is significantly different. Vegetation cover has slightly degraded in most southern and western parts of the area under study, and that in the central region is almost unchanged. The vegetation cover shows an improving trend in some areas including the border of Inner Mongolia, Lanzhou, Gansu and Yulin region of Shaanxi. Vegetation covers of different slopes are good. Degrees of changes in areas with different slopes are ranked in a descending order as above 28°, 20°~28°, 14°~20°, 9°~14°, 4°~9°. The vegetation cover in sloping fields of middle part of the Loess Plateau is most improved. Slight improvement of the vegetation cover has been found in areas including eastern Gansu, junction of Gansu and Qinghai provinces, southern Shaanxi, and the cities Linfen, Yuncheng and Sanmenxia of Shanxi, showing positive results in controlling loss of water and soil.
