Vegetation change and its influence on runoff and sediment in different landform units, Wei River, China

China has been ranked first in its contribution to the newly increased green areas in the world since 2000. Because of the significant differences in natural and geographical conditions in China, the spatial variation of vegetation varies greatly. We selected the Wei River basin in the transition zone between the Qinling Mountains and the Loess Plateau as the research object. Based on MOD13Q1 normalized difference vegetation index (NDVI) data, the characteristics of and factors influencing vegetation restoration, and their effects on the process of runoff-sediment under various landform units (Loess Plateau, Guanzhong Plain and Qinling Mountain areas) were studied using statistical and trend analyses. Our results indicated that the NDVI in 68.02% of the total research area had significantly increased (p < 0.05), with the fastest vegetation restoration in the loess area. The Theil Sen slopes in the loess, mountain and plain areas were 0.0095/year, 0.0049/year and 0.0013/year, respectively, with significant differences among them (p < 0.05). In the loess area, vegetation restoration rate was slightly higher in the zone with an elevation of 1000-2500 m than 500-1000 m. The fastest vegetation restoration in the mountains was in mountainous areas at low altitude, and the rate tended to decrease with greater elevation. In addition, the fastest vegetation restoration in the loess and mountain regions was in areas with gentle slopes of 5-15 degrees. The relationship between vegetation and precipitation was stronger in the mountains than in the loess area. The results also revealed that improvement in vegetation coverage of cultivated and grassland was the leading cause of vegetation restoration in loess and mountain areas, but urbanization was the main factor affecting vegetation degradation in the plain. In the context of vegetation restoration, change in precipitation was still the dominant factor affecting variation in runoff and sediment load. In the loess area, vegetation restoration has a greater impact on sediment load, while in the Qinling Mountains, vegetation restoration mainly affects runoff.