Spatio-temporal variation of vegetation and its correlation with soil moisture in the Yellow River Basin

Analyzing the temporal and spatial changes of vegetation and its correlation with soil moisture and quantitatively revealing the impact of soil moisture on vegetation growth are of certain significance for drought monitoring and ecological protection. However, previous quantitative researches on soil moisture and vegetation growth are not enough. Based on MOD13Q1 NDVI products and global land data assimilation system (GLDAS) soil moisture data, Sen + Mann Kendall trend test and correlation analysis are used to analyze the temporal and spatial variation characteristics of vegetation and the impact of land use change on vegetation growth in the Yellow River Basin from 2000 to 2020. The correlation between vegetation and soil moisture at different depths in growing season is explored. The results show that (1) the vegetation growth is characterized by “high in the south and low in the north”. Along the runoff direction of the Yellow River, the vegetation growth on the right bank of the upper reaches is significantly better than that on the left bank, and there is no significant difference in the vegetation growth on the two banks of the middle and lower reaches. The NDVI increases by 22.19% from 2000 to 2020, with the highest value of 0.435 and the lowest value of 0.356. (2) The order of NDVI value of different land use types from large to small is woodland>cultivated land>grassland>unused land. The order of NDVI value of different seasons from large to small is summer>autumn>spring>winter. (3) Most of the vegetation is in the state of improvement and stability, and a small part is in the state of degradation. The main reason for the degradation is grassland degradation, and urban expansion leads to the degradation of cultivated land and the conversion of cultivated land to construction land. (4) NDVI is positively correlated with soil moisture at different depths (0−10 cm, 10−40 cm, 40−100 cm, 100−200 cm), with the correlation coefficients of 0.535, 0.647, 0.681 and 0.619, respectively. The correlation between NDVI of different land use types and soil moisture of different depths is different. The positive correlation area between NDVI and soil moisture of cultivated land, grassland and unused land is the largest at the depth of 10−40 cm, while the positive correlation area between NDVI and soil moisture of forest land is the largest at the depth of 40−100 cm.