Evolution of vegetation index and contribution quantification of influence factors in the middle and lower reaches of the Heihe River basin

Understanding the evolution of vegetation cover and associated driving mechanisms are crucial for ecological security in arid inland river basins. Impacts of natural factors and human activities are particularly crucial for vegetation restoration. This study investigated the middle and lower reaches of the Heihe River Basin. Spatiotemporal variations of the Normalized Difference Vegetation Index (NDVI) from 2000 to 2022 and associated driving factors were studied using the Mann-Kendall test, Theil-Sen Median trend analysis, and time-lag correlation analysis. Contributions of natural factors and human activities to NDVI changes were quantified by multiple regression models. The spatial variability of NDVI changes in the study area was found significant, with slight improvement in the irrigation area in the middle reaches and the right bank in the lower reaches, accounting for 76.6%. Slight degradation in the left bank was identified in the lower reaches, accounting for 23.4%. Climate warming and increasing runoff from the upper reaches was found to have a positive effect on vegetation restoration. There was a 4-month lag effect of surface and root soil moisture on NDVI. The proportion where natural factors contribute more than 65% to NDVI changes in the study area was 26.7%. The proportion where human activities contribute more than 65% was 58.0%. Human activities contributed more to NDVI changes on the right bank in the lower reaches, while natural factors contributed more to the left bank.