1982—2022 年黄河中上游旱涝急转事件变化特征及其对植被覆盖的影响

Global climate change has led to frequent occurrence of compound extreme events. Clarifying the temporal and spatial characteristics and their effects on vegetation in ecologically fragile areas of compound events such as dry-wet abrupt alternations can provide scientific basis for safeguarding regional ecological security. By establishing the Long-cycle Drought-Flood Abrupt Alternation Index (LDFAI) and applying the Theil-Sen slope estimation, Mann-Kendall test and Pearson correlation analysis, we reveal the temporal and spatial change characteristics of dry-wet abrupt alternations and NDVI in the upper and middle reaches of the Yellow River from 1982 to 2022, and explore the impacts of these events on vegetation cover. The results indicate that: 1) An dry-wet abrupt alternations occurred in the upper and middle reaches of the Yellow River every 2-3 years on average, and the frequency was higher in the Loess Plateau and the eastern Qinghai-Tibet Plateau. The intensity of these events decreased significantly after 2003. 2) The annual average NDVI of vegetation in the upper and middle reaches of the Yellow River increased at a rate of 0.0132/10a, especially with a significant growth rate after 2003 (P<0.05). The vegetation coverage in the Loess Plateau and Fenwei Plain continued to increase, and the vegetation degradation and improvement coexist in the Yellow River source area and the Ordos Plateau. 3) The dry-wet abrupt alternations had different effects on different types of vegetation in the upper and middle reaches of the Yellow River. In arid and semi-arid regions, drought-to-flood events can rapidly alleviate drought stress on herbaceous plants, thereby promoting their growth. Although herbaceous plants possess certain long-term drought tolerance, the sharp deterioration in water availability caused by flood-to-drought events still significantly compromises their growth potential. Forests and shrublands in areas such as the Loess Plateau and the Yin Mountains exhibit strongly constrained growth during flood-to-drought events. In regions with greater anthropogenic intervention, such as the Fenwei Plain, cultivated vegetation exhibits a relatively weaker degree of impact from Dry-Wet Abrupt Alternation events. This resilience is primarily attributable to the beneficial effects of human activities and high-quality conservation projects.