Quantifying the superimposed effects of drought-flood abrupt alternation stress on vegetation dynamics of the Wei River Basin in China

Understanding the response mechanism of vegetation growth to hydro-meteorological events is essential at the global or regional scales, especially in context of global warming and anthropogenic interventions. Previous studies have focused mainly on the responses of vegetation vulnerability to single drought or flood extremes but rarely on those of drought-flood abrupt alternation (i.e., coexistence and rapid transformation of drought and flood, DFAA) events on vegetation growth. Here, this study proposed a binary copula evaluation model to quantitatively explore the internal mechanism of vegetation activity response to DFAA events (flood to drought (FTD) and drought to flood (DTF)) from probabilistic perspective. The Wei River Basin (WRB), the largest tributary of the Yellow River, was selected as a case study. Normalized Difference Vegetation Index (NDVI) was used as a proxy to reveal the spatial-temporal dynamics of vegetation coverage. Then, the response time of vegetation vitality to spring-summer and summer-autumn DFAA events was diagnosed. Finally, the conditional possibility of different vegetation states under DFAA stress was examined. Results showed that: (1) the lag time of optional vegetation response to DFAA events was more than 4 months; (2) FTD and DTF showed drought-flood synergism and antagonism by strengthening and weakening single stress, respectively; and (3) the Jing River Basin is an FTD-sensitive area, whereas as a summer-autumn DTF-compensatory area, the decision-makers should control the stress degree of the upper of the WRB to have a favorable impact on vegetation. Generally, this study sheds new insights into vegetation response to DFAA events, which provides effective theoretical support for decision-makers to formulate effective strategies for DFAA mitigation and sustainable development of ecosystems.