Driving mechanisms of drought triggers in response to vegetation loss across the Weihe River Basin.

The interplay between vegetation and meteorological drought within the growing season involves complex dynamics. A key aspect of evaluating ecosystem stability lies in precisely determining the drought trigger threshold that leads to vegetation degradation. However, the dynamic evolution pattern and driving mechanisms of the drought trigger threshold remain unclear. To address this, the present study was conducted in the Weihe River Basin (WRB), with the core growing season (defined as May to September) serving as the temporal scope. A novel assessment framework was established based on the Copula joint function to determine the drought threshold triggering vegetation loss, where the drought trigger threshold was defined as the Standardized Precipitation Evapotranspiration Index (SPEI) at which the conditional probability of vegetation loss first reached or exceeded 0.5. Subsequently, the dominant drivers of the threshold's regional differentiation were investigated by combining the CatBoost model with the SHapley Additive exPlanations (SHAP) method. The results show that: (1) Vegetation response time to drought shows clear seasonal and spatial patterns. Seasonally, the share of 1-6 month rapid response rises from 22% in May to 52% in June-July, before shifting to 9-24 month longer lags in August-September. Spatially, rapid response is found in the northwest loess hilly area, whereas prolonged lags are found in the Guanzhong Plain and Qinling Mountains. (2) The drought trigger threshold for vegetation loss shows clear spatial and seasonal patterns. Upstream forests endured severe drought down to - 1.9, whereas downstream agriculture was sensitive to mild drought above - 0.9. Drought tolerance peaked in midsummer at - 1.67 during June-July and weakened to - 1.27 in May and - 1.23 in September. (3) Water factors are the main drivers of the drought threshold. Precipitation (PRE) dominates throughout the growing season. The findings establish a scientific foundation for managing ecological risks and optimizing water resources within river basins.