Responses of stable vegetation to drought accumulation timescales and time lags in the Weihe River Basin

Abstract

This study aimed to distinguish drought accumulation timescale from response timing in stable vegetation in the Weihe River Basin. We analyzed MODIS enhanced vegetation index (EVI) and ERA5-Drought standardized precipitation evapotranspiration index (SPEI) data for 2003–2024 on the native 0.25° drought grid. Persistent land-cover classification required at least 18 valid annual classifications and a modal-class fraction of at least 80%. Four accumulation timescales (1, 3, 6, and 12 months) and 13 window-end lags (0–12 months) formed 52 response–exposure combinations. We controlled for growing-season month effects and a linear trend. We applied maximum-statistic tests with synchronized 2-year moving-block resampling and controlled the false discovery rate across cells using the Benjamini–Hochberg procedure. The area-weighted mean of grid-cell partial correlations peaked at SPEI-12, lag 0 (r = 0.3216), with a mean slope of 0.00971 EVI units per SPEI unit. Of 190 eligible cells, 126 showed a corrected positive association, covering 69.68% of eligible stable-vegetation area. Grasslands and high-elevation areas had stronger descriptive responses. Relaxing valid-area coverage from 0.70 to 0.50 retained the basin-wide peak. Across detected cells, the median bootstrap reselection frequency of the observed best timescale–lag combination was 53.1%. Across 2,252 SPEI-3 grid-cell drought events, the mean EVI residual reached −0.00909 one month after onset (95% onset-year block-bootstrap interval, −0.01448 to −0.00439). Among 2,105 uncensored events, meteorological drought termination was confirmed a median of 3 months after the last dry month (interquartile range, 2–3 months). The dominant association links vegetation condition to the 12-month water balance ending in the response month. These findings support monitoring that combines long-term water balance, early vegetation responses, and continued vegetation observations after meteorological drought termination.