State-dependent drought propagation from soil moisture to vegetation under a non-stationary climate

Abstract

Drought propagation from soil moisture to vegetation is a complex process influenced by ecosystem characteristics and climate. The combined effects of ecosystem characteristics and a non-stationary climate on this process remain poorly understood. In this study, the dynamics of drought propagation across Iran from 2000 to 2025 were analyzed via ERA5-Land soil moisture and MODIS normalized difference vegetation index (NDVI) data. Ecosystem-specific root-zone soil moisture was estimated for forests, grasslands, and croplands, and propagation dynamics were quantified with lagged cross-correlation analyses under both stationary and moving-baseline frameworks. State-dependent responses were examined under dry, normal, and wet conditions. Propagation behavior differed markedly among ecosystems. Croplands exhibited the strongest association and the shortest observed lag ( r  = 0.138, lag = 0 months), grasslands showed an intermediate response ( r  = 0.091, lag = 1 month), whereas forests displayed the weakest but most delayed coupling ( r  = 0.042, lag = 12 months). Under the 10-year moving-baseline framework, peak correlations increased from 0.138 to 0.254 in croplands, from 0.091 to 0.200 in grasslands, and from 0.042 to 0.161 in forests, while the dominant propagation lags remained unchanged for forests and croplands. These results indicate that drought propagation is strongly state-dependent and that evolving climatological baselines primarily modify propagation strength rather than propagation timing.