Dense vegetation as a natural buffer: Mitigating and decelerating drought propagation.

Effective management of soil drought requires a comprehensive understanding of drought propagation dynamics, particularly in the context of increasing vegetation. This study investigates the relationship between meteorological and soil droughts in the Red River Basin (RRB), Southwest China, using monthly Standardized Precipitation Evapotranspiration Index (SPEI) and Soil Moisture Index (SMI) data from 2000 to 2021. A novel peak backtracking method was developed to quantify key propagation metrics: propagation time (PT), triggering threshold (TR), and propagation rate (PR). Granger causality analysis was applied to identify factors driving these dynamics. The key findings are as follows: (1) The average drought propagation rate in the RRB is 64.64 %, with a mean propagation time of 3.3 months and a triggering threshold of approximately -0.72. (2) Vegetation factors, particularly NDVI, are the dominant driver of changes in drought propagation characteristics, demonstrating stronger explanatory power than NPP. (3) Enhanced vegetation cover intensifies the decoupling between soil moisture and meteorological droughts, leading to slower propagation rates. High-vegetation areas experience a 13 % decrease in propagation rate and a 0.34-month increase in propagation time compared to low-vegetation areas. (4) The turning points in non-linear analysis results occurred at NDVI value of 0.419 and 0.514 respectively. Alleviation can only appear when the vegetation cover surpasses the turning point. These findings underscore the role of vegetation restoration in improving soil properties such as water retention and porosity, thereby mitigating drought propagation. They highlight the importance of afforestation and sustainable land management in reducing drought risks, particularly in ecologically sensitive regions like Southwest China.