Spatiotemporal Evaluation of Vertical Dynamics Propagation of Flash Drought and Driving Mechanisms in the Indus Basin in South Asia (1970–2023)

Abstract. Flash drought (FD) leads to relatively short periods of anomalously low and rapid decreasing soil moisture (SM), which can significantly affect vegetation growth and ecosystem. However, the vertical propagation characteristics and driving mechanisms of FD through soil columns remain largely unknown, which is crucial for guiding agricultural and ecological disaster prevention and reduction. Here, we present a multi-layer FD evaluation method to explore the vertical propagation of FD at different soil depths (0–10 cm, 10–40 cm, and 100 cm), and based on the GLDAS data we comprehensively evaluate the spatiotemporal dynamics and driving mechanisms of FD during 1970–2023 in the Indus Basin, a highly climate-sensitive region. We find that the frequency of FD decreases with increasing soil depth, while the relationship between the rate of intensification (RI) and drought severity varies with soil depth, with stronger correlation (r²>0.9) in the middle and root zone soil layers than in the upper layer. We further identify 2148 simultaneous events (‘t’) and 1154 subsequent events (‘t+1’) between the upper and middle soil layers, which mostly occur during spring and early summer. The temporal differences in the ‘t+1’ FD events are closely related to the persistence of meteorological conditions. In contrast, the ‘t’ events are caused by the simultaneous depletion of SM from the upper layer to the deeper layer, indicating the rapid development of FD conditions due to deeper moisture loss. The analysis also highlights the significant spatial heterogeneity of FD characteristics, with the humid and sub-humid regions in the middle Indus basin being the most sensitive to FD, and precipitation deficit and high temperature are the dominant driving forces for FD occurrence.