Study on spatiotemporal heterogeneity and driving factors of soil drought in Sandy Lands based on the HMS-DF: A case study of the Mu Us Sandy Land

Mu Us Sandy Land, China. The increasingly severe risk of drought poses a serious threat to the sustainable development of agriculture. Precise drought monitoring and ecological risk management are crucial for optimizing ecological restoration and water resource allocation. However, research on the soil drought characteristics at different soil depths in the Mu Us Sandy Land (MUSL) is still insufficient. By constructing the Hierarchical Multi-scale Drought Framework (HMS-DF), analyzes the temporal and spatial evolution, response characteristics, and driving mechanisms of drought in MUSL. Key findings reveal a vertical stratification of drought processes: (1) A significant wetting trend is strongest and most stable in deep layers, which act as a hydrological memory bank. (2) Drought duration and intensity exhibit an inverse vertical relationship: surface droughts are short but intense, while deep droughts are prolonged with high cumulative severity. (3) The root zone layer is the key hydrological interface, showing the strongest coupling with deep soil moisture, a linkage that strengthens over longer time scales. (4) Drivers are scale- and depth-dependent: surface drought is controlled by local hydrology and rapid atmospheric forcing, whereas deep drought is governed by accumulated soil water and low-frequency climate variability. These insights underscore the necessity of a stratified, multi-scale framework for accurate drought monitoring in this region. The research process of this paper is divided into the following parts: (1) Conducting spatio-temporal analysis of the drought index through MK test and EOF method; (2) Extracting features based on the operational theory; (3) Determining the vertical propagation time characteristics based on multiple time-scale drought indices, and exploring the response and driving relationship of drought. • Spatial-temporal analysis of droughts employs M-K and EOF methods. • Analysis of SSMI based on the hierarchical multi-scale drought framework. • Surface droughts are short and intense, while deep-layer ones are prolonged and extreme. • The rhizosphere layer serves as the hub and is most closely coupled with the deep zone. • The causal relationship of drought varies with depth and scale.