Analysis of the vertical propagation process from meteorological drought to soil drought in the southern margin of the Mu Us Sandy Land

To investigate the vertical propagation of meteorological drought into vadose zone soil drought at the southern margin of the Mu Us Sandy Land in the Yellow River Basin, this study utilized the Hydrus-1D model to simulate soil moisture dynamics at 0-150 cm depth from 1980 to 2020, based on in-situ monitoring data from grassland and bare land as well as long-term meteorological data. Using the standardized precipitation evapotranspiration index (SPEI), the past four decades were categorized into alternating wet-dry periods (S1), drought periods (S2), and wet periods (S3). Combined with the soil moisture deficit index (SMDI), methods including maximum correlation coefficient analysis, wavelet coherence analysis, and drought propagation indices were employed to explore drought propagation processes in grassland and bare land under different climatic regimes. The results indicated that meteorological drought propagation time to soil drought increased with soil depth. Propagation intensity generally weakened with depth, but the root zone of grassland exhibited weaker drought propagation intensity compared to other depths due to root distribution. Under different land cover conditions, grassland soil drought during the same period was more severe than bare land, characterized by longer drought duration, higher intensity, and deeper affected soil layers. Grassland responded more sensitively to meteorological drought-wet shifts, with drought propagation times 1-2 months faster than bare land across depths, and stronger propagation intensity. Drought propagation processes were influenced by climatic regimes. During S3, propagation time was shortest due to high precipitation and temperature, but propagation intensity was strongest in S1, followed by S3 and weakest in S2, owing to variations in evapotranspiration. Vertical drought propagation was jointly regulated by hydrological connectivity, root distribution, and evapotranspiration intensity. Given the stronger propagation intensity in grassland, vegetation density was suggested to be controlled in desertification control and ecological restoration projects to mitigate soil water overconsumption. The research results reveal the vertical propagation process from meteorological drought to soil drought, and illustrate the synergistic influence of vegetation cover and climate model on drought propagation process, which provides theoretical reference and practical basis for the work of desertification control in Mu Us Sandy Land.