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The Yan'an area, located on the Loess Plateau in Northern Shaanxi Province, China, experienced heavy and prolonged rainfall in July 2013, which induced 8,135 slope failures (loess landslides and loess flow-slides), destroying approximately 10,000 cave dwellings and killing 45 people. Field investigations have indicated that the occurrence of loess flow-slides is closely related to infiltration depth, slope angle, slope morphology, rainfall intensity, and loess strength. The results from this study showed that (a) most of the loess flow-slides in Yan'an were small scale, ranging in volume from tens of cubic meters to hundreds of cubic meters; (b) the loess flowslides only occurred at a depth of\2 m, corresponding to a surface layer of completely saturated loess; (c) most slope failures of loess flow-slides occurred on loess slopes with angles between 35°and 50°; and (d) a reduction in the soil strength of saturated loess was a primary cause of the occurrence of loess flow-slides. However, tree roots may reinforce deeper soil mass ([2 m) and can effectively prevent loess flow-slides.

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The key objective of this paper is to advance our present understanding of how surface water infiltrates in thick unsaturated loess, which is found in arid and semiarid regions of the world, considering the ground‐atmosphere interaction. In situ data for a period of 1 year in thick loess layer at a site in the Loess Plateau of China that has groundwater table at 97.5 m depth were collected for achieving this objective. Climate factors, mainly rainfall and actual evaporation, were measured. In addition, variations of soil temperature and water content at different depths in the unsaturated zone were also measured. The data were used to interpret the water percolation characteristics by dividing the thick unsaturated zone into three zones; namely, (i) surface zone, which constitutes the top 1.0 m, (ii) unsteady zone, which is from 1.0 to 7.0 m, and (iii) steady zone, which is below 7.0 m. In the surface zone, soil temperature and water content are sensitive to climate factors. There is a variation of water content associated with the cumulative influence of infiltration and evaporation in the precipitation and nonprecipitation periods, respectively. In the unsteady zone, the water content is relatively constant; however, temperature varies in different seasons. Water percolation in this zone is both in liquid and vapour phases. In the steady zone, both soil temperature and water content are constant during the entire investigation period. The percolation velocity in this zone is approximately 1.23 × 10−8 m/s or 0.39 m/year, which suggests that it will take approximately 230.8 years for surface water to pass through the thick unsaturated zone and recharge the groundwater.

No abstract