Although vegetation is increasingly used to mitigate landslide risks, how vegetation roots affect the landslide threshold of slope has rarely been explored, particularly in the case of lateral runoff. In this study, we established a two-dimensional saturated-unsaturated infiltration equation considering the hydraulic effects of vegetation roots. The analytical solution for the shallow unsaturated two-dimensional coupled infiltration of vegetated slope (VS) was obtained by a Fourier transform technique. The numerical method was used to evaluate the stability of VS caused by four root architectures, the rainfall amount, and the rainfall duration. Subsequently, the transformation law in runoff, vegetation evaporation, and landslide threshold was analyzed. The results indicate that the factor of safety (FOS) increases with increasing drying time and decreases with increasing depth; the minimum FOS is at the junction of the root-rootless zone. Runoff and vegetation evaporation are favorable for the shallow stability of VS. The time of the safe area is 35 h for rainfall amount 500 m in the uniformly root clay slope. Moreover, four landslide threshold curves that reflected the root architecture, rainfall amount, and rainfall duration are developed, which are more realistic than those created using one-dimensional instability modeling.

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As an ecological subsystem, a small watershed is mainly located upstream from lakes, rivers, or other water bodies. The characteristics of non-point source (NPS) pollution in a small watershed are random and complex. Rainfall is the direct driving force of NPS pollution, and different land-use types are the main factors affecting NPS output in small watersheds. At present, the NPS pollution of small watersheds is serious, and the problem of eutrophication of watershed water is prominent. Nitrogen (N) and phosphorus (P) are essential nutrients for aquatic organisms, but excessive amounts can lead to water pollution and ecological imbalances. The study of N and P loss in small watersheds can provide a decision-making basis for NPS pollution control in small watersheds. This paper introduces the research progress on small watersheds in detail, focusing on the main influencing factors of N and P output in small watersheds, including rainfall, different land-use types, N and P loss prevention, and control measures; it also provides a prospective view of the current problems, hoping to provide references for the study of NPS pollution in small watersheds.