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The development of agriculture in the arid and semiarid regions of China mainly depends on agricultural irrigation. Until 2016, water required for agricultural irrigation has accounted for more than 90% of the total water consumption. But traditionally extensive broad irrigation causes frequent loess geological hazards in irrigation area and it threatens security of local life and property. In this paper, we selected the Heifangtai irrigation district in Yongjing County, Gansu Province, where frequent instabilities occur, as the study area. We used laboratory tests and numerical simulation to examine the mechanism of loess landslides owing to the irrigation hydrological cycle. Irrigation changes the local natural hydrogeological conditions because of the loose and macroporous structure of loess. The numerous pores and fissures constitute preferential migration pathways of irrigation water; thus, irrigation can increase the groundwater level and hydraulic gradient. Broad irrigation is the main inducing factor of geological hazards (including landslides, collapses, and soil salinization) in arid and semiarid regions, and the development of fissures and sinkholes increases the risk of landslides. New water-saving irrigation methods need to replace the traditional irrigation methods and improve the utilization of water resources. A monitoring and warning system in susceptible areas should be established to ensure the sustainability of local agriculture. [ABSTRACT FROM AUTHOR]

Abstract Agricultural irrigation in the Loess Plateau induces many loess landslides. Long-term flood irrigation is frequent on the top platform of loess slopes, where many landslides occur. To confirm the relationship between irrigation and landslides, we monitored the soil moisture content and groundwater level in Heifangtai Platform, and performed tests on unsaturated soil, including TRIM (Transient Release and Imbibition Method) and triaxial shear test. Then, we simulated the changes of the seepage field on slopes for flood irrigation at different locations and analyzed the effect of flood irrigation on slope stability. Monitoring results show that flood irrigation affects soil moisture content far more than rainfall. With the increasing depth, the response time of the change of soil moisture content increases gradually. The lag effect is obvious. During irrigation, at a depth of 0.1–0.2 m, the soil moisture content increases sharply; at a depth of 0.5–3 m, it increases gradually under different degrees; and at a depth of 5 m, it increases little. After irrigation, the soil moisture content decreased rapidly at a depth of 0.1 m and 0.2 m and returned to the average level nearly a month later. However, the soil moisture content continued to increase slowly over a period at a depth of 0.5–3 m. During the monitoring period, the cumulative rainfall was <110 mm, and the depth affected by rainfall is not >0.5 m. These data show the change of groundwater level is out of sync with irrigation activities and has a visible lag effect. According to the results of seepage and stability analysis, short-term irrigation has little impact on the groundwater level. However, it affects the flow field and causes the matric suction of the superficial loess to decrease. Long-term irrigation causes the groundwater level to increase locally. Compared with single irrigation of high intensity, the reduction of slope stability is more evident under long-term irrigation of low intensity. Changing the irrigation methods to reduce deep infiltration might be one useful precaution to minimize sliding. The closer the distance of the irrigation location to the slope edges is, the higher the effect of flood irrigation on the distribution of pore pressure and matric suction will be. Irrigation near potential slide sites has an apparent cumulative effect and lag effect on slope stability. The longer the irrigation time and the closer to the edge of the slope the irrigation is, the higher the effect on slope stability will be. Avoiding flood irrigation near or at the potential sliding region may reduce the frequency of landslides. Highlights • Data of irrigation infiltration is obtained and infiltration process is described. • A new method for unsaturated stability analysis is proposed. • The effect of long-term irrigation of low intensity on stability is identified. • Safe distance of irrigation to the slope edge is proposed. [ABSTRACT FROM AUTHOR]