Abstract: Agricultural irrigation in loess platform has led to a continuous rise of the groundwater level and triggered a series of loess landslides. To gain a better understanding of the infiltration behavior of irrigation water in loess, a field infiltration test with a diameter of 20 m was conducted on the South Jingyang tableland, Shaanxi province. The spatiotemporal characteristics of moisture content and matric suction were monitored. Then the infiltration laws under the conditions of soaking and intermittent irrigation were simulated. Results show that the soak infiltration can be divided into three stages, namely, uniform infiltration, preferential flow infiltration, and stable infiltration, respectively. Preferential flow associated with the vertical cracks in the Malan loess was observed when the width of the crack was more than 2 mm and the soil above the crack was saturated. The hydraulic conductivity of the lower part of paleosol layer (S1) was relatively weak compared to the upper part of S1, and a transient perched water was observed above the lower part of S1. The three-dimensional infiltration numerical model was established to reappear the soak infiltration process. Due to the multiple irrigation events, the downward percolation of water was significantly promoted by the superposition effect of wetting front. The infiltration capacity decreased with the increase of depth. The velocity of infiltration below 5.6 m depth was less than the hydraulic conductivity of the soil, the infiltration was driven solely by the gravity, and the moisture content and matric suction were almost unchanged. This finding explains the phenomenon that water infiltration in deep loess is difficult to be monitored in the previous studies.

Key words: loess, field monitoring, irrigation infiltration, moisture content, matric suction