Field infiltration of artificial irrigation into thick loess

Frequent agricultural irrigation triggers numerous landslides in the loess platform, northwest China. We conducted a field infiltration experiment with a diameter of 20 m on the South Jingyang tableland in Shaanxi province, China to better understand the infiltration process in thick unsaturated loess. Soil moisture probes, tensiometers, and differential pressure transducers were installed at various depths to monitor the infiltration process. The results showed that the initially high infiltration rate gradually decreased and finally approached a constant value less than that of the saturated hydraulic conductivity of the top soil. Matric suction decreased rapidly, and the volumetric water content increased to a nearly saturated state with the arrival of the wetting front. The soil water characteristic curve obtained from field monitoring data agreed with that from laboratory tests performed on undisturbed specimens. Preferential flow associated with vertical cracks parallel to the margin of the platform was observed in the late Pleistocene Loess layer (L1), and an empirical model was developed from monitoring recordings to explain preferential flow formation. A transient perched water table formed above the lower part of the first paleosol layer (S1). The monitored pore-air pressure increased and then fluctuated before the arrival of the wetting front. The maximum value of pore-air pressure at different depths was less than 5 kPa and tended to increase linearly with dry density. The results of this research help understand the migration of irrigation water in thick unsaturated loess and the recharge mechanism of the deep groundwater table. •A large-scale instrumented irrigation experiment was implemented in unsaturated thick loess.•Infiltration process was revealed by recordings of infiltration rate, moisture content, matric suction and pore-air pressure.•The first paleosol layer was permeable, whereas a transient perched water above the lower part of paleosol was observed.•The maximum value of pore-air pressure has a linear relation with dry density of soil.