Calculation and validation of wetting depth of infiltration in aeolian sand in arid areas based on particle model

Abstract: In Badain Jaran Desert, an extremely arid region with annual precipitation of 100 mm and annual evaporation of 3 000 mm, the sand underneath a dry surface layer of about 20 cm is relatively wet. Even at the top of the giant dunes, which is about 300 m high, the moisture content at the depth of 2 m is more than 3% in a volumetric fraction. The provenance of soil water in the wet sand layer has always been in dispute over decades. In order to give a better understanding of the contribution of local rainfall to the soil water in unsaturated sand layers, the theoretical analysis and model calculation was conducted. The physical and mechanical mechanisms of moisture movement in the process of water infiltration were analyzed, and the states of moisture in unsaturated sand layers were taken into consideration. Focusing on the microscopic physical model, by the assumptions of spherical particles, the descriptions for the formation mechanism on pendular water and water retention attributed by pendular water in particle materials were presented. And the analytical expressions for rainfall infiltration depth were derived based on the cubic particle model, in which the pendular water was the main water storage unit. To verify the applicability of the derived expressions in sandy soil, the field artificial water infiltration experiment was conducted in sandy soil of the Badain Jaran Desert. Based on the physical and mechanical analysis, sandy particle surface was difficult to hold the relatively thicker film water because of the relatively large particle size and the small surface charge density. The most part of liquid water in unsaturated sand soil was located between particles in the form of pendular water. The derived expressions of wetting depth of infiltration indicated that the wetting depth depended on the amount of precipitation and the wetting angle of pendular water. The initial moisture content of sand soil also had an effect on the depth of infiltration. Based on the balance between gravity and surface tension in the pendular water, it was concluded that the wetting angle of pendular water was negatively related to the grain size of sand soil. The result of field infiltration experiment indicated the expressions for the wetting depth of infiltration was applicable to the estimation of wetting depth of infiltration in sandy soil. However, further study was needed to investigate the mathematical relationship between particle size and the wetting angle of pendular water. In addition, the contribution of the local precipitation to the soil water in the wet sand layer was evaluated based on the study of wetting depth of infiltration and the ratio was 11.5 when the wetting angle was π/4. According to the calculated result, the wetting depth of infiltration was 115 cm after a 100-mm rainfall event in the Badain Jaran Desert. In consideration of the great evaporation in this area and the rare rainfall events, it was concluded that the local precipitation had little contribution to the soil water in the wet sand layer. The provenance of soil water in this area may be the groundwater.