Effects of the Sand Characteristics on the Dynamic Capillary Pressure during the Downward Infiltration.

Infiltration is the process of water moving from the soil surface into the soil which is important to irrigation, groundwater recharge, and chemicals transmission process in the soil. Many methods have been proposed to model the infiltration process, such as Green-Ampt model (GAM) and Richard's equation. The capillary pressure is often assumed to be constant in the GAM during the infiltration process, and the capillary pressure-saturation curve is assumed unique when applied in the Richard's equation. However, studies have showed that the capillary pressure-saturation curve depends on the velocity of water movement. The velocity-dependent capillary is called dynamic capillary pressure. In this study, we performed a series of infiltration experiments in sand columns with different types of sands and under different boundary conditions. To show the effects of dynamic capillary pressure, we compared the results of the experiments with the simulations from the GAM and the modified GAM which takes the dynamic capillary pressure into account. We found that the dynamic effects are more significant during the infiltrations in sands with large grain size than the ones with small grain size. In addition, we also showed the dynamic effects of the capillary pressure during the oil infiltration into air-dry and pre-wetted sands.