Abstract:

Water infiltration of the vadose zone plays a key role in the water cycle. A better understanding of the relationships among rainfall, ponded water, and soil infiltration processes is critical to estimate groundwater replenishment and redistribution in desert environments. However, research on the moisture distribution in the vadose zone of sandy soil is still limited. In particular, few studies have examined the water transport processes in the desert vadose zone. Therefore, we aimed to determine the soil moisture distribution and variation in homogenous sandy soil under ponded infiltration and to validate models using data from these observations. This study was conducted in a nature dune slack through dynamic in situ observations using profile infiltration experiment and continuous soil moisture measurements at 1 min intervals. A single-ring infiltrometer (diameter, 20 cm; insertion depth, 5 cm) was used to keep a constant water head. Kostiakov-Lewis, Green-Ampt, Philip, Hydrus-1D/2D/3D numerical software based on Richard’s equation were used to describe the vertical infiltration and soil water movement processes and to identify suitable models for ponded infiltration processes in sandy soil. The validation indices included the sum of squared error and root mean squ are error. A comparison between the simulated and measured results indicated that the Philip model could predict the infiltration rate, cumulative infiltration, and wetting front advancement correctly, with an RMSE value of 0.0003, 0.04, and 0.24 cm·min^-1 and R ² of 0.95, 0.99, and 0.94, respectively. These values were clearly less than those of the Kostiakov-Lewis, Green-Ampt, and Hydrus-1D models, although the other models also showed good overall agreement with the field measured cumulative infiltration. The Hydrus-3D model yielded a better fit to the simulated soil moisture than the Hydrus-2D in the wetted zone based on the coefficient of determination, average RMSE (0.018 cm³·cm^-3) and R ² (0.93). Altogether, it appears that the combination of the Philip and Hydrus-3D models is a highly effective approach to estimate ponded infiltration parameters and simulate the water transport process in variably saturated sandy soil. The three-dimensional soil water diffusivity must be considered to predict water infiltration in sandy soils. Our results also demonstrate that the integration of numerical simulation and field measurements can improve the research efficiency of soil water infiltration in dry environments. In practice, the accurate prediction of soil water infiltration can be done by the selection of the proper models based on the soil properties of the particular sites.

Key words: vadose zone, ponded infiltration, numerical simulation, sandy soil