Experimental Study on Wetting Front and Air Counterflow in Unsaturated Sand Columns During Ponded Water Infiltration

This study investigates water–air coupled transport characteristics during ponded water infiltration in unsaturated sand columns through systematic laboratory experiments. The experiments considered three soil textures, two initial dry densities (1.50 and 1.60 g/cm3), and four initial saturations (0% to 41%), with synchronous monitoring of pore pressure and volumetric water content using pressure sensors (P1–P7) and moisture sensors (W1–W5) to track dynamic changes in wetting front, pressure, and saturation. The results reveal four distinct stages of pore pressure variation during ponded water infiltration: pressure soars (Stage I), pressure ascends with air compression (Stage II), pressure surges due to air breakthrough (Stage III), and pressure stabilization (Stage IV). The duration, intensity of these stages, and wetting front migration rates are significantly influenced by soil texture, initial dry density, and initial saturation. Specifically, lower dry density and clay content shorten the time for the wetting front to reach the column bottom, while higher initial saturation promotes entrapped air bubble breakthrough, triggering Stage III. This study enhances understanding of water–air coupled transport in unsaturated sandy soils, providing insights for optimizing irrigation and soil-water conservation strategies.