Investigation and quantification of hydraulic gradient and precipitation effect on flow and solute transport in the capillary fringe and saturated zone

Abstract

Although the capillary fringe (CF) zone plays a critical role in water flow and solute transport within porous media, its influence is often neglected in numerical models. In this research, the HYDRUS‐2D model was validated with data from a physical experiment. The calibrated model was then used to investigate how variations in hydraulic gradient and precipitation affect flux and flow patterns in the CF. Results showed that the CF can be segmented into three horizons aligned with the flow direction: an upward horizon near the inlet, a middle horizon roughly parallel to the groundwater table slope, and a downward horizon near the outlet. Findings indicated that the CF's contribution to water flow and solute transport near the outlet was up to twice that near the inlet, mainly due to higher water velocities. On average, 11%–25% of the total flow passed through the CF, with the rest occurring in the unsaturated zone and saturated zones (SZ). The study demonstrated that HYDRUS‐2D effectively simulates water movement in porous media when accurate observational data are available. It also emphasized the CF's vital role in overall water movement. Four scenarios were compared: normal gradient, high gradient without rainfall, and high and low gradients with rainfall. The results showed that increasing the hydraulic gradient (i) reduced the CF's contribution to water flow and solute transport. At high gradients, especially during rainfall, most flow occurred through the SZ beneath the groundwater table.