Abstract: In numerical simulations of unsaturated soil water infiltration,conventional approaches typically prescribe a single hydraulic boundary condition and ignore the transition between different boundary types induced by rainfall intensity or variations in soil hydraulic conductivity. In this study,a one-dimensional unsaturated seepage model for loess that explicitly accounts for hydraulic boundary transformation was developed based on the boundary treatment proposed by van Dam et al.(2000). The model determined boundary switching by comparing the infiltration flux,the instantaneous maximum infiltration capacity,and the saturated hydraulic conductivity,and employed the Richards equation as the governing flow equation. Based on an implicit finite difference scheme,the proposed model was numerically solved to simulate the water infiltration process in loess under switching conditions between flux and head boundaries,and was validated using existing soil column experiments. The results show that hydraulic boundary switching has a significant influence on the infiltration process in loess. Neglecting the ponding head after a flux-to-head transition leads to an underestimation of soil water infiltration,whereas assuming complete infiltration of rainfall results in overestimation of the total infiltrated water and the wetting-front depth. Moreover,the transition from flux to head boundary does not occur instantaneously but experiences a short"head-flux-head" oscillatory stage. In addition,boundary switching influences only the infiltration process itself and does not affect the final water content distribution of the soil column after rainfall.