###### Abstract  
The rainfall infiltration model plays a key role in the fields of seepage theory and geological hazard evaluation. To reflect the change in the moisture content in soil during rainfall infiltration, the infiltration process was segmented into stages, and an improved unsaturated wetting front extension model was proposed by introducing unsaturated soil parameters on the basis of the traditional infiltration model. The variation rules of surface moisture content and wetting front depth were revealed under the condition of uniform or non-uniform moisture content distribution in soil. The results indicate that the surface moisture content and wetting front depth increase nonlinearly with time during the infiltration process, and the proposed model is in good agreement with the numerical simulation results, the maximum error is less than 5%, while the growth rate under two hypotheses shows obvious differences for different rainfall intensities. Moreover, due to the variation in the moisture content with time, the proposed model calculates a lower moisture content than that of the traditional model under the same conditions, but the expansion depth of the wetting front is higher and the range of influence on soil is greater.  
os 2022 2022 15 June 20221  
overall. Nevertheless, it is difficult for soil to reach a saturated moisture content under the condition of non-ponding infiltration, and the calculation based on the saturated permeability coefficient may result in a great error. Therefore, many scholars have improved the model.  
In the past, most studies used the hypothesis of the horizontal wetting front in the Mein-Larson model. For example, [PERSON] et al. [9] established a non-ponding infiltration model based on the hypothesis of the horizontal wetting front by introducing the unsaturated permeability coefficient of soil, but the effect of matric suction was ignored. [PERSON] et al. [10] considered the influence of matric suction on this basis, reflecting the change in soil moisture content with time, but their method is mainly suitable for soils with strong matric suction. [PERSON] et al. [11] studied the rainfall infiltration process when the initial soil moisture content was in linear distribution, but its scope of application is small. The hypotheses above are too simplified, limit the calculation accuracy and are inconsistent with the actual conditions. Some scholars [12,13] found that the wetting front is not horizontal through experiments and field tests, so some hypotheses of other shapes were proposed. [PERSON] et al. [14] believed that the shape of the transition layer is an ellipse, and its thickness is about half of the wetting depth, but the changing process of the transition layer was not considered. [PERSON] et al. [15] also adopted the ellipse shape hypothesis and segmented the rainfall process, taking into account the role of the soil moisture profile in the model, but the application of unsaturated soil parameters was imprecise. [PERSON] et al. [16] adopted the equivalent unsaturated soil permeability coefficient on this basis, which achieved good results, but mainly aimed at soil with a uniform initial moisture content.  
In view of the problem above, this study segmented the infiltration process, introduced unsaturated parameters, and put forward an improved unsaturated wetting front expansion model under different initial moisture content hypotheses, meaning that it reflects the non-ponding infiltration process more realistically and provides theoretical support for engineering disaster prevention.