The infiltration of muddy water in soil has a complex change of upper boundary process. Among them, the hydraulic conductivity of soil has been one of the most important indicators for the infiltration characteristics of muddy water in the soil. In this study, a series of infiltration tests (9 orthogonal test treatments and 8 groups for the model validation) were performed on the muddy water in the saturated soil column. The hydraulic conductivity of the formed dense layer was also determined during the infiltration of muddy water. The dynamic model was then constructed for the hydraulic conductivity of the sand under multiple factors （sand content of muddy water, clay content, and infiltration time） using multiple regression. A verification test was made on the dynamic models of the hydraulic conductivity of the saturated soil columns in the muddy sandy and silt loam soil. It was assumed that the muddy water was infiltrated into the saturated soil column. The results showed that there were highly significant effects of the muddy water sand content, clay content, the gradation of sediment particles, and infiltration time on the hydraulic conductivity of the sand （ P<0.01）. The first factor was the infiltration time, followed by the sand content, and the last was the clay content. The coefficient of determination （R²）was 0.853 in the dynamic model for the hydraulic conductivity of sand, and the root mean square error was 0.004 cm/min, indicating the better-represented relationship between various factors and hydraulic conductivity. Meanwhile, the values of root mean square error was less than 0.01 cm/min, and the mean absolute value of relative error was less than 7%, indicating the highly reliable dynamic model of hydraulic conductivity. The R² values of the dynamic model were 0.912 and 0.930 for the sandy and silt loam, respectively, and the values of root mean square error were 2×10⁻³ and 5×10⁻⁵ cm/min, respectively. The values of root mean square error were less than 0.01 cm/min, and the mean absolute value of relative error was less than 17% in the model validation, indicating the excellent agreement between the calculated and measured. The finding can greatly contribute to the theoretical basis of the infiltration process of muddy water in the soil, together with the barrier mechanism of the dense layer.

Muddy water irrigation, an effective water-saving irrigation method, has been widely used in the Yellow River Basin in China. To investigate the effect of sand content on water infiltration and dense layer formation under one-dimensional vertical infiltration of muddy water, muddy water infiltration experiments were performed in the laboratory, and five sand contents of muddy water (S=0%, 3%, 6%, 9%, and 12%) were used. Models were established to describe the relationship between the cumulative infiltration amount [I(t)] and the infiltration duration (t); the relationship among the migration distance of the wetting front (Z), S, and t; the thickness of the sedimentary layer [H(t)]; and the relationship between S and t. The results revealed that I(t) and Z decreased significantly with the increase of sand contents, while H(t) increased significantly with the increase of sand contents. I(t) and Z were in the range of 7 cm and 20 cm for each treatment, respectively. The variation in I(t) with t fitted Kostiakov and Philip models, and the coefficients of determination were all greater than 0.99. With the increase in S, the infiltration coefficient gradually decreased, the infiltration index gradually increased, and the sorptivity gradually decreased. The particle composition of the sedimentary layer was similar to that of the argillaceous sediment, and the content of particles with a size of less than 2 mm in the sedimentary layer was lower than that of the argillaceous sediment. Compared with the original soil, the content of particles with a size of less than 0.05 mm and physical clay particles (diameter less than 0.01 mm) in the soil with an infiltration depth of 0-2 cm increased. The retention layer was from the topsoil to the infiltration depth of approximately 2 cm. This study can provide a scientific basis for further research on soil infiltration mechanisms under muddy water.