Background The soil erosion of purple soil sloping cultivated land in the Three Gorges Reservoir area is serious, and the annual soil erosion amount is as high as 9 854 t. The infiltration properties of purple soil and its influencing factors have been widely studied, but there are few reports on the quantitative calculation of the matrix and preferential inflow of purple soil. In view of this, the purple soil in sloping cultivated land was chosen as the research object, and the characteristics of soil matrix and preferential inflow in purple soil plough layer were quantitatively discussed, which provided a scientific basis for the mechanism of infiltration process in purple soil plough layer.

Methods Taking the purple soil slope farmland in Beibei, Chongqing as the research object, the soil matrix and the total infiltration rate of the purple soil plough layer were quantitatively measured by using the surface ring infiltration meters, and then the preferential infiltration rate was calculated. The variation of soil matrix infiltration rate to preferential infiltration rate of this area with time was analyzed. Philip infiltration model and Kostiakov infiltration model were used to fit the matrix infiltration process measured at different test sites.

Results 1) The initial infiltration rate of soil matrix infiltration is 8.38 to 32.66 mm/min, the stable infiltration rate is 3.15 to 4.2 mm/min, the average infiltration rate is 3.62 to 6.26 mm/min. The initial infiltration rate of preferential flow is 37.01 to 42.34 mm/min, the stable infiltration rate is 0.04 to 1.59 mm/min, the average infiltration rate is 2.31 to 9.67 mm/min. 2) The cumulative infiltration of soil preferential flow in the purple soil layer of the study area is 0.6-1.5 times that of the soil matrix infiltration. The ratio of matrix infiltration rate to preferential infiltration rate is 0.3-0.5 in the initial infiltration stage; the ratio of matrix infiltration rate to preferential infiltration rate is gradually increasing during the process of infiltration. After reaching the stage of steady infiltration, preferential infiltration rate decreases, the ratio of matrix infiltration rate to preferential infiltration rate is 1.5-16.5.

Conclusions Soil matrix and preferential flow played the same important role in the hydrological process of purple soil slope farmland. The fitting results of soil matrix infiltration rate were good, and the determining coefficient was above 0.93.The results of this study may provide a scientific basis for studying the mechanism of infiltration process of purple soil.