Effect of precipitation intensities on preferential flow and its numerical modeling

Abstract: The preferential flow is common in the soil with preferential paths, which affects soil water infiltration greatly. Although previous studies have made a great progress in preferential flow, the effect of preferential paths on preferential flow has not been reported, and a great effort is necessary to understand how precipitation intensities affect preferential flow. Therefore, in this paper, five undisturbed soil columns were sampled at the depth of 0-100 cm with different distribution of preferential paths from the Experimental Station of the Institute of Hydrogeology and Environmental Engineer, Chinese Academy of Geological Science (Hebei, China) and from Luancheng Experimental Station of Chinese Academy of Science (Hebei, China). Tests of different precipitation intensities, i.e., 0.23-0.24, 0.07-0.11, and 0.008-0.042 cm/min, were done to investigate preferential flow of the five soil columns. And the Hydrus-1D software coupling a dual-permeability model was applied to simulate the preferential flow of the soil columns under differential rainfall intensities. The results showed that the preferential paths played a vital role in development of the preferential flow since the levels of preferential flow were higher with more preferential paths in the soils. The relative transport abilities of preferential paths, which were defined by the ratio of velocity of preferential flow to rainfall intensity, increased with heavier rainfall. When the rainfall intensities were larger than the maximal coefficients of hydraulic conductivity of the intact soil columns, the velocities of preferential flow increased remarkably with increase of rainfall intensities. However, when the rainfall intensities were smaller than the maximal coefficients of hydraulic conductivity, the velocities of preferential flow increased little or no change. The effect of rainfall intensities on the ν and increased ranges of ν was related to the development of preferential paths in the intact soil columns, i.e., the effect can be stronger with the higher degree of preferential paths. In the soil columns with better development of preferential paths, the initial times of soil water flowing out at the bottom of the soil columns were shorter when rainfall intensities were larger, while in the soil columns with well developed preferential paths, the initial times of soil water flowing out at the bottom of the soil columns decreased firstly with a higher rainfall intensity, and then increased. The dual-permeability model described preferential flow well but couldn't simulate the process that soil water in the soil matrix flowed out horizontally to preferential paths after the rainfalls. As such the model was not suitable for the soils with plenty of soil fissures. The percentage of preferential flow obtained from the results of modeling was 93.6%-99.9%, which was in agreement with the tested results in undisturbed to disturbed soil. It suggested the model could simulate the preferential flow well. However, the results may be affected by size of columns since the characteristics of preferential flow varied with study scales. Therefore, further studies were needed to improve the understanding of preferential flow in larger scales. The results here will be useful to evaluate the groundwater recharge, water saving methods and movement of soil pollutants in arable land.