Characteristics of preferential flow of liana and secondary arbor forests along riverside based on Ground Penetrating Radar

The riparian zone is a transition zone between river ecosystem and forest ecosystem. The characteristics of preferential flow in forest belt along riverside have an important influence on the surface hydrological process of the watershed. Therefore, study of the relationship between soil heterogeneity and preferential flow is of great significance to local groundwater security and water ecological security of upstream and downstream rivers. In this paper, using the technique of Ground Penetrating Radar（GPR） combined with the traditional staining tracing method, the liana and secondary arbor forest sample plots along Yongding Riverside（the junction of Beijing and Hebei） were detected. The single channel oscillogram of the GPR image was extracted, and then the parameters of reflected wave and dye area ratio were fitted. Finally, the key step was to analyze the preferential flow staining features. The staining morphological characteristics showed that the preferential flow segment of liana sample plot was 5-30 cm below the soil surface, while the secondary arbor forest sample plot was 10-25 cm below the soil surface. The average dye area of the secondary arbor was 1.68% smaller than that of liana. The GPR lines results showed that the maximum amplitude and the total amplitude area decreased with the increase of soil depth at 0-15 cm. At 15-35 cm below the soil surface, the two parameters began to fluctuate. The maximum amplitude and total amplitude area of liana plot both were larger than that of secondary arbor plot. It was found that the dye area ratio was logarithmic with the maximum amplitude and the total amplitude area. The logarithm relationship between the dye area ratio and the maximum amplitude was y=25.104 lnx+25.137（R~2=0.8697）, and the logarithm relationship between the dye area ratio and the total amplitude area was y=23.874 lnx-26.862（R~2=0.6173）. The soil compactness of secondary arbor forest was better than that of liana. Therefore, there was less electromagnetic signal lost in the secondary arbor forest during the physical process. The electromagnetic signal of liana plot fluctuated greatly and lost more signals. It could be seen that the difference of soil structure between different layers of liana vegetation was more distinct than that of the secondary arbor, which led to preferential flow more frequently under liana coverage. In order to protect the safety of groundwater, it is necessary to cut down lianas appropriately and moderately in