Effects of developmental stages and parts of collapsing gully on soil hydraulic properties in southern Jiangxi

Abstract: The investigation site is located in Jinggouxing watershed (from 26°10′31″N to 26°12′33″N and115°9′3″E to 115o11′39″E) in the north of Gan County, Jiangxi Province, China. The soil type is mainly red soil developed by granite. The collapsing gully area is 95.8 hm2, which belongs to the intensity erosion area of the hills and mountains in the south of Jiangxi. Due to the sudden, violent, serious damage, and difficult to manage characteristics of collapsing gully, it has severely affected the local ecological environment and residents' lives, so it has become an important research direction for soil and water conservation. Collapsing gully is affected by hydraulic and gravity erosion. Among them, the hydraulic erosion of collapsing gully is mainly affected by surface runoff. The instability of the collapsing gully wall gradually increases through the undercutting effect of runoff, and then develops under the combined action of hydraulic and gravity erosion. Soil infiltration is the premise and basis of surface runoff. By analyzing the infiltration characteristics of collapsing gully soil, we can further understand the process of soil erosion by collapsing gully soil. In recent years, the disc infiltration meter has been widely used to determine soil hydraulic characteristics due to its fast and easy operation. In this paper, a disc infiltration instrument was carried out under four pressure heads (-9, -6, -3, and 0 cm) on the collapsing gully soil at three different developmental stages (initial stage, active stage and stable stage) and three parts (catchment, slope and channel). The basic physical and chemical properties of collapsed hills at different developmental stages were studied, and the correlation between the developmental stages and parts of collapsing gully to the basic physicochemical properties was analyzed. The variation of soil infiltration rate with different infiltration heads at different developmental stages and parts of the collapsing gully was plotted. According to the measured values of soil hydraulic conductivity and Gardner's constant α, the effects of collapsing gully at different developmental stages and their parts on the soil hydraulic conductivity and soil pore classification contribution to flow were analyzed. The results showed that soil infiltration properties were affected by soil sand content, pressure head, soil pore size distribution, soil erosion and space parts. The higher the sand content in the soil, the more severe the soil erosion would be, which would affect the soil infiltration characteristics. When the pressure head was 0, the stable infiltration rate at the collapsing gully in the developmental stable period was the highest, 1.01 and 1.40 times higher than that in the collapsing gully at the initial stage and active stage of the collapsing gully. With the decrease of the pressure head, the stable infiltration rate of the soil in the three different developmental stages gradually decreased. The soil hydraulic conductivity of different parts of the collapsing gully was basically similar under different pressure head conditions. There was no significant difference in soil hydraulic conductivity and Gardner's constant α at three different developmental stages. Except for the hydraulic conductivity at a pressure head of -9 cm, the soil hydraulic conductivity and the Gardner constant α values at different parts of the collapsing gully were significantly different. As the degree of erosion increased, the contribution rate of macropores to flow at different developmental stages increased. The parts of collapsing gully significantly affected the contribution rate of macropores, mesopores 2 and micropores to flow, it could be seen that the part of collapsing gully was an important factor which affected the hydraulic properties of soil. The results could provide some references for the soil erosion process of the collapsing gully soil at different developmental stages in the southern china