Assessment of the size selectivity of eroded sediment in a partially saturated sandy loam soil using scouring experiments

Highlights

• Sorting and transport of particles in partially saturated sandy loam soil were investigated.

• Stress characteristic of soil particles is important in affecting transport process.

• Eroded sediments transport with suspension/saltation by a unimodal distribution.

• Distribution, sorting of sediments are controlled by aggregation state of particles.

Abstract

Infiltrating water accumulates in the plough pan because of the low permeability of this layer, which results in saturation or partial saturation (soil water contents between the field water capacity and saturated water content) of the cultivated layer during continuous rainfall. Even in sloping farmlands, soil infiltration via large pores created by roots, wormholes and straw rot continuously increases, and the infiltrating water forms subsurface flow above the plough pan, causing the soil in the cultivated layer to become partially saturated. The processes and characteristics of soil particle sorting and transport in partially saturated soil were investigated in the laboratory using scouring experiments. The results showed that the stability of the aggregates was destroyed when the soil was in a soaking state and tended to be saturated, which produced a unimodal distribution of eroded sediment finer than 0.021 mm, and the main transport mechanism along the partially saturated soil was suspension/saltation. The eroded sediment in the erosion process was mainly composed of silt-sized particles, accounting for approximately 44–60% of the sediment load. Destruction of aggregates during erosion can easily enrich the soil in fine particles, especially clay-sized particles. Most fine and coarse sand-sized particles were predominantly aggregates of other finer particles; the aggregation of fine sand-sized particles was relatively poor, and a certain number of fine sand-sized particles were transported as single particles. The results reported in this study have important implications for an in-depth understanding of sediment transport in partially saturated soil.

Introduction

The natural processes of erosion, transport and sedimentation have been active throughout geological time and cause severe engineering and environmental problems (Julien, 2010, Warringtonet al., 2009). Soil erosion destroys land resources, thins the soil surface layer of agricultural farmland, reduces nutrient and clay contents, increases soil degradation and decreases land productivity, which affect agricultural sustainability (Kiani-Harchegani et al., 2019). Sediment-laden runoff is not only a water pollutant but also a catalyst, carrier, and storage agent of other forms of pollution (Julien, 2010). The transport of eroded sediment changes the content and composition of soil carbon, nitrogen and phosphorus (Shi et al., 2017), thus affecting the global biogenic element cycle.

The detachment-transport-deposition processes of soil erosion by water depend on the interaction between the external force of erosion (the external factor) and the soil erosion resistance (the internal factor). Erosional forces are caused by rainfall and runoff, while soil erosion resistance is mainly controlled by the soil structure. Although many soil properties directly or indirectly affect the water erosion process, the soil structure is considered to be the most important and direct factor (Bryan, 2000). The quality of the soil structure can effectively reflect the ability of soil to resist erosion and is one of the important evaluation indexes of soil erodibility (Rabot et al., 2018). Soil sand particles, clay particles and organic compounds become cemented and bonded to each other, forming aggregates of different sizes, which are further organized and arranged in three-dimensional space to form the macroscopic soil structure. Hence, soil aggregates are the basic soil structure units. Aggregate breakdown is important in the detachment process because it provides a substantial material supply for erosion (Wuddivira et al., 2013). When aggregates are broken down by raindrop impact and/or slaking processes, the disaggregated particles are deposited within soil pores to begin the detachment and transport processes (Han et al., 2019). The sediments eroded during the processes of erosion, transport and sedimentation exhibit different size distributions, and the particle size distribution (PSD) of eroded sediment particles can reflect changes in the erosion process. The size distribution of sediments during the transport process is affected by many factors, such as (a) the particle size distribution (PSD) of the original soil, (b) the aggregate stability, (c) the aggregate breakdown and (d) the settling velocities of different sized particles or aggregates (Asadi et al., 2007a). Due to the unsteady water flow and mobility of the boundaries, the type of sediment movement shifts among the different movement states. The movement of sediment particles during erosion can be divided into static, suspension and bed load (saltation, contact (rolling) load) (Moss et al., 1979). In the process of slope erosion, the form and distance of sediment movement depend not only on the hydrodynamic characteristics of the slope runoff but also on the properties of the sediment itself, especially the size and density of the sediment particles (Asadi et al., 2011). Eroded sediment contains both individual and aggregate particles, and the PSD of these soil particles can significantly affect the processes of erosion detachment and transport (Rienzi et al., 2013). In addition, the transport conditions differ among sediment particles of different sizes. The sand content in the eroded sediment is dependent on the rainfall duration, which indicates restricted transport conditions. The contents of clay and silt decrease with increasing rainfall duration, and the transport conditions are supply-restricted conditions (Durnford and King, 1993). The transport processes of coarse and fine particles may be different. Therefore, with the in-depth study of erosion mechanisms, the sorting characteristics and transport mechanisms of sediment particles in the process of erosion have received increasing attention (Hanet al., 2019, Haoet al., 2019, Kiani-Harcheganiet al., 2019, Rienziet al., 2013, Wanget al., 2018, Warringtonet al., 2009). Several well-known soil erosion models, such as the CREAMS (Foster et al., 1985), WEPP (Nearing et al., 1989) and GUEST (Misra and Rose, 1996) models, also consider the role of the eroded sediment particles in erosion, thus resulting in more accurate soil erosion predictions (Saygin and Erpul, 2019).

On slopes, infiltrating water accumulates in the plough pan because of the low permeability of this layer, and the soil moisture of the cultivated layer therefore increases and approaches saturation during continuous rainfall (Holthusenet al., 2018, Huanget al., 2018). Compared with non-saturated soil, partially saturated soil has a higher soil water potential in terms of the maximum soil matrix potential and high-pressure potential inside the soil. At the same time, the gravity-induced movement of free water in the saturated soil layer and formation of interflow in the surface soil further exacerbate soil erosion (Martínez et al., 2013). Soil aggregates are continuously subjected to the dispersal effects of water and air entrapped during the process of soil approaching saturation. Under these conditions, structural breakdown occurs, resulting in attendant runoff and erosion and in a reduction in agricultural soil productivity (Wuddivira et al., 2013). Partial saturation results in a greater ability to transport sediment particles at higher soil moisture and runoff velocity levels, and the pollutants attached to sediment particles will enter downstream waterways along with runoff during the erosion process, thereby causing serious nonpoint source pollution in river basins. Most studies have only focused on how soil physical properties change after saturation, but the more serious problems of erosion and sediment transport caused by partial soil saturation urgently require attention. The present study aimed to i) analyse the characteristics of the eroded sediment particle distribution and sorting during the erosion process, ii) examine how eroded sediment particles are transported in the erosion process of partially saturated soil, and iii) characterize the erosion particularity of partially saturated soil.